Understanding the Substitution Reactions

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Substitution Reactions Transcript:

Let’s take a look at an example of such a reaction; here we have hydroxide reacting with chloromethane. Our product has, where the chlorine used to be on the carbon chain, we’ve replaced it now with the OH group; and the chlorine is now on its own as chloride. This is described as a substitution since we have replaced one group with another group; we’ve substituted; and let’s define some of the players in this reaction.

Nucleophile

This first species I’ve labeled Nu:–that stands for nucleophile; so I am going to be using the abbreviation; Nu with a lone pair. A nucleophile is defined as something that is electron rich; so typically we will see a lone pair on a nucleophile; we will also see examples where a ? bond can behave as a nucleophile. It can have a negative charge; like our nucleophile in this case is hydroxide; we see a negative charge. But a very nice abbreviation for nucleophile is Nu: with a lone pair; that lone pair right away gives you some clue as to the behavior of this nucleophilic species. Because it is electron rich, a nucleophile is going to be seeking an electron deficient species.

Electrophile

The next part in this reaction is described as the electrophile; a very handy abbreviation for electrophile is E⁺. Again that tells us something about the species; because electrophiles are things that are electron poor. It might be possible that you will see a positive charge to show the electron deficiency; or more likely what we will usually have is a partial positive (?+). That is true for this electrophile here; it is a neutral species; there are no charges. But because this chlorine is pulling electron density away from the carbon, the carbon is in fact partially positive; and that is what makes him a good electrophile. An electrophile is something that seeks an electron rich species; oh, I should mention, where does the name electrophile come from? It is called an electrophile because it is electron loving; right?–and that makes sense; if it electron deficient, it is going to be seeking electrons, loving electrons. A nucleophile is called such because it is nucleus loving; and what sort of things do we have in the nucleus of an atom?–we have protons; we have positively charged species. A nucleophile is something that is seeking out electron deficient species; and electrophiles are something that are electron deficient and seeking out electron rich species. What we have here with the nucleophile-electrophile pair really is just a perfect match. We are going to see that nucleophiles and electrophiles coming together are going to explain the vast majority of the organic reactions we are going to be seeing down the road.

Leaving Group

The other part in this equation is in this case played by the chloride; it is called the leaving group; which I am going to abbreviate LG; sometimes it is just abbreviated with an L. This as the name implies is a group that leaves; and as it leaves, it takes its two electrons with it. You can see that this bond ended up leaving with the chloride because now it has four lone pairs of electrons. That is going to be true of all of our leaving groups; they will take electrons with them. We are also going to be learning about what makes a good leaving group; we will find that stable groups, things that are stable on their own, make very good leaving groups. Things that are weak bases are typically pretty stable; so we will look for that, those sorts of features. A general reaction… and I should also mention that this leaving group we had here is a chloride. For example, X^– makes a very good leaving group; we will see chloride, bromide, iodide as a very common leaving group that we can have; these are all weak bases because it is conjugate. Look at HCl; how do we describe HCl?–we know that is a strong acid; so that tells us that the conjugate base chloride is a very weak base; very stable; and so they make very good leaving groups.

General Reaction for Substitution Reactions

Overall the general reaction for the substitution reactions we are going to be studying in organic chemistry look like this. We have some kind of nucleophile reacting with some kind of carbon chain; this R represents a carbon chain; that has a leaving group on it. When the reaction is done, that R group is going to have the nucleophile bonded to it in place of the leaving group. The leaving group is going to now be on its own; and typically with an extra lone pair and a negative charge. If this our general reaction, what are some things we can study about it? What if we wanted to understand the mechanism by which these starting materials get converted to these products?–what are the possible mechanisms for a substitution reaction? There is really two reasonable possibilities; one possibility would be a simultaneous mechanism. Or in other words, a concerted mechanism in which the nucleophile attacks the carbon, starts to form a bond with the carbon group, at the same time as the leaving group leaves.

Substitution Reaction Mechanism: Simultaneous

That would be just a single step mechanism that would accomplish the transformation; and this mechanism actually does exist.  We will be studying this; this mechanism is called the Sn2 mechanism–a simultaneous concerted single step mechanism.

Substitution Reaction Mechanism: Stepwise

But another possibility is that we have a stepwise mechanism in which the leaving group leaves first. If the leaving group leaves, what does that leave behind on this carbon?–this carbon is now missing a bond so this would end up being a carbocation. Then that carbocation could combine with the nucleophile to give our substitution product; and in fact that mechanism happens as well; this mechanism is known as the Sn1 mechanism. We are going to be learning both about this concerted mechanism and this stepwise mechanism; and they are called the Sn2 and the Sn1.

Example of SN2 Mechanism

We will start with the Sn2 mechanism; and here is an example of such a mechanism; we said this is the one-step mechanism; so what does it look like? We could show arrows to follow the electron path, the electron movement, the bonding; our nucleophile… I’m sorry, we should label these guys. This group that is coming in and doing the substituting, we describe a nucleophile; here again we have something, we have the lone pair and a negative charge. The alkyl halide here is going to be our electrophile; again why is it electrophilic?–I don’t see any electron deficient species. Well sure, that carbon bearing the leaving group, bearing the halogen, is in fact partially positive. There is going to be an attraction between this electron rich nucleophile and this electron deficient carbon; and that is what is going to help form this bond. As that bond is formed, this carbon-iodine bond is going to break; and those electrons are going to go to the iodide. There is our mechanism; just two arrows; single step; and we get our two products–this is our substitution product and this is our leaving group; remember we labeled him a leaving group. The direction of attack of the nucleophile onto the carbon bearing the leaving group is described as backside attack. The nucleophile has to come in from the opposite face of where the leaving group is; that is the only possible attack to get that leaving group to leave and have the orbitals interact properly. We are going to see the consequences of that shortly; let’s talk about the kinetics of the reaction–things that affect the rate of the reaction.

SN2 Kinetics

The rate of the Sn2 is given by the following rate expression: the rate is proportional to both the concentration of the methoxide, the CH₃O^–… sorry. The CH₃O^–, our nucleophile, and the concentration of the ethyl iodide; let’s just abbreviate this CH₃CH₂–we could abbreviate that as ethyl; ET for ethyl. In other words, in the rate expression, we find both the nucleophile and the electrophile; that is why this reaction is described as a bimolecular reaction. You add up the exponents; each of these are to the first power; so it is a bimolecular reaction; and that is where the Sn2 comes from. That is what the 2 represents in the name–is that it is the bimolecular reaction; it has both the nucleophile and the electrophile coming together in the one step of the reaction. We will find that sterics play a big role in the rate of the Sn2; so let’s see some examples of that.

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