Organic Chemistry Alkane Structures

Welcome back to Educator.0000

Next we are going to talk about alkane structures.0002

How do we name alkanes (that would be their nomenclature), what kind of physical properties do they have, what kind of shapes to they have?--that sort of thing.0005

We'll start with the nomenclature of alkanes.0013

Unfortunately, this is the one part of organic chemistry where you can't avoid doing some memorization.0016

Because just like we have to memorize the alphabet in order to learn how to spell and communicate with the written language, we need to know the names of organic molecules if we are to communicate about them.0022

I've shown here the first ten alkanes--C1 through C10.0035

These ten names--you will most definitely need to commit to memory; and so this is maybe a good time for some flashcards and some practice.0043

But of course the more you practice with these, these are going to become second nature to you much like the alphabet is to you.0052

The very simplest alkane with one carbon is called methane.0059

Two carbons is ethane; propane; butane.0062

Pentane is the five carbon just like a pentagon has five sides so that prefix isn't too hard to know.0066

Same with hexane just like a hexagon; hexane has six carbons.0073

Heptane has seven; octane has eight carbons just like the octopus or the octagon has eight sides or eight legs.0077

Nonane is nine; and decane is ten; again, that is a pretty easy one to remember since a decade is ten years.0085

Out of these ten that you... at least these ten you will need to know.0091

Some of them are quite easy and others you will quickly become familiar with as you start working with them.0096

Now what we are going to find when we go to name an alkane is we are going to first identify a parent carbon chain.0104

We are going to give that parent carbon chain one of these names--methane, propane, decane, and so on.0111

Attached to that carbon chain are going to be additional groups; we can call those substituents because they are substituted onto the carbon chain.0117

If one of those substituents is another carbon chain, we are going to name that substituent with the same sort of nomenclature.0124

But we are going to replace... instead of having an -ANE ending like we do for our parent chain, we are going to have a ?YL ending.0135

A one-carbon carbon group is called a methyl group.0144

Two carbons... I'm sorry, three carbons is called a propyl group.0150

Two carbons is called an ethyl; and we could have hexyl and octyl and so on.0155

It is also common to have halogens attached to our alkaness; so let's talk about that nomenclature as well.0161

We are going to add an -O... use an -O suffix for these groups.0168

So a fluoro, chloro, bromo, iodo is what we call it when it's a halogen attached.0172

It is also quite common to find a benzene ring attached to a parent chain.0176

Whenever we encounter that we are going to call that a phenyl group; so that is good to know as well.0180

The rules we are going to follow in doing our nomenclature is--the very first thing we are going to do is to inspect the structure and identify the longest carbon chain.0187

If we have more than one choice, we are going to identify the one with the maximum # of substituents.0196

And we are going to name that parent, again, using one of these names shown--propane, butane, and so on.0202

The next thing we need to do is we need to number the carbon chain.0209

And we are going to... we have to number from one end or the other.0213

And we are going to choose the end that is closest to the first substituent.0217

Then we are going to name and number the substituents that are attached.0222

We are going to see how you can use prefixes like di- and tri- and tetra- and so on in just a moment.0225

Finally, we are going to alphabetize all of our substituent names and list them out in front to come up with a complete IUPAC name.0230

IUPAC stands for the International Union of Pure and Applied Chemists; this is the committee that has developed these names.0238

The beauty of nomenclature for organic chemistry (although it might not be the most exciting subject in the world) is that all around the world all chemists use this same nomenclature rules.0248

Whether you are English-speaking or not, when you see this molecule and follow these same systematic rules, every person around the world should come up with the same name for that molecule.0262

That is what makes it a very important thing to know.0274

For example, we can look at this very simple example.0279

We have a three carbon chain; so a three carbon chain is going to be called a propane.0283

But it is not just propane, right?--because we have this chlorine attached to it; and we need to figure out where it is attached.0291

The way we are going to do that is--we are going to number the carbon chain.0298

In this case, it doesn't matter which end we start numbering from; we will get the same result--one, two, three.0301

We see that the chlorine is on the second carbon; so we are going to call this a 2-chloropropane.0306

And this all becomes all one word; there is no space here; we are going to put a dash in between numbers and letters; so this molecule is called 2-chloropropane.0315

How about the next one?; our first step is to find the longest carbon chain.0325

I see a one, two, three, four-carbon chain here; so this is going be a butane derivative.0331

Then we need to number the chain; if we numbered from this end--one, two, three; our substituent would be on the third carbon.0345

If we numbered from this end--one, two; our substituent would be on the second carbon.0352

We are going to number it in such a way to give the substituents the lowest possible numbers.0357

What do we have attached to... sometimes it is really useful to put a box around or a circle around your parent chains so you remember which carbons you have accounted for just by naming the parent.0365

We now have an additional carbon off of carbon 2.0377

We are not going to call that a methane group--there is no such thing; we call it a methyl group; we use the ?yl.0381

So we have a 2-methyl... butane is the name of the structure; 2-methylbutane.0386

The next one is a little more challenging because we have to search for our longest carbon chain.0396

If we look straight across--one, two, three, four; we would find a four-carbon chain.0400

But if we follow a different path and go up here, we see that we can come up with a longer carbon chain.0405

Remember a molecule can be drawn in any way so you want to look very closely to find the longest continuous carbon chain.0413

As long as you are still connecting to another carbon, you don't stop till you hit an end--a carbon that has no other carbons attached.0422

So let's see: this one, two, three, four, five, six carbons; so this is a hexane derivative.0429

We need to number those carbons; we can either start from this end or start from this end.0438

But because this is closer to the first group--we would get a group on carbon 2; we will number from there--one, two, three, four, five, six.0446

Again we have methyl here... we have two methyls; we have a methyl on carbon 2 and a methyl on carbon 3.0459

Rather than list those repeating substituents separately, our names would get very long, very cumbersome.0464

What we do is we take the common substituents and we group them together; and we call it instead a dimethyl.0471

And what we do is we list where those two methyls occur; one is at carbon 2, one is at carbon 3; so we call it a 2,3-dimethyl... a 2,3-dimethylhexane.0477

