In our discussion of the SN1 reaction above, we proposed the t-butyl carbocation as a reason able intermediate formed by loss of bromide from t-butyl bromide. We now need to explain the evidence we have that carbocations can indeed exist, and the reasons why the t-butyl carbocation is much more stable than, for example, the n-butyl cation. In Chapter 12 we introduced the idea of using a reaction energy profile diagram to follow the progress of a reaction from starting materials to products, via transition states and any intermediates. The energy profile diagram for the SN1 reaction between t-butyl bromide and water looks something like this:

The carbocation is shown as an intermediate—a species with a finite (if short) lifetime for reasons we shall describe shortly. And because we know that the first step, the formation of the carbocation, is slow, that must be the step with the higher energy transition state. The energy of that transition state, which determines the overall rate of the reaction, is closely linked to the stability of the carbocation intermediate, and it is for this reason that the most important factor in determining the efficiency of an SN1 reaction is the stability or otherwise of any carbocation that might be formed as an intermediate.

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

? 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