Amines, Amino Acids and Polymers

Amines, Amino Acids and Polymers

Amines

Amines are organic derivatives of ammonia (NH₃) in which one or more hydrogen atoms are replaced by alkyl or aryl groups.

Classification:

• Primary amine (1°): RNH₂ (e.g. ethylamine, CH₃CH₂NH₂)
• Secondary amine (2°): R₂NH (e.g. diethylamine, (CH₃CH₂)₂NH)
• Tertiary amine (3°): R₃N (e.g. triethylamine, (CH₃CH₂)₃N)
• Quaternary ammonium ion: R₄N⁺ (e.g. tetraethylammonium, permanently charged)

Preparation of Amines

From halogenoalkanes: Nucleophilic substitution with excess ammonia in ethanol, heated in a sealed tube:

CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br

The excess ammonia minimises further substitution (the primary amine is itself a nucleophile and can react again to give secondary, tertiary amines, and eventually a quaternary ammonium salt).

From nitriles: Reduction with LiAlH₄ in dry ether, followed by hydrolysis:

RCN + 4[H] → RCH₂NH₂

From nitrobenzene (aromatic amine): Reduction with tin (Sn) and concentrated HCl, then NaOH:

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

This produces phenylamine (aniline), which is a weaker base than aliphatic amines.

Basicity of Amines

Amines are bases because the nitrogen lone pair can accept a proton:

RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻

Basicity order: Secondary aliphatic > Primary aliphatic > Ammonia > Phenylamine

• Alkyl groups are electron-releasing (positive inductive effect), which increases electron density on nitrogen, making the lone pair more available to donate → stronger base
• In phenylamine, the nitrogen lone pair is delocalised into the benzene ring, making it less available for protonation → much weaker base

Reactions of Amines

As nucleophiles — the nitrogen lone pair attacks δ⁺ centres:

• With halogenoalkanes → further substitution (see above)
• With acyl chlorides → amides: RNH₂ + R'COCl → R'CONHR + HCl
• With acids → salts: RNH₂ + HCl → RNH₃⁺Cl⁻

Amino Acids

Amino acids have both an amine group (–NH₂) and a carboxylic acid group (–COOH) on the same molecule. In α-amino acids, both groups are attached to the α-carbon:

H₂N–CHR–COOH

Where R is the side chain that distinguishes the 20 common amino acids.

Zwitterions

In solid form and in solution at their isoelectric point (pH at which the molecule has no net charge), amino acids exist as zwitterions:

⁺H₃N–CHR–COO⁻

The amino group is protonated (–NH₃⁺) and the carboxyl group is deprotonated (–COO⁻). This explains the high melting points and solubility in water of amino acids (ionic interactions between zwitterions).

In acidic solution (low pH): The COO⁻ group is protonated → ⁺H₃N–CHR–COOH (overall positive charge)

In basic solution (high pH): The NH₃⁺ group is deprotonated → H₂N–CHR–COO⁻ (overall negative charge)

Optical Isomerism in Amino Acids

All amino acids except glycine (R = H) have a chiral α-carbon with four different groups (H, NH₂, COOH, R). They exist as two enantiomers (D and L forms). Biological proteins are made exclusively from L-amino acids.

Condensation Polymers

Condensation polymerisation occurs when monomers join together with the elimination of a small molecule (usually water). This contrasts with addition polymerisation, where monomers simply add together with no loss of atoms.

Polyamides (Nylons)

Formed from a diamine + a dicarboxylic acid (or diacyl chloride):

nH₂N(CH₂)₆NH₂ + nHOOC(CH₂)₄COOH → –[NH(CH₂)₆NHCO(CH₂)₄CO]n– + 2nH₂O

This is nylon-6,6 — the numbers refer to the carbons in each monomer.

The peptide bond (–CONH–) is the same amide linkage formed between amino acids in proteins.

Properties: Strong, flexible, high melting point (extensive hydrogen bonding between chains along the N–H and C=O groups).

Hydrolysis: Polyamides can be broken down by heating with dilute acid or alkali (the amide bonds are cleaved):

• Acid hydrolysis → diamine salt + dicarboxylic acid
• Alkaline hydrolysis → diamine + dicarboxylate salt

Polyesters

Formed from a diol + a dicarboxylic acid:

nHO(CH₂)₂OH + nHOOC–C₆H₄–COOH → –[O(CH₂)₂OOC–C₆H₄–CO]n– + 2nH₂O

This is PET (polyethylene terephthalate), used in plastic bottles and polyester fibres.

The linkage is an ester bond (–COO–).

Hydrolysis: Broken down by heating with acid or alkali.

Addition Polymers

Formed from alkene monomers — the C=C double bond opens and monomers add together:

nCH₂=CH₂ → –[CH₂–CH₂]n– (poly(ethene))

No small molecule is lost. The repeat unit is derived directly from the monomer.

Examples:

• Poly(chloroethene) / PVC — from CH₂=CHCl
• Poly(propene) — from CH₃CH=CH₂
• PTFE (Teflon) — from CF₂=CF₂

Comparing Addition and Condensation Polymers

Feature	Addition	Condensation
Monomer type	Alkenes (C=C)	Difunctional (diols + diacids, diamines + diacids)
Small molecule lost	None	Water (or HCl with acyl chlorides)
Linkage	C–C only	Ester (–COO–) or amide (–CONH–)
Biodegradability	Generally non-biodegradable	Can be hydrolysed (more biodegradable)
Examples	Poly(ethene), PVC	Nylon, PET, proteins

Proteins as Condensation Polymers

Proteins are polyamides formed by condensation of amino acids. The amide bond between amino acids is called a peptide bond. Two amino acids form a dipeptide:

H₂N–CHR–COOH + H₂N–CHR'–COOH → H₂N–CHR–CONH–CHR'–COOH + H₂O

Hydrolysis of proteins (by refluxing with 6M HCl) breaks them back into their constituent amino acids.

Exam Tips

• When drawing condensation polymers, show the repeat unit in square brackets with bonds extending through the brackets, and write the small molecule eliminated
• To identify the monomers from a polymer, look for the linkage (ester or amide) and mentally insert H and OH to regenerate the monomers
• For amine basicity questions, always explain using electron density on the nitrogen lone pair
• Remember phenylamine is a weaker base because of lone pair delocalisation into the ring — draw the delocalised structure