So, substitution reactions at saturated C go via one of two alternative mechanisms, each with a very different dependence on the nature of the nucleophile. It’s important to be able to predict which mechanism is likely to apply to any reaction, and rather than doing the kinetic experiments to find out, we can give you a few simple pointers to predict which will operate in which case. The factors that affect the mechanism of the reaction also help to explain why that mechanism operates. The most important factor is the structure of the carbon skeleton. A helpful generalization is that compounds that can form relatively stable carbocations generally do so and react by the SN1 mechanism, while the others have no choice but to react by the SN2 mechanism. As you will see in a moment, the most stable carbocations are the ones that have the most substituents, so the more carbon substituents at the reaction centre, the more likely the com pound is to react by the SN1 mechanism. As it happens, the structural factors that make cations stable usually also lead to slower SN2 reactions. Heavily substituted compounds are good in SN1 reactions, but bad in an SN2 reaction because the nucleophile would have to squeeze its way into the reaction centre past the substituents. It is better for an SN2 reaction if there are only hydrogen atoms at the reaction centre—methyl groups react fastest by the SN2 mechanism. The effects of the simplest structural variations are summarized in the table below (where R is a simple alkyl group like methyl or ethyl).

The only doubtful case is the secondary alkyl derivative, which can react by either mechanism, although it is not very good at either. The fi rst question you should ask when faced with a new nucleophilic substitution is this ‘Is the carbon electrophile methyl, primary, secondary, or tertiary?’ This will start you off on the right foot, which is why we introduced these important structural terms in Chapter 2. Later in this chapter we will look in more detail at the differences between the two mecha nisms and the structures that favour each, but all of what we say will build on the table above.

مواضيع ذات صلة

A closer look at steric effects

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

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