If we really want to make aromatic compounds undergo nucleophilic substitution in a general way, the way to do it is to use absolutely the best leaving group of all—nitrogen gas. In fact, the diazonium compound below is so good at nucleophilic aromatic substitution that is does so even without activating groups. On warming, the nitrogen molecule just departs, leaving behind a cation, which is captured by a nucleophile, in this case water. Do you find this reminiscent of the SN1 reaction? We hope so.

Before we talk about this group of aromatic SN1 reactions in more detail, let’s consider how to make the diazonium salt. The reagent we need is the reactive nitrogen electrophile NO⁺. You met NO⁺ in Chapter 20, but to remind you, it forms when the nitrite anion (usually sodium nitrite) is treated with acid at around 0 °C. Protonation of nitrite gives nitrous acid, HONO; protonation again gives a cation, which can lose water to form NO⁺. Butyl nitrite (or other alkyl nitrites) can also be used as a source of NO⁺.

A diazonium salt is formed when NO⁺ reacts with an amine. The lone pair of the amine attacks the NO⁺ cation, and then water is lost. The mechanism is actually quite simple, but it does involve a lot of proton transfers. There is, of course, an anion associated with the nitro gen cation, and this will be the conjugate base (Cl− usually) of the acid used to form NO⁺. This reaction is known as diazotization.

If the amine is an alkyl amine, this diazonium salt is very unstable and immediately loses nitrogen gas to give a planar carbocation, which normally reacts with a nucleophile in an SN1 process, loses a proton in an E1 process, or rearranges. It may, for example, react with water to give an alcohol:

If the amine is an aryl amine, then the reaction you saw at the beginning of this section will take place and a phenol will form. This is in fact rather a useful reaction as it is difficult to add an oxygen atom to a benzene ring by normal electrophilic substitution: there is no good rea gent for OH⁺. A nitrogen atom can be added easily by nitration, and reduction and diazotization provide a way of replacing the nitro group by a hydroxyl group.

However, an aryl carbocation is much less stable than an alkyl carbocation because its empty orbital is an sp² rather than a p orbital. This makes the loss of nitrogen slower. If the diazotization is done at temperatures around 0 °C (classically at 5 °C), the diazonium salt is stable and can be reacted with various nucleophiles other than water.

مواضيع ذات صلة

How to reduce carboxylic acids to alcohols

How to reduce amides to amines

The benzyne mechanism

Other nucleophiles

The activating anion-stabilizing substituent

The leaving group and the mechanism

The intermediate in the addition–elimination mechanism

The addition–elimination mechanism

Nucleophilic aromatic substitution

Nucleophilic epoxidation

Conjugate substitution reactions

Unsaturated nitriles and nitro compounds