Now we are going to put a comma in between the numbers just like you would if you write the number 1,000 or 1,000,000; put a comma between numbers and always a dash between numbers and letters.0493

These rules are using the IUPAC nomenclature; in addition to the IUPAC rules, there are a small number of very commonly used names for commonly seen groups; we do need to be familiar with these as well.0504

For example, if you think about a three-carbon chain that you want to attach to a parent, there is two possible points of attachment.0523

You can either attach it from one of the end carbons; so you would end up... if this is your parent, you would have a CH₂CH₂CH₃ substituent; we call that a propyl group.0534

Its common name--it is called an n-propyl just because it's a normal propyl; it is a normal straight chain carbon.0550

If you attached at the middle carbon instead, the group that you've attached would be a CH with two CH₃s on it.0558

This is a different arrangement; this is an isomer of the original arrangement so we call this isopropyl.0567

You may have heard of the word isopropyl when something like isopropyl alcohol is an astringent that you might find in your medicine cabinet; and it means you have an isopropyl group.0576

If we come back to this first structure, this 2-chloropropane also has an isopropyl group.0588

It is a three-carbon chain with something attached to the middle carbon; and so a common name for this compound could be isopropyl chloride.0593

Isopropyl alcohol would have an OH group attached in this position.0612

There are a few common names that you will need to be familiar with; isopropyl is one example.0618

Another commonly encountered substituent is if we had a four-carbon chain that we wanted to attach.0624

Now again if we attach just at the first position... so this was the end position; this was the iso position.0632

Here if we attached at the first carbon, we would have a CH₂CH₂CH₃; so just a normal butyl group; so this is called an n-butyl.0639

And if we were to attach at the second carbon which is described as a secondary carbon because it has two carbons attached, we would get the isomer that is known as sec-butyl.0656

What would that look like?--you have one carbon... I'm sorry, you have a three-carbon chain CHCH₂CH₃ with one carbon group attached.0670

Here you see we have a four carbon chain that is attached at the second carbon or the secondary carbon of that.0683

This group is known as the sec-butyl group... this is known as the sec-butyl group.0690

Again you could have sec-butyl alcohol or sec-butyl chloride and so on.0696

Finally, if we... we could have another arrangement of carbons like this--of four carbons; so these are commonly encountered.0703

If we attach at this middle carbon, this middle carbon is described as a tertiary carbon because it has three carbon groups attached.0712

That is where the name tert-butyl comes from; we use the prefix tert- to describe one carbon with three methyl groups attached to it.0725

This is still a butyl group because it has four total carbons; but we've attached at the middle carbon with this arrangement; this is called tert-butyl.0739

Or you could just call it t-butyl; sometimes we just use the first letter of the abbreviation; so tert-butyl.0751

The last possible arrangement for four-carbons is if we attached over here; this one is known as iso... the isobutyl.0759

What does that look like?--it has a CH₂... it has a CH₂.0771

Then we have this isopropyl group; so isobutyl is kind of like an isopropyl with an extra carbon.0776

So then we have what we saw for isopropyl; and this is called isobutyl.0783

So just a brief introduction onto some common names that we will be encountering when we take a look at a lot of the names that you are going to be encountering throughout your reading.0791

What would we do if we had a slightly more complex structure?--let's take a look at this one.0806

We might have a line drawing.0818

Looks like we have a one, two, three, four, five, six, seven, eight-carbon chain here.0827

You might be tempted to take that as your parent.0832

But if you look more closely, you see you have a one, two, three, four, five, six, seven, eight-carbon chain in that direction as well.0835

How do we distinguish between these two?0843

We come back here, and if we see that if there is a tie between two possible carbon chains, we are going to pick the one with the maximum # of constituents.0845

We are going to take the more complex path here... we are going to take the more complex path to have a simpler name.0855

Now we need to number our chain; if we number from this end, we get our first substituent at 2 and then the next one is at 3.0873

If we number from this end, we get our first substituent at 2 and our second substituent at 2; so that is going to be a better choice.0881

So this is carbon #1, 2, 3, 4, 5, 6, 7, 8; so this looks like an octane derivative.0889

What does this octane have attached to it?--well, this is where it is very useful to draw a circle around your parent chain because now you can see all the other groups that you need to account for.0905

We have a chlorine at 7 and a chlorine at 6; we will combine those together because it is the same group; we have a 6,7-dichloro... we have two chloryl groups.0915

At carbon 3, we have a two-carbon chain; a two-carbon compound is called ethane; so a two-carbon substituent is called ethyl; so on carbon 3, we have an ethyl.0936

At carbon 2, we have... what is this sticking out here?--it must be just a one-carbon group so we have a one-carbon... we have a methyl here and a methyl here.0951

Again, we have two methyls so we are going to combine them; we are going to call this a 2,2-dimethyl... a 2,2,-dimethyl.0961

If you are saying that it's a dimethyl, that means you have two methyl groups so there has to be two numbers appreciating that--showing where both those methyls are.0973

Here they happen to be on the same carbon so we call that a 2,2-dimethyl.0980

Now how do we organize all this information--we are going to just alphabetize it.0985

We are going to list... when you alphabetize, you ignore the prefixes that you added; so you are going to ignore the di- in this case.0991

The first letter that would come first is the C for chloro; so we would have a 6,7-dichloro... took care of that one.1002

Then we have an ethyl and a methyl; ethyl comes first; then we do a dash 3-ethyl... took care of that one.1014

Now we have our methyl; so then we go -2,2-dimethyl; and then we go right into octane; no space there.1022

You can see these names can get pretty long pretty quickly; this is called 6,7-dichloro-3-ethyl-2,2-dimethyl octane.1033

Let's take a look at the molecule formula that we expect for an alkane and something known as a degree of unsaturation.1046

A general formula for an alkane is C_nH_2n+2.1054

If we have that formula, we describe that as being a saturated hydrocarbon--that means we have the maximum # of hydrogens.1061

Being saturated describes the relationship: it is saturated with hydrogen atoms.1072

