Sn1 Reaction – Haloalkanes

00:00 Right. Now, we talked about SN2 reactions, but there is the so called SN1 reaction. This is substitution nucleophilic unimolecular.

00:10 This takes place in the absence that I said of actually an attacking nucleophile and we’ll go through that. But, before I do, I just want to talk about the steric chemistry. If we said, or if we look at the previous SN2 reaction, you can see we get inversion of the steric chemistry for SN2. An S, or sinister, stereochemistry under Cahn-Ingold-Prelog rule is inverted to an R or recto stereochemistry. However, when an SN1 reaction takes place, you can actually convert an enantiomer into a racemic mixture, where you have approximately equal amounts of S and R enantiomers. And that is the basis of an SN1 reaction in terms of the stereochemistry. So, why is that? Let’s have a quick look.

01:00 In the SN1 mechanism, the reaction coordinate is actually not dependent upon the concentration of your nucleophile. In an SN1 reaction, the loss of halide takes place first, thus generating a carbocation. As you can see, in the center of this particular reaction series, you’ve got here a plain old mirror that show because what you have, as I showed you before in the case of the alkanes, I am sorry, alkenes, is a p-orbital, which is perpendicular to that trigonal planar arrangement. This means, after that carbocation is formed, which is an intermediate, is that the nucleophile, in this case our hydroxide, can attack from either side. By being able to attack from either side, which is a fast step in this particular reaction, you can convert an enantiomerically pure compound into its racemate, which, in fairness, is often not particularly desirable. The reality, when it comes to SN1 reactions, is that you tend to see more of one enantiomer than the other. This is due to the ion pair effect. But, that goes beyond our terms of reference.

02:15 The reaction is said to be unimolecular, that is, the number 1 in SN1 stands for unimolecular.

02:22 Only the halogeno-alkane is participating in the slow step of the reaction and therefore, the rate is dependent solely on the concentration of the haloalkane. Therefore, the reaction kinetics are thus where the reaction rate, k, is proportional to the concentration of the haloalkane. Bear in mind, this is just to do with kinetics; nothing to do with thermodynamics.

02:49 Now, let’s have a look at some of the reaction parameters relating to SN1 reactions, specifically, the nucleophilicity of the nucleophile, the solvent, the alkyl group and the leaving group.

03:05 Let’s have a look at the stability of the carbocations first because as we indicated, the slow step is actually the formation of the carbocation. The nucleophilicity of the nucleophile is to a large extent irrelevant in this case, since it is the fast step of the reaction that it participates in. Unlike the SN2 reaction, the order of stability or the order of reactivity for this particular mechanism is reversed.

03:30 As you can appreciate from having looked at the way in which addition reactions take place over alkenes, you can see that a positive charge surrounded by electron donating alkyl groups, shown as R in this particular case, can better stabilise that carbocation and therefore, the conversion of a haloalkane to a carbocation... carbocation by the loss of its halide is more likely to occur for a tertiary haloalkane.

03:57 The next, and most stable, is the secondary, shown there as 2, primary - 1 and then finally methyl. The methyl group is a very unstable carbocation, due to a slight inductive effect of the alkyl groups that partially stabilise the carbocation.

04:16 All that was said previously regarding the stability of the nucleophiles is true for SN1 reactions. However, also, to facilitate the charge separation in the first step, a good ionising solvent will be needed. So, this effectively alters our parameter set for the SN1 reaction. So, ideally, what we want is something that stabilises the carbocation in the first case. So, unlike with the SN2, solvents such as water and formic acid and alcohols are generally considered better ionising solvents than some organic solvents such as DMSO and dimethylformamide.