Amines are derivatives of ammonia. The lone pair on th nitrogen atom is responsible for amines having three important properties:
- They can act as bases
- They are nucleophiles
- They can act as ligands
Amines can be primary, secondary or tertiary. In primary amines the nitrogen atom is bonded to one alkyl or aryl group. In secondary amines the nitrogen atom is bonded to two alkyl groups and in tertiary amines the nitrogen is bonded to three alkyl groups.
CH3NH2 = Methylamine
CH3CH(NH2)CH3 = 2-aminopropane
Primary amines react with halogenoalkanes to form secondary amines in alkylation reactions. Amines can also react with acyl chlorides in acylation reactions to form a secondary amide.
Primary amides can be made when an amine reacts with a carboxylic acid. This produces a primary amide. Secondary amides can be made when amines react with acyl chlorides. Polymers are examples of polyamides, in which a molecule has several amide groups present.
Like esters, amides can be hydrolysed by acid or alkali and water. The C-N bond breaks and dependent on the catalyst the products will differ.
Acid hydrolysis: