We have already pointed out that having more alkyl substituents at the reaction centre makes a compound more likely to react by SN1 than by SN2 for two reasons: firstly, they make a carbocation more stable, so favouring SN1, and secondly, they make it hard for a nucleophile to get close to the reaction centre in the rate-determining step, disfavouring SN2. Let’s look in more detail at the transition state for the slow steps of the two reactions and see how steric hindrance affects both.

In the approach to the SN2 transition state, the carbon atom under attack gathers in another substituent and becomes (transiently) five-coordinate. The angles between the substituents decrease from tetrahedral to about 90°.

In the starting material there are four angles of about 109°. In the transition state (enclosed in square brackets and marked ‡ as usual) there are three angles of 120° and six angles of 90°, a significant increase in crowding. The larger the substituents R, the more serious this is, and the greater the increase in energy of the transition state. We can easily see the effects of steric hindrance if we compare these three structural types:

• methyl: CH₃–X: very fast SN2 reaction

• primary alkyl: RCH₂–X: fast SN2 reaction

• secondary alkyl: R₂CH–X: slow SN2 reaction.

 The opposite is true of the SN1 reaction. The rate-determining step is simply the loss of the leaving group, and the transition state for this step will look something like the structure shown below—with a longer, weaker, and more polarized C–X bond than the starting mate rial. The starting material is again tetrahedral (four angles of about 109°) and in the intermediate cation there are just three angles of 120°—fewer and less serious interactions. The transition state will be on the way towards the cation, and because the R groups are further apart in the transition state than in the starting material, large R groups will actually decrease the energy of the transition state relative to the starting material. SN1 reactions are therefore accelerated by alkyl substituents both for this reason and because they stabilize the cation.

مواضيع ذات صلة

Adjacent C=C or C=O π systems increase the rate of SN2 reactions

An adjacent C=C π system stabilizes a carbocation: allylic and benzylic carbocations

Alkyl substituents stabilize a carbocation

Shape and stability of carbocations

A closer look at the SN1 reaction

? How can we decide which mechanism (SN1 or SN2) will apply to a given organic compound

Significance of the SN1 rate equation

Significance of the SN2 rate equation

Mechanisms for nucleophilic substitution

Resolutions using diastereoisomeric salts

Separating enantiomers is called resolution

Chiral compounds with no stereogenic centres