Question 1

What is the mechanism of amine acylation to form amides?

Accepted Answer

Amine acylation involves a nucleophilic attack where the amine approaches the carbonyl carbon of a carboxylic acid derivative, forming a tetrahedral intermediate. The leaving group is then lost, reconstructing the C=O bond. A second amine molecule acts as a Brønsted base, abstracting a hydrogen from the protonated amide to yield the free amide product.

Question 2

Why does acylation of amines require two equivalents of amine?

Accepted Answer

The first amine equivalent functions as a nucleophile, attacking the carbonyl carbon and participating in tetrahedral intermediate formation. The second amine equivalent serves as a Brønsted base, deprotonating the quaternary amide intermediate to generate the neutral amide product. Both roles are essential for reaction completion.

Question 3

What carboxylic acid derivatives can be used for amine acylation?

Accepted Answer

Various carboxylic acid derivatives are suitable for amine acylation, including acid chlorides, esters, and anhydrides. These compounds contain a carbonyl group with a leaving group attached, allowing nucleophilic substitution by the amine to produce amides efficiently.

Question 4

How do amides differ from amines in chemical reactivity?

Accepted Answer

Amides are stabilized by resonance between the nitrogen lone pair and the carbonyl π-system, making the nitrogen less nucleophilic and less basic than in amines. This reduced reactivity prevents polyacylation and makes amides useful protecting groups in electrophilic aromatic substitution reactions.

Question 5

Why is aniline bromination different when the amino group is acylated first?

Accepted Answer

Direct bromination of aniline yields a tribrominated product because the amino group is strongly activating and directs electrophilic aromatic substitution. When the amino group is first acylated to form an amide, the resulting acetyl group is less activating, allowing selective monobromination instead of polysubstitution.

Question 6

What role does resonance play in amide stability?

Accepted Answer

Resonance stabilization in amides occurs through delocalization of the nitrogen lone pair into the carbonyl π-system. This electron delocalization makes the positively charged nitrogen less reactive, reducing nucleophilicity and basicity compared to amines and preventing unwanted side reactions like polyacylation.

Question 7

How can amides be used to control regioselectivity in aromatic substitution?

Accepted Answer

Amides serve as protecting groups that reduce the activating effect of amino groups in electrophilic aromatic substitution. By converting an amine to an amide before bromination or other electrophilic reactions, you can achieve monobromination instead of tribromination, allowing precise control over substitution patterns on aromatic rings.

Aminas em Amidas: Acilação de Aminas

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