The high reactivity of aniline can actually be a problem. Suppose we wanted to put just one bromine atom onto the ring. With phenol , if bromine is added slowly to a solution of phenol in carbon disulfide solution and the temperature is kept below 5 °C, the main product is para-bromophenol. Not so if aniline is used—the main product is the triply substituted product.

How then could we prevent over substitution from occurring? What we need is a way to make aniline less reactive by preventing the nitrogen lone pair from interacting so strongly with the π system of the ring. Fortunately, it is very simple to do this. we saw how the nitrogen atom in an amide is much less basic than a normal amine because it is conjugated with the carbonyl group. This is the strategy that we will use here—simply acylate the amine to form an amide. The lone pair electrons on the nitrogen atom of the amide are conjugated with the carbonyl group as usual but their delocalization into the benzene ring is weaker than in the amine. The amide nitrogen donates less electron density into the ring, so the electrophilic aromatic substitution is more controlled. The lone pair is still there, but its power is tamed. Reaction still occurs in the ortho and para positions (mainly para) but it occurs once only.

After the reaction, the amide can be hydrolysed (here, with aqueous acid) back to the amine.

● Anilines react rapidly with electrophiles to give polysubstituted products. Their amide derivatives react in a more controlled manner to give para-substituted products.

مواضيع ذات صلة

Halogens show evidence of both electron withdrawal and donation

Some substituents withdraw electrons by conjugation

Electron-withdrawing substituents give meta products

The sulfonation of toluene

Selectivity between ortho and para positions

A nitrogen lone pair activates even more strongly

Oxygen substituents activate a benzene ring

Reaction conditions

Conjugated alkenes can be electrophilic

Alkenes conjugated with carbonyl groups

Two reasons to avoid a Friedel–Crafts alkylation

Two or more substituents may cooperate or compete