For example, if we have heptane... using this formula we could determine the molecular formula for heptane.1079

Heptane means that we have seven carbons; and so how many hydrogens are there?1087

Must be 2n+2; so 2n is 14; plus 2; so the formula for heptane is C₇H₁₆.1094

That is another very good formula to know C_nH_2n+2; that is going to come in handy.1109

Why does it always end up being 2n+2?--well, let's take a look at just a regular carbon chain and let's think about how many hydrogens this carbon chain is going to require.1116

We know that a carbon is going to want four bonds; so every one of these carbons has two hydrogens.1127

You will see that we always have twice the # of hydrogens as we do carbons; but the structure isn't complete, is it?1133

At the end, we need to cap it off with a hydrogen on each end; and that is where we get our plus two; 2n+2.1141

It turns out that no matter how I arrange these carbons or if I had ten carbons and put them all different connectivity, we are always going to have that same number of hydrogens.1149

What if I wanted to add a ring however?--what if I wanted to connect the ends of these carbons?--what would I have to do?1160

I would have to remove a hydrogen here and remove a hydrogen here in order to connect these; so addition of a ring is something that causes me to lose two hydrogens.1168

What if I wanted to add a pi(π) bond to this structure?--again in order to form a double bond here, I'd have to remove a hydrogen from each carbon.1182

The addition of a π bond also causes a loss of two missing hydrogens.1193

So when we inspect a formula and we identify that there are two missing hydrogens--that gives us an important clue.1197

Every two missing hydrogens is called a degree or a site of unsaturation; we would describe that as having one DU--if a formula had two missing hydrogens.1206

For example, let's took a look at the formula C₉H₁₆ and ask how many degrees of unsaturation there are.1218

A lot of textbooks have pretty complicated formulas for these; and really the only formula you ever need to know is this one right here.1227

As long as you know this formula, doing these types of problems is very straightforward.1234

The question you ask yourself is: if it was saturated, what would the formula be?1238

Well, there is nine carbons; so how many hydrogens does that require?1244

2n+2; so 9 times 2 is 18; plus 2 is 20; there is our 9 times 2 plus 2.1250

If it was saturated, our formula should be C₉H₂₀; but it is not; it is actually C₉H₁₆.1263

All we have to do very simply is compare those two formula; and what do we find?--there are four hydrogens missing.1270

Because every two missing hydrogens is one degree of unsaturation; four missing hydrogens means we have two degrees of unsaturation.1284

Let's try another one--how do we determine the degrees of unsaturation if our formula contains something other than carbons or hydrogens?1296

How about a halogen?--if I wanted to put a halogen on this structure, what would I have to do?1305

I would have to remove one of the hydrogens and put a halogen in its place; because just like hydrogens, halogens require just one sigma(σ) bond.1310

Every time we see a halogen, we are going to count as a hydrogen; we are going to count it as if it were a hydrogen because they take the same place.1320

If we see an oxygen in the formula, we get to ignore these; it turns out that adding an oxygen to this structure has no effect on the # of hydrogens.1331

If you think about an oxygen, it can form two bonds.1340

If I wanted to put in an oxygen here, I would just squeeze it in between one of the bonds that are already here; and I would not have to replace any hydrogens.1345

So I get to ignore the oxygen; and so that means this formula is the equivalent of having C₆H₁₂.1352

Because we drop the oxygen and we included the chlorine with our hydrogen count; and now we proceed as usual.1364

What do we do?--we ask: if it was saturated, what would the formula be?--for 6 carbons, we need 14 hydrogens; 2n+2.1372

We compare that to the actual formula C₆H₁₂; and we see that there are two missing hydrogens.1384

Does that mean we have two degrees of unsaturation?--no, every two hydrogens missing is one DU.1393

That is how we can work with a formula of an alkane or a related compound to determine unsaturation.1403

You could even look at a structure and determine the degrees of unsaturation.1410

Without considering the formula, let's take a look at the structure of benzene and determine how many degrees of saturation are there.1415

Well, a degree of unsaturation is defined as either a ring or a π bond; because those are the structural units that cause hydrogen to be missing.1423

What does benzene have?--it has one ring; plus one, two, three π bonds; so it must have four DU.1436

So determining degrees of unsaturation given a structure is very simple; we just count up the rings and the π bonds.1451

If you wanted to check the formula and practice your calculation there and confirm that based on the formula you would also determine that it is four DU.1457

The benzene formula is C₆H₆; each of these carbons has just one hydrogen attached.1468

So it is C₆H₆; and if you do your calculation, you should come again with four DU for benzene.1474

One way that understanding these degrees of unsaturation can be very helpful is if you have an exercise such as the one shown here.1483

If I asked you to draw the isomers of C₅H₁₂, the very first thing you should do is calculate the DU.1490

That is going to tell you something about the structure; so let's do that.1497

If it was saturated, what would the formula be?--for five carbons, we need 2n+2; we need twelve hydrogens.1500

Guess what?--our structure has all twelve hydrogens; so that means there are zero DU; there are no degrees of unsaturation.1513

What clue does that give us about our structure?1522

It tells us that all isomers of formula C₅H₁₂ can have no rings and they can have no π bonds... no rings and no π bonds.1526

Because those structural elements would not allow twelve hydrogens to fit onto the structure.1542

This frees us from ever having to count the hydrogens in our structure.1548

All we have to worry about now are the carbons; so let's deal with our five carbons; how can we put them together?1552

One way is to connect them all in a straight line; this would be one isomer of C₅H₁₂.1559

I know that when I fill in the hydrogens, I will have twelve; I don't need to do that initially while I am working this problem.1567

A systematic way of approaching the problem would be to say--OK, well how about having a four-carbon chain?--and then taking that one extra carbon and attaching it somewhere on that chain.1575

What if I attached it to this first carbon?--what do you think about that?--does that look like a new isomer of C₅H₁₂?1586

Well, no; because this is really the same structure I've drawn; it is still five carbons all in a line.1594

Just because I've drawn it differently on the page doesn't mean it is something new.1600

That is something you are really going to have to be on the lookout for when you are drawing isomers--that you don't have any repeat structures.1604

