Química Orgânica 2 · Revisão de substituição aromática para a prova
Why alkylation over-reacts and acylation does not
Condições: Benzene (1 equiv) with chloroethane/AlCl₃ gives ethylbenzene together with diethyl- and triethylbenzenes. Benzene (1 equiv) with acetyl chloride/AlCl₃ gives acetophenone and essentially no diacetylbenzene.
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A pergunta
Why does alkylation give polysubstitution while acylation stops at one group?
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Resposta e explicação
Resposta correta The ethyl group activates the ring, so the product reacts faster than benzene; the acetyl group deactivates it, so the product reacts more slowly.
Each substitution changes the reactivity of the product. An electron-donating alkyl group makes ethylbenzene a better nucleophile than benzene; an electron-withdrawing ketone (further deactivated by binding AlCl₃) makes acetophenone a worse one.
Esta não Acylium ions are too large to approach a ring that already has a substituent.
Size is not the reason: acylium ions acylate substituted rings such as toluene easily. The ketone product is simply deactivated.
Esta não Ethyl cations rearrange, and the rearranged cation attacks a second time.
An ethyl cation has no more stable isomer to rearrange to; polyalkylation comes from the activated product, not from rearrangement.
Esta não AlCl₃ is consumed in acylation, so no catalyst remains for a second reaction.
AlCl₃ is bound by the ketone, but a slight excess is used; even with extra AlCl₃, acetophenone does not acylate again because its ring is deactivated.
Solução explicada
In alkylation the product is more reactive than the starting material, so using the arene in large excess is the only way to limit over-alkylation. In acylation the product is less reactive, so monoacylation is clean. This is one reason chemists often acylate and then reduce the ketone when they need an alkylbenzene.
O que você vai praticar
Explain the difference in over-reaction between Friedel–Crafts alkylation and acylation from the effect of the group introduced.
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