In this next section we are going to consider reactions related to conjugate substitutions but in which the double bond is part of an aromatic ring . explaining that aromatic rings are nucleophilic: electrophiles attack them, and typical aromatic reactivity is to undergo electrophilic substitution. In general, nucleophilic substitutions of aromatic halides—such as the one proposed here in which hydroxide is attempting to displace bromide—do not happen. You might well ask, ‘Why not?’ The reaction looks all right and, if the ring were saturated, it would be all right. This is an SN2 reaction, and we know that attack must occur in line with the C–Br bond from the back, where the largest lobe of the σ* orbitals lies. That is perfectly all right for the aliphatic ring because the carbon atom is tetrahedral and the C–Br bond is not in the plane of the ring. Substitution of an equatorial bromine goes like this:

But in the aromatic compound, the C–Br bond is in the plane of the ring as the carbon atom is trigonal. To attack from the back, the nucleophile would have to appear inside the benzene ring and invert the carbon atom in an absurd way. This reaction is of course not possible. This is another example of the general rule:

● SN2 at sp2 C does not occur.

If SN2 is impossible, what about SN1? This is possible but very unfavourable unless the leaving group is an exceptionally good one (see below for an example). It would involve the unaided loss of the leaving group and the formation of an aryl cation. All the cations we saw as intermediates in the SN1 reaction were planar with an empty p orbital. This cation is planar but the p orbital is full—it is part of the aromatic ring—and the empty orbital is an sp2 orbital outside the ring. Yet some aromatic compounds do undergo nucleophilic substitution. Just as normally nucleophilic alkenes can be made to undergo conjugate substitution if they carry electron withdrawing substituents, so normally nucleophilic aromatic rings also become electro philic if they have the right substituents. The mechanism by which they undergo nucleophilic substitution also closely parallels that of conjugate substitution which you have just seen.

مواضيع ذات صلة

How to reduce carboxylic acids to alcohols

How to reduce amides to amines

The benzyne mechanism

Other nucleophiles

The SN1 mechanism for nucleophilic aromatic substitution: diazonium compounds

The activating anion-stabilizing substituent

The leaving group and the mechanism

The intermediate in the addition–elimination mechanism

The addition–elimination mechanism

Nucleophilic epoxidation

Conjugate substitution reactions

Unsaturated nitriles and nitro compounds