To start this chapter, let us take you back to one of the first reactions we introduced: nucleophilic addition to carbonyl groups. Here are two examples, both of which give products which you should fully expect. We’ve included details of the IR spectra of the products to confirm firstly that the product has no carbonyl group and secondly that the alkene is still there.

Now let’s tweak the conditions: we repeat the first reaction at a higher temperature, and we add to the second a small amount of a copper salt. Now the products are different:

Both products A and B have kept their carbonyl group (IR peak at 1710 – 1715 cm⁻¹) but have lost the C=C. Yet A, at least, is unquestionably an addition product because it contains a C≡N peak at 2250 cm⁻¹. Well, the identities of A and B are revealed here: they are the products of addition, not to the carbonyl group, but to the C=C bond. Here’s a mechanism, for both reactions of cyanide: firstly the direct addition to C=O and secondly addition to the C=C bond.

This type of reaction, where a nucleophile adds to a C=C double bond, is called conjugate addition, and this chapter is about the sorts of alkenes (and arenes) that do this kind of thing. We will also explain how such small differences in reaction conditions (temperature, or the presence of CuCl) manage to change the outcome so dramatically. But fi rst we need to place these conjugate additions into context. alkenes are nucleophilic. Almost regardless of their substituents, they react with electrophiles such as bromine to form adducts in which the π bond of the alkene has been replaced by two σ bonds.

Even when the alkene is conjugated with an electron-withdrawing group, as the alkenes on the last page were, bromine addition still occurs, although less readily. Never lose sight of this: alkenes are nucleophilic.

But as we have just seen, this last type of alkene also reacts with nucleophiles (such as cyanide, Grignard reagents, and, as you see below, more besides), and we now need to consider why.

مواضيع ذات صلة

Reaction conditions

Conjugated alkenes can be electrophilic

Two reasons to avoid a Friedel–Crafts alkylation

Two or more substituents may cooperate or compete

Phenols react rapidly with bromine

Alkyl and acyl substituents can be added to a benzene ring by the Friedel–Crafts reaction

Sulfonation of benzene

Nitration of benzene

Benzene and its reactions with electrophiles

enols and phenols

Reactions of silyl enol ethers with halogen and sulfur electrophiles

Reactions of enol ethers Hydrolysis of enol ethers