But if I had four carbons and attached it to the second carbon, that would be a unique carbon chain from this; so that would be a new isomer.1611

Again I am guaranteed that they would have 12 hydrogens when I fill in all the hydrogens.1621

How about four carbons and then I move it over to the third carbon--that extra carbon?1626

Wow, that looks like it is a repeat of this again, right?--all I did was flip the molecule over.1634

This is where understanding nomenclature can come in handy; because these both would have the same name--this would be 2-methylbutane; this would be 2-methylbutane.1638

Checking the name of a compound is a great way to determine whether you've drawn a repeat or not.1650

How about going to a three-carbon chain and having two extra carbons to work with?--well, I could maybe put one here and put one here.1658

Here is five carbons; do you think that is something new?--or do we still have a four-carbon chain with an extra carbon on the second carbon?--still 2-methylbutane.1671

Three carbons and maybe I can do it as a two-carbon chain?--nope, it's still a four-carbon chain.1684

So it is going to be... you are really going to have to look very closely.1692

The last arrangement we can have is having the three carbons and then putting both carbons on the middle carbon; this now would be a unique arrangement of the carbon atoms.1695

It is just these three possible isomers; they are three isomers; they are all C₅H₁₂.1706

I was able to do this problem so much more quickly by just thinking about degrees of unsaturation and not ever counting out my hydrogens.1715

You can go ahead and fill in those hydrogens and convince yourself that you have 12.1722

But what I know immediately is that I don't have to consider a structure like this--how about if I put the five carbons in a ring?1727

Could that be one of my isomers?--it can't be because it has a ring; and that means it is not going to be C₅H₁₂.1736

It is going to be C₅H₁₀ because putting in a ring causes it to lose two hydrogens.1743

So very useful to know this technique when you are working on isomer problems.1751

Let's talk about how alkanes will behave; what sort of physical properties will they have?1759

Alkanes just have carbons and hydrogens so they are going to be nonpolar molecules.1765

That makes them not very soluble in water... not at all soluble in water because water is a very polar molecule.1771

We know like dissolves like; so we can describe alkanes as being hydrophobic... as being hydrophobic because they have very poor interactions with water.1779

They are isolated from petroleum reserves and crude oil.1794

Part of the distillation process in the refinery separates out the alkanes based on their boiling points and so this is where we get our alkanes commercially.1798

It turns out that alkanes are fairly unreactive; remember these have all strong σ bonds.1810

An alkane can't have any π bonds; it has no other atoms in there other than carbon and hydrogen.1818

Because it has no π bonds and it has no lone pairs, there really aren't too many reactions that they are going to want to undergo.1827

There are few exceptions; one reaction is that alkanes can be used as fuel.1840

In other words, it can undergo combustion; and that of course is what we do with many of the alkanes we get from crude oil.1846

If you looked at the names, propane is the fuel we use for our gas barbeques; methane is the fuel we use to heat our homes; butane are for disposable lighters; octane is a component in gasoline.1855

So we might be familiar with many of these alkanes in our everyday lives because of their role as being a fuel.1875

There is a single reaction that alkanes can undergo that we typically study in organic chemistry--is called a halogenation reaction.1882

That is where we take an alkane, and we convert it to an alkyl halide.1890

We replace one of the hydrogens on the alkane with a halogen like chlorine or bromine; and that is one of the reactions we will be studying for alkanes in just a bit.1896

Let's talk about their physical properties; here we have a series of alkanes.1907

We have a three-carbon chain and then a six-carbon chain and then a thirty-one carbon chain.1911

What do we expect?--what do we observe happening here?--we see an increase in our boiling points.1917

If you have a boiling point of -42 degrees, that means it is going to be a gas at room temperature.1923

Room temperature is somewhere around 20 degrees C so this is going to go to a liquid form all the way down at -42.1930

This three-carbon chain is propane, and we know propane is a gas; that makes sense.1939

When we have six-carbon chains, we now have a boiling point of 69; so hexane is a liquid.1944

That can be used as a solvent; again components in gasoline; we have hexanes in there so that is a liquid.1951

When you have a boiling point above 300 degrees, that means it is going to be very hard even to melt it and then to boil it.1961

This is a solid; it is a waxy solid; kind of like beeswax or candle wax might have a formula like this; very, very long carbon chains; completely hydrophobic; greasy, waxy, that sort of thing.1970

So what is the trend here? What explains this increase in boiling points?1986

Simply as we increase our molecular weight... they all have the same polarity of course, they are all nonpolar.1993

But as you increase your molecular weight, you increase your surface area.1998

You increase your van der Waals attraction or London forces; and that is going to increase your boiling point.2007

So the trends we saw in looking at the various intermolecular nonbonding interactions.2013

How about the shape of alkanes?--what do they do look like in three-dimensions?--we can have different shapes known as conformational isomers.2023

I put isomers in quotes here because these are not isomers in the traditional sense of isomers that are non-interconvertible unique compounds.2034

Conformational isomers (also called conformers) can be interconverted; and so that is why I prefer not to call them isomers; I prefer to call them conformers instead.2045

The way they are related is they are structures which differ only by rotation about σ bonds; so conformers... conformers are different forms of the same molecule.2060

They are interconvertible; because they way we go from one to another is simply by rotation around a σ bond.2075

For example--conformations of ethane; if we take a look at an ethane molecule, we have just a two-carbon structure.2080

This is a CH₃ (one, two, three); this is a CH₃.2089

The various conformations that ethane can have are these: we can simply rotate around this carbon-carbon bond; and in doing so, this molecule is going to have a different 3D shape.2093

The relationship of this conformer to this conformer--they are called conformational isomers or conformers.2104

I have shown here examples of two extreme conformations; one conformation has two hydrogens pointing up in the same direction.2113

If we rotate this just 60 degrees, we will see in our 3D sketch that one hydrogen is pointing up and one is pointing down.2123

Or we can rotate it this way and get that same conformation where one hydrogen is up and one hydrogen is down.2132

We have names for these conformations; when the two hydrogens in the plane are pointing up--we call this the eclipsed conformation.2139

