Amines, Amino Acids and Polymers

This lesson covers the chemistry of amines (organic nitrogen compounds), amino acids (the building blocks of proteins), and condensation polymers (polyesters and polyamides). These topics connect organic functional group chemistry to biological and materials science applications.

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Amines

Amines are organic compounds containing nitrogen. They are derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups.

Classification of Amines

Type	Structure	Example
Primary (1°)	RNH₂ (one H replaced)	CH₃NH₂ (methylamine)
Secondary (2°)	R₂NH (two H replaced)	(CH₃)₂NH (dimethylamine)
Tertiary (3°)	R₃N (three H replaced)	(CH₃)₃N (trimethylamine)

Note: Do not confuse the classification of amines with alcohols. In amines, classification refers to how many R groups are on the nitrogen. In alcohols, it refers to how many R groups are on the carbon bearing the –OH.

Preparation of Amines

Method 1: From halogenoalkanes and excess ammonia

CH₃Br + 2NH₃ → CH₃NH₂ + NH₄Br

Conditions: Excess concentrated ammonia in ethanol, heat in a sealed tube.

The excess NH₃ is essential to favour formation of the primary amine. Without excess, further substitution occurs:

CH₃NH₂ + CH₃Br → (CH₃)₂NH + HBr (secondary amine) (CH₃)₂NH + CH₃Br → (CH₃)₃N + HBr (tertiary amine) (CH₃)₃N + CH₃Br → (CH₃)₄N⁺Br⁻ (quaternary ammonium salt)

Method 2: Reduction of a nitrile

CH₃CN + 4[H] → CH₃CH₂NH₂

Reagent: LiAlH₄ in dry ether, followed by dilute acid work-up. Or: H₂ with Ni catalyst at high temperature and pressure.

This is useful because nitriles are made from halogenoalkanes + CN⁻ (extending the chain by one carbon), and the subsequent reduction gives a primary amine with one extra carbon.

Method 3: Reduction of nitrobenzene (aromatic amines)

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

(nitrobenzene → phenylamine)

Reagent: Tin (Sn) + concentrated hydrochloric acid (conc. HCl), reflux. Then add NaOH to liberate the free amine.

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Basicity of Amines

Amines are bases because the lone pair on nitrogen can accept a proton (H⁺).

CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻

CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻ (methylammonium chloride — a salt)

Comparing Basicity

Base	Relative basicity	Explanation
NH₃	Baseline	Lone pair on N available for protonation
Aliphatic amines (e.g. CH₃NH₂)	Stronger than NH₃	Alkyl groups are electron-donating (inductive effect), increasing electron density on N, making the lone pair more available
Phenylamine (C₆H₅NH₂)	Weaker than NH₃	The lone pair on N is partially delocalised into the benzene ring, making it less available for protonation

Order of basicity: aliphatic amines > NH₃ > aromatic amines (phenylamine)

Common Misconception: Students sometimes say phenylamine is a weak base because it has fewer lone pairs. The correct explanation is that the nitrogen lone pair is partially delocalised into the aromatic ring (overlapping with the π system), reducing its availability to accept a proton. The lone pair is still there — it is just less available.

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Amino Acids

Amino acids contain both an amino group (–NH₂) and a carboxyl group (–COOH) in the same molecule. The naturally occurring alpha-amino acids have both groups bonded to the same carbon (the α-carbon).

General Structure

H₂N–CHR–COOH

where R is the side chain (variable group) that distinguishes the 20 naturally occurring amino acids.

Zwitterions

In aqueous solution at neutral pH, amino acids exist as zwitterions — molecules carrying both a positive and negative charge:

⁺H₃N–CHR–COO⁻

The –NH₂ group is protonated (acting as a base) and the –COOH group is deprotonated (acting as an acid). The overall molecule has no net charge at the isoelectric point.

Acid-Base Behaviour