Amines, Amino Acids and Polymers

Structure and Classification

• Primary amine: RNH₂ (one organic group attached to N)
• Secondary amine: R₂NH
• Tertiary amine: R₃N
• Quaternary ammonium ion: R₄N⁺

Preparation of Amines

From halogenoalkanes: $\text{CH}_3\text{Br} + 2\text{NH}_3 \rightarrow \text{CH}_3\text{NH}_2 + \text{NH}_4\text{Br}$

Excess NH₃ minimises further substitution.

From nitriles (reduction): $\text{RCN} + 4[\text{H}] \xrightarrow{\text{LiAlH}_4} \text{RCH}_2\text{NH}_2$

Properties

As bases: Amines accept protons (lone pair on N): $\text{CH}_3\text{NH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{NH}_3^+ + \text{OH}^-$ $\text{CH}_3\text{NH}_2 + \text{HCl} \rightarrow \text{CH}_3\text{NH}_3^+\text{Cl}^-$

As nucleophiles: Lone pair on N attacks δ+ carbon (e.g. in halogenoalkanes).

Amino acids have the general structure: $\text{H}_2\text{N-CHR-COOH}$

Zwitterions

At neutral pH, amino acids exist as zwitterions — with both a positive and negative charge: $^+\text{H}_3\text{N-CHR-COO}^-$

This explains their high melting points and solubility in water.

Acid-Base Behaviour

• In acid: $^+\text{H}_3\text{N-CHR-COOH}$ (cation)
• At isoelectric point: $^+\text{H}_3\text{N-CHR-COO}^-$ (zwitterion)
• In base: $\text{H}_2\text{N-CHR-COO}^-$ (anion)

Optical Isomerism

All amino acids (except glycine) have a chiral centre (4 different groups on the α-carbon) and exist as enantiomers — non-superimposable mirror images that rotate plane-polarised light in opposite directions.

Addition vs Condensation Polymerisation

Polyesters

Formed from a dicarboxylic acid + diol: $\text{HOOC-R-COOH} + \text{HO-R'-OH} \rightarrow \text{-OC-R-COO-R'-O-} + \text{H}_2\text{O}$

PET (polyethylene terephthalate) = terylene: from benzene-1,4-dicarboxylic acid + ethane-1,2-diol.

Polyamides

Formed from a dicarboxylic acid + diamine: $\text{HOOC-R-COOH} + \text{H}_2\text{N-R'-NH}_2 \rightarrow \text{-OC-R-CONH-R'-NH-} + \text{H}_2\text{O}$

Nylon-6,6: from hexanedioic acid + hexane-1,6-diamine.

Proteins (Natural Polyamides)

Proteins are condensation polymers of amino acids: $\text{H}_2\text{N-CHR-COOH} + \text{H}_2\text{N-CHR'-COOH} \rightarrow \text{H}_2\text{N-CHR-CONH-CHR'-COOH} + \text{H}_2\text{O}$

The peptide bond (−CONH−) links amino acids.

• Addition polymers: generally NOT biodegradable (no functional groups for enzymes to attack)
• Condensation polymers: can be hydrolysed (ester/amide bonds broken by water, acid, base, or enzymes) → potentially biodegradable

• Amines: bases (lone pair on N accepts H⁺) and nucleophiles
• Amino acids: zwitterions at neutral pH; optically active (chiral)
• Condensation polymers form by losing H₂O: polyesters (−COO−), polyamides (−CONH−)
• Proteins are natural polyamides linked by peptide bonds
• Condensation polymers are hydrolysable → potentially biodegradable