When one is up and one is down--we call this the staggered conformation.2150

These are rapidly interconverting... rapidly interverting conformers; so these are not two different molecules; it is simply one molecule that is rotating around very rapidly.2157

One way that you can really see why we call these eclipsed and staggered conformations is--when we view the molecules not sideways like we did in the 3D sketch.2175

But we view it in this way--where we are viewing down the carbon-carbon bond.2183

When we do that now the way we are going to see the eclipsed conformation is when the three groups in the front exactly align with the three groups in the back.2189

So this has one straight down... or they both have one straight down or maybe they both have one straight up.2199

We called this the eclipsed conformation; just like the solar eclipse or lunar eclipse or when these things line up with one another.2206

And the staggered conformation is when you have the bonds in the back are intersecting the bonds in the front; and so it goes front, back, front, back; and they are staggered in that way.2212

This point of view is called a Newman projection; it is a very valuable projection.2224

The way we can see that is by standing over here and viewing the molecule like this: looking down the C₂C₃... I'm sorry, the carbon-carbon bond.2228

What do we see on this front carbon?--on this front carbon, we see a hydrogen pointing straight up; and then we have hydrogens down one is to my right.2240

If I'm looking here--the wedged bond is to my right and the other one is to my left; so this is what we see on the front carbon.2252

What do you see on the back carbon?--well, the back carbon you see the exact same thing--there is a hydrogen straight up and the other two hydrogens are down into the left and right.2263

The way we show the back carbon on a Newman projection is we draw it as a big circle so that we can see them both.2270

So this is the back carbon, and this dot is the front carbon.2278

The back carbon in this eclipsed conformation... we recognize that the back hydrogen is exactly behind the front hydrogen.2288

But what we do is we tilt it just a little tiny bit so that we can see it; so it is visible.2296

So in our drawing we move it slightly; but we keep them very close to each other because we recognize that in fact they are exactly aligned.2302

This is what the eclipsed conformation looks like in a Newman projection.2313

If we rotate the front carbon slightly like we did here... so we rotated around this carbon-carbon bond to get this structure.2318

Now by rotating it 60 degrees, we take this front hydrogen and move it 60 degrees clockwise; this front hydrogen and this front hydrogen--all three groups rotate as we twist the molecule.2331

What happens now is--on this front carbon when we take the same perspective... now on the front carbon, we have a hydrogen pointing straight down; and the other two are up and to the right and left.2345

On the back carbon, we see a hydrogen pointing straight up; and then the other hydrogens are down to the right and left.2358

So here we see the staggered conformation--a little easier to see in this Newman projection.2367

Are these conformations equal in energy?--they are not; the eclipsed conformation is higher in energy.2374

It is less stable because of the interactions between the electrons in the CH bond in the front and the CH bonds in the back being perfectly aligned.2385

We get some interactions there; we call that a torsional strain in having those groups aligned.2396

When they are spread out, we have a staggered conformation which is lower in energy.2408

While the staggered conformation is lower in energy, again, this is just rapidly rotating, freely rotating around quite easily.2415

Let's call this eclipsed conformation A and the staggered conformation B; and what we can do is we can trace the energy of this molecule as it goes through the various conformations.2425

If we start with the A conformation which is staggered, a 60 degree rotation... again if we are rotating, it looks like we are rotating this front carbon because the back carbon stays the same.2442

We go from being staggered to being eclipsed; that's structure B.2452

Another 60 degree rotation would bring me right back to another staggered conformation; it will bring me right back to a conformer that is equivalent to A.2456

And so on; the next rotation brings me back to B, and so on.2466

So if we were to trace our energy curve, at 0 degrees initially, we would be at staggered conformation; that is a low energy conformation.2470

Then after 60 degrees, we will be at a higher energy conformation--the highest energy conformation in fact; the maximum energy you can have; the least stable is when it is completely eclipsed.2481

But then as you continue rotating, you come back down to your staggered conformation and then back up to eclipsed; down to staggered and so on as you rotate.2494

If you were to track just a single hydrogen all the way through 360 degrees, you would come back to A.2507

What happens if you trace the energy of this?--the staggered is the absolute most stable conformation.2514

As you start to bring those hydrogens closer and closer and closer, they become eclipsed and then we are at the maximum conformation energy.2520

Then as they slide past one another, then we start to decrease in energy and we go down to a fully staggered and so on.2527

We go up to eclipsed and down to staggered and up to eclipsed; and this is what our energy diagram looks like as we rotate through the various conformations of ethane.2534

Butane is a more interesting molecule because instead of just having two carbons and three hydrogens on the front carbon and back carbon.2550

If we take a look at butane which has four carbons... if we view it again as a Newman projection, take a look at what is going to happen.2560

As you do your conformations, what you can see pretty quickly is that we have a methyl group on the front carbon and a methyl group on the back carbon.2571

That relationship of those two methyls is continuously changing as we rotate it 360 degrees.2581

So what do you think is going to be the absolute worst conformation of butane to have?2586

Looks like it is probably right here where those two methyl groups are exactly aligned to each other; they are eclipsed with each other.2592

In fact, you can even see that they are crashing into each other; we will talk about that in a second.2598

And what would be the best possible conformation--the lowest energy?2602

It is going to be the one where these large methyl groups are as far apart from each other as possible; so we will take a look at that as well.2606

For the Newman projection, what we do for butane--the most interesting point of view is to look down the carbon-carbon bond between the middle two carbons; so we call that viewing between the C2-C3 bond.2615

Looking from this perspective, what do we see on the front carbon?--we see a methyl group pointing straight up.2632

So we have a methyl group pointing straight up, and we have hydrogens down into our right and left.2641

What do we see on the back carbon?--we see a methyl group going straight down.2647

This line now is drawn from the circle because the circle represents the back carbon.2651

Then we have a hydrogen up into the right and up into the left; so this is a Newman projection of butane.2656

If we were to trace the energy as we go from one conformation to the other, let's take a look at the forms that we are going to have.2671

