Amines, Amides, Amino Acids

Detailed Explanations

A. Basicity of amines (typical equation)

RNH₂ + H₂O ⇌ RNH₃ + + OH⁻

With acids, amines form salts: RNH₂ + HCl → RNH₃ + Cl⁻

Key writing point:

• “Amines are weak bases (they only partially react with water).”

B. Why amides are much less basic

In an amide, the nitrogen lone pair is delocalised into the carbonyl group. Because the lone pair is less available to accept H⁺, therefore amides are much less basic than amines.

C. Making amides (acylation)

From an acyl chloride: RCOCl + 2NH₃ → RCONH₂ + NH₄Cl

From a carboxylic acid (less reactive, requires strong heating / dehydrating conditions in general):

• exam questions often focus on the acyl chloride route because it is clean and fast

D. Hydrolysis of amides

Acid hydrolysis (forms carboxylic acid + ammonium ion): RCONH₂ + H₂O → [H + /heat] RCOOH + NH₄ +

Alkaline hydrolysis (forms carboxylate + ammonia): RCONH₂ + OH⁻ → [heat] RCOO⁻ + NH₃

E. Amino acids: zwitterions and reactions

Zwitterion form (example: glycine) includes both -NH₃ + and -COO⁻.

Reactions to know:

• With acids: acts as a base (protonation): H₂NCH₂COOH + H⁺ → H₃N + CH₂COOH
• With bases: acts as an acid (deprotonation): H₂NCH₂COOH + OH⁻ → H₂NCH₂COO⁻ + H₂O

Because an amino acid contains both an acidic group (-COOH) and a basic group (-NH₂), therefore it can react with both acids and bases (amphoteric behaviour).

F. Workflow: decide what happens to an amino acid in acid/base

1. Identify the two functional groups: -NH₂ and -COOH.
2. In acidic conditions, add H⁺ to the basic site (amine) → -NH₃ +.
3. In alkaline conditions, remove H⁺ from the acidic site (carboxylic acid) → -COO⁻.
4. Write the equation with charges shown clearly.

Mini example: Glycine + OH⁻ forms H₂NCH₂COO⁻ and water.

G. Preparing amines and comparing basicity

Heat a halogenoalkane with excess ethanolic ammonia in a sealed vessel to form a primary amine. Excess ammonia reduces further substitution. Reduction provides two other routes: nitriles form primary amines with one more carbon than the original halogenoalkane, while amides form amines with the same carbon skeleton as the amide.

In the gas phase, alkyl groups donate electron density and generally make an alkylamine lone pair more available than ammonia’s. In aqueous solution, hydration of the protonated species also matters, so use supplied data when comparing substituted amines. Phenylamine is less basic than ammonia because its nitrogen lone pair is delocalised into the benzene ring. Amides are much less basic because their lone pair is delocalised towards the carbonyl group.

H. Reactions of phenylamine and primary amines

Phenylamine reacts readily with bromine water to form 2,4,6-tribromophenylamine as a white precipitate. The amino group donates electron density into the ring, so no halogen carrier is needed.

A primary amine reacts with an acyl chloride to form an N-substituted amide. For example:

CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃Cl

An amide can be reduced by LiAlH₄ in dry ether, followed by water, to form an amine. Track the carbonyl carbon: it becomes a -CH₂- group rather than being lost.