The preparation and reactions of amines, amino acids and proteins, and the formation and properties of addition and condensation polymers including their disposal.

What this topic is asking

Edexcel Topic 19 wants you to describe the preparation, basicity and reactions of amines, explain the amphoteric nature of amino acids and how they form proteins, and account for the formation, properties and disposal of addition and condensation polymers.

The answer

Amines

The strength of an amine base depends on how available the lone pair is. Aliphatic amines such as ethylamine are stronger bases than ammonia because the alkyl group releases electron density onto nitrogen, making the lone pair more available. Aromatic amines such as phenylamine are weaker than ammonia because the lone pair is partly delocalised into the benzene ring and so is less available to bond a proton.

Amino acids and proteins

In acidic solution the zwitterion gains a proton to become a cation ($^+\text{H}_3\text{N-CHR-COOH}$); in alkaline solution it loses a proton to become an anion ($\text{H}_2\text{N-CHR-COO}^-$). This is why amino acids can be separated by electrophoresis according to their charge at a given pH.

Polymers and disposal

Addition polymers form from alkene monomers by opening the $\text{C=C}$ double bond, leaving a saturated carbon backbone with no small-molecule by-product, for example $n\,\text{CH}_2=\text{CH}_2 \rightarrow [\text{-CH}_2\text{-CH}_2\text{-}]_n$. This backbone is chemically inert and non-biodegradable, so disposal relies on landfill, incineration (which can release toxic gases) or recycling.

Condensation polymers form when two monomers join with loss of a small molecule (usually water or $\text{HCl}$):

• Polyesters (e.g. Terylene) form from a diol and a dicarboxylic acid, linked by ester bonds.
• Polyamides (e.g. nylon) form from a diamine and a dicarboxylic acid (or diacyl chloride), linked by amide bonds.

Because these links can be hydrolysed (by acid, alkali or enzymes), condensation polymers break down more readily than addition polymers, which is an advantage for disposal.

Examples in context

Example 1. Kevlar. Kevlar is an aromatic polyamide made by condensation of a benzene-1,4-diamine and benzene-1,4-dicarboxylic acid (or its diacyl chloride). The amide links allow extensive hydrogen bonding between adjacent chains, aligning them into sheets that give Kevlar its enormous tensile strength, used in body armour. The chemistry is exactly the condensation polymerisation of Topic 19, with the strength arising from the same hydrogen bonding studied in earlier topics.

Example 2. Thalidomide and chirality of drugs. Many amino-acid-derived and amine drugs are chiral. Thalidomide, prescribed in the late 1950s, was given as a racemate; one enantiomer relieved morning sickness while the other caused birth defects. This tragedy is the reason the pharmaceutical industry now strives to make single enantiomers, and it links amine and amino-acid chemistry to the importance of stereochemistry covered in the synthesis and chirality topic.

Try this

Q1. Explain why amines act as bases. [1 mark]

• Cue. The lone pair on nitrogen accepts a proton.

Q2. State one reason addition polymers are difficult to dispose of compared with condensation polymers. [2 marks]

• Cue. Their inert C-C backbone is non-biodegradable; condensation polymers have hydrolysable ester or amide links.