We start out in this first conformation where the methyl groups... first of all, it's a staggered conformation and the methyl groups are as far apart from each other as possible.2678

When we rotate 60 degrees, what we are going to do here is we are going to rotate the front carbon clockwise.2687

As usual, that is going to bring us from a staggered conformation to an eclipsed conformation.2694

Continued rotation brings us to the staggered conformation; we see our methyl group traveling down.2700

Now it is going to be eclipsed where it is at the bottom; and it is staggered again; and then it is eclipsed.2705

Then our final rotation is going to bring us back to our first structure A; so our last rotation to get 360 degrees brings us back to A.2710

We have names for these staggered conformations.2722

When the two methyl groups are as far apart from each other as possible, we call that the anticonformation; anti meaning opposite--they are 180 degrees from each other in this dihedral angle.2725

When the two methyl groups are right next to each other, we call this interaction a Gauche interaction; let me write the name down first--Guache.2737

What happens in a Gauche interaction is we start to get some interaction between the bulky methyl group on the front carbon and the methyl group on the back carbon.2748

We describe that interaction as steric hindrance or just sterics for short--and this is the interaction we have between very large or bulky groups.2759

It is not a good thing to have large groups near each other; that will cause some instability--this steric hindrance; so a Guache conformation has some steric hindrance.2768

This is another Guache conformation--when the two methyl groups are in adjacent positions; wWe describe that as being Guache; there is two Guache conformations.2781

When we look at our three staggered conformations--A and C and E, what is going to be the most stable conformation?2793

The best one is going to be the anti where the methyl groups are as far apart from each other as possible; so let's put that on the lowest possible level.2801

Then when we come over to C... if we imagine this A level here--this energy level.2811

When we come over to C, we expect that to be... because there has some steric hindrance, we expect this to be a little higher in energy than A.2820

E has the same exact Gauche interaction that C does; so E is going to be at the same energy as C.2835

Then when we go 360 degrees, we come all the way back to A; and that is full circle.2844

Now let's compare our eclipsed conformations; our eclipsed conformations are all going to be bad; but which one is the worst?2851

Here we have two interactions of methyls with hydrogens; here we have an interaction between two methyls.2857

When our two largest groups are eclipsing each other--that has the worst sterics; and that is going to be the highest energy--eclipsed.2865

We said that the staggered... we mentioned that up here... is the lowest energy.2881

So when we come to our first... when we come to B, it is going to be a high energy conformation.2888

But when we get to D--the eclipsed one, it is going to be the highest energy--the least stable; we are going to put this all the way up here.2896

Then when we come to our final eclipsed structure F, it has the same interactions that B does--the methyls with the hydrogens; so we are going to put that at the same level--the exact same energy as B.2904

So the energy of B equals F; these are the two eclipsed conformations with equal energy.2923

And the energy of C is equal to E; those have equal energies as well.2933

Now when we connect the... when we follow the energy path, we are going to go up to an eclipsed conformation, back down to a staggered.2941

Up to the worst eclipsed, down to a staggered, up to eclipsed, and back down to the most stable conformation.2951

When we follow this path with our model, we are starting with the front methyl group straight up and the back methyl group straight down.2961

Then we are going to be rotating this clockwise; so we are going to go up in energy to eclipsed; and then back down in energy to a staggered.2971

But this staggered is not going to be as good because we have this Guache interaction; we have a little steric interaction between those two methyls.2980

Then we are going to continue and we are going to go up in energy to the highest eclipsed because we have these two methyl groups interacting with steric hindrance; so that is the worst.2987

But as they swing past each other, we go back down to a staggered; and then back up to an eclipsed; and then back up to our best staggered.2996

We are always going to be alternating in energy between maxima and minima; what is real important is to note that all eclipsed conformations are high energy.3005

Even the best eclipsed is still bad compared to any of the staggered; and all the staggered ones are low energy; so these are some things to point out.3020

You also want to keep in mind that there is a significant difference between the eclipsed ones and the staggered ones.3035

The difference between the various eclipsed conformations is on the order of 1 kcal where the difference in energy between the worse eclipsed and the best staggered is about 5 kcal.3042

Likewise the difference between the Guache and the anti is around 1kcal; so not a huge difference between these staggered energies or between the eclipsed energies.3066

But between the staggered and eclipsed, that is where you want to see the biggest jump when you are trying to do these sorts of energy diagrams for conformations.3076

If we take a look at cycloalkanes, when we try and form a ring and we try and do it with either a three-carbon chain or a four-carbon chain, and we try to form it into a ring.3088

Even with this model, you can see that we are meeting some resistance; that is not an easy thing to do; in fact, that is a good way to break a model kit.3102

The problem is that if we are to try and form a triangle--a cyclopropane, this bond angle would have to be 60 degrees in order to be a triangle.3113

It is very difficult for it to make that; carbon wants to be 109.5 because it is sp3 hybridized; and it can't do that.3128

Even with a cyclobutane, this would require a 90 degree angle; and that would also squeeze or compress that angle together; we describe that strain as angle strain.3136

In addition, when you take a look at the structures... if you do form this and you look at the Newman projection, you see that the front hydrogens exactly line up with the back hydrogens.3149

We call those eclipsing each other; and we describe that as torsional strain; and there is no way to alleviate that unlike the regular butane where we can rotate in various conformations.3162

This is locked into this rigid structure and so there is no way to get around that.3173

So when you take a look at cyclopropane, you have this wedge and dash hydrogen and this wedge and dash hydrogen; and they are all eclipsing each other; and so that offers some torsional strain as well.3177

So both cyclopropane and cyclobutane are unstable molecules.3200

Together we describe this combination of torsional strain and angle strain as having a high amount of ring strain; we call it ring strain when we try to form these small rings of three and four members.3210

It is possible that these compounds do exist but they are very high energy; they are highly reactive and difficult to form.3229

When you form a five-membered ring, however, it is going to be easier to bring these together because we have more room.3236

You can see actually even with this model kit it is quite easy to bring these ends together and connect it.3245

This has very little strain; the angles can naturally be very close to 109.5.3251

If we were to hold the molecule completely planar, then we would have this eclipsing interaction again with the front and the back.3258

But if we bend the molecule slightly, we can alleviate some of that torsional strain and get some better interactions.3265

This conformation is known as the envelope conformation--where you have four carbons in one plane and then the fifth carbon is bent out of that plane; and that is what cyclopentane looks like.3273

Cyclohexane is a compound that we will spend quite a bit of time studying.3298

Because when you have a six-membered ring and you bring it together, the structure you end up with has absolutely no ring strain.3304

It is completely flexible; it is just as flexible and just as stable as it would have been if it were in a straight chain; and so six-membered rings are very common in organic chemistry.3312

What we will find is that because this can rotate about the σ bonds, there are various conformations that cyclohexane can have; the two extreme conformations are shown in the drawing here.3325

One is called a chair conformation, and that is where you have--four carbons are in one plane; that is kind of like the seat of the chair; and then you have this carbon pointing up and this carbon pointing down.3339

Or vice versa; you have four here and the one on this side is pointing down; the one on this side is pointing up; this is called a chair conformation.3360

The other extreme conformation is where both carbons on either end are pointing in the same direction.3368

Here we have our four carbons are in one plane, and then we have this carbon up and this carbon up; this kind of is a boat shape; we call this the boat conformation.3374

As shown here with this equilibrium, you can see that it is favoring the direction of the chair; the chair is the more stable conformation; and the boat is less stable.3383

There are a number of reasons that the boast is less stable.3399

One is that if you look at the hydrogens on these top carbons, you see that they are facing in near one another; and so we have some steric interactions; these are called flagpole hydrogens.3402

In addition, if you take a look at the hydrogens on these two front carbons, you see that these are eclipsing; so we have some torsional strain as well.3421

In fact, the boat is so unstable that a cyclohexane really never hangs out in a boat conformation.3436

What happens instead to alleviate some of these interaction rather than having these two hydrogens pointing in right towards each other.3443

It twists a little bit to avoid that interaction; we call that the twist-boat conformation.3450

You will see this drawing sometimes as a way to represent something that is not as good as a chair but not quite as bad as the boast.3455

These two conformations are interconverting because we can rotate around; and so the chair kind of goes through a twist boat as it is going to the boat and back to the chair.3463

Drawing a cyclohexane chair is going to be a skill that you want to develop in organic chemistry; and let's go through how to do that.3480

The first thing you want to do is draw a wedge; draw a little angle here.3489

Then you are going to draw two bonds coming down that are parallel to each other; and then you are going to draw another wedge pointing up.3495

Wherever this bond was drawn, this bond is going to be parallel to that; and however this bond was drawn, you are going to draw a bond parallel to that.3506

This is the general form that a chair is going to take; with some practice, you get something good there.3516

What you want to avoid is just drawing a zigzag thing like this because then it is going to be very hard to show any detail on that chair.3524

Now what you will notice when you look at a chair conformation... notice that I've put some different colored atoms on this to illustrate the two different types of positions you have on a chair.3532

Some of these atoms are pointing straight up... let me get a good looking... my model is a little too wobbly here.3545

Here you see that the red atoms are pointing straight up and straight down.3554

Notice that the chair is a zigzag; it goes up down, up down, up down, kind of like we draw our zigzag line drawings.3560

Every carbon that is a zigzag up has a hydrogen atom or a position that is straight up; every one that has a zigzag down has one that is going straight down.3567

These are the first ones we should draw onto our chair--this is a zigzag up so it has a hydrogen going straight up; this is a zigzag down so it has one going straight down.3576

And then we go up; and then we go down; and notice that they are alternating up and then down.3589

The perspective that we take is that this carbon up on the top is behind these carbons in the front.3594

We are looking at the chair from an angle; and we are tipping it down a little bit so that these front are downward and the back carbons are the ones that are closer to the top of the page.3602

So we have these six positions that are all straight up and down; we call these axial positions; so let's label these as axial hydrogens.3614

Six that are straight up... I'm sorry, three that are straight up and three that are straight down; those are the easiest ones to put on first.3625

Then on each of these positions, each of these is a CH₂; so there is another hydrogen on each of these positions but take a look at these.3638

These are all pointing around like the equator of the molecule; these are called the equatorial hydrogens.3646

Notice that none of them are pointing straight out; they are not at a 90 degree; they are either slightly down or slightly up... slightly down or slightly up.3653

When we drawn in the equatorial hydrogens, we take a look at the axial we've drawn and then we know that it is a 109.5 angle; so we draw it slightly down and then slightly up.3664

We could draw these on this end; and then over here we could draw slightly down and then slightly up.3683

Then we have these in the middle as well; this is where we will draw this equatorial hydrogen; and this is where will draw this equatorial hydrogen.3693

Again with practice, you will get better at this.3701

Every bond that you draw... the axials are straight up and down; every other bond that you draw is going to be parallel to some other bond.3704

See that this equatorial is parallel to this bond in this cyclohexane; and in a nicely drawn cyclohexane, you will see that you can see this W here or you can see an M here.3713

Those are some of the features you can look for as you are drawing it and try to decide where to put your equatorial and your axial.3727

A Newman projection of a cyclohexane is interesting because as you view down one carbon-carbon bond, you are simultaneously viewing down the parallel carbon-carbon bond.3734

If we view the molecule in this direction looking down this carbon-carbon bond, we are at the same time viewing this carbon-carbon bond.3749

On this front carbon, I see a hydrogen straight down; and a hydrogen up into my right; and up into my left, I have this CH₂.3757

On the back carbon, I have a hydrogen straight up; and a hydrogen down into my left; and down into my right, I have a CH₂.3771

There is our Newman projection of the carbon on the left.3786

The carbon on the right, I see a hydrogen straight down and then a CH₂ up into my left that is connecting this CH₂.3789

On the back carbon, we have a hydrogen straight up; hydrogen down into the right; and down into the left, I have this CH₂ that was down into the right from the other back carbon.3806

So a lot to go through pretty quickly; but as you take your time and try it on your own, what this illustrates very nicely is that this is an all staggered conformation.3821

Coming back to why the chair conformation is so stable, not only does it avoid the flagpole hydrogens, but it also avoid the eclipsing interactions that you have with the boat.3830

Everything is very nicely staggered.3842

When you draw a chair conformation or you look at a chair conformation for cyclohexane, what we will find is that there is actually two possible chairs that any cyclohexane derivative can have.3851

Right now the molecule I have has all the red atoms in the axial position.3863

If I perform a little magic trick here, I can convert it to the conformation where all of the white atoms are in the axial position; what I just did there is called a chair flip.3869

I've drawn a model here where I just have an X representing the axial positions.3883

What happens in a chair flip is that you take this end carbon and flip it up; and you take the other end carbon and you flip it down.3889

Let's take a look at that with the model; there is four carbons in the middle--that is the seat of my chair.3898

I take the one on the far left that is down, and I bring it up into a boat conformation temporarily.3910

Then I take the one on the far right, and I bring it down; and I have a new chair.3918

The way we can translate it to the next chair is--let's number our carbons.3926

Let's call this 1, 2, 3, 4 (those are the front carbons) 5, 6.3929

This carbon that is on the far left is still on the far left; he was pointing down, but now he is pointing up; this is 1.3937

Then because we numbered counterclockwise here, we are going to continue numbering counterclockwise.3944

The front carbons are still in the front; 1, 2, 3, 4, 5, 6; and on carbon 1, the X was in the down position.3949

If you think of a cyclohexane, if you flatten out the ring--totally flatten it out, six of the atoms are pointing up and six are pointing down, right?3965

Because he started out in the down position, he is still going to be in the down position; where is the down position?--it is right here.3975

Carbon 2 has an X up; carbon 2 still has an X up; and they alternated--down and then up, down and then up.3983

Look what happened: all of our axial groups are now equatorial; and all of our equatorial groups have become axial; so this called a chair flip.3993

Let's see an example of it; here we have methylcyclohexane.4005

If we wanted to do a chair flip of methyl cyclohexane, what we do is we would take this carbon on the left and bring him up; bring the carbon on the right and bring him down.4008

Then the chair that we have cannot have the same shape anymore; instead of starting down and going up, it is going to start up and go down; and then we draw our two parallel lines.4019

So this is our new chair; they should have this mirror relationship to one another.4035

This carbon on the far right is now this carbon on the far right; the methyl was up; the methyl still is up.4042

Where are the two positions on this carbon?--because it is a zigzag down, that means it has an axial straight down and an equatorial slightly up.4051

Where does the methyl go?--he was up and in the axial position; now he is up and in the equatorial position.4064

So these are the two chair conformations of methylcyclohexane.4071

The next question we can ask is: are each of these conformations equal in energy?--and the answer is no.4076

This is the more stable conformation; and this is the less stable conformation.4083

That is because the large methyl group has some sterics when it is in the axial position as is here; it has steric interactions called 1,4-diaxial interactions.4093

When we have an axial group, it sees these axial hydrogens; and they can crowd each other and have some sterics; so the sterics are called 1,4-diaxial.4117

That makes this less stable; so this equilibrium lies in the forward direction here where the larger group is put in the equatorial position.4129

Let's just try one example of this; how about if we are asked to draw the chair conformations of cis-1-t-butyl-4-methylcyclohexane?4142

Cis means that the groups are both up or both down.4151

Let's draw a chair; let's just draw it flat first.4159

In one position, we have a methyl group; in the other position, we have a tert-butyl group.4167

Just so we get oriented, this is what we are trying to draw.4174

We want to draw chair conformation; so let's draw one chair... OK, and... just trying to straighten out my lines here.4178

Let's makes these our end carbons the opposite carbons here.4192

Here we have a methyl group up; so let's put our methyl group up on this end.4196

Then on the far end on the fourth carbon, we have a t-butyl group up; that would be one chair conformation.4203

The other chair conformation where we bring this down and we bring this up--would look like this; here is our other chair conformation.4215

Our methyl group is up here in the axial position; and our t-butyl group is up here in the equatorial position.4229

What if we have to make a choice?--we know that large groups prefer to be in the equatorial position.4240

But how do we decide if they both can't be in the equatorial position?--it would be great if they could, but they can't here.4246

Because the tert-butyl group is a larger group, the larger group prefers the equatorial.4253

Larger group prefers the equatorial position; so our equilibrium is going to lie in this direction.4269

Remember the problem with having the larger group in the axial is that this group crowds the other axial groups--hydrogens in this case; called 1,4-diaxial interactions.4274

If you take a look at what a methyl group looks like compared to a tert-butyl group, a tert-butyl group is notorious for being a very bulky group; it has three methyls attached to a single carbon.4285

So this would have a lot of steric hindrance where a small methyl group wouldn't mind as much being in the axial position.4296

We can look at a variety of substituents and see what their preference is for being in the axial versus equatorial position.4303

We call this the A value--it is the energy difference between the axial and equatorial.4313

Of course, for a hydrogen, there is no difference; a chlorine is slightly larger, so it has a small energy difference.4318

Methyl and ethyl are close to the same; remember an ethyl group can just rotate out of the way to avoid this interaction.4324

But when you get to t-butyl, look at this huge number; a t-butyl group is so bulky that it essentially locks; we could say that this is locked in this chair.4331

It is so difficult to flip that t-butyl group up into the axial position; so this is a way to lock a single conformation.4343

One interesting example is how about this alkyne--this triple bond group; why is that such a small number?--can you think about the geometry of the triple bond?4350

If I were to put this in an axial position, what is that going to look like?--it goes straight up, doesn't it?4361

So how much interaction is it going to have with these other axial hydrogens?--not very much compared to a tert-butyl group... let's put this on here while I have the model.4371

A tert-butyl group in this position--there is no way you can alleviate that steric strain; so there is a very high energy there.4382

But this is so small because it is linear; and so when we put that in an axial position, it goes straight up; and there are very little steric interactions so it doesn't mind being it.4390

So when we have to make a choice between chair conformations, we are going to pick the one with the largest group in the equatorial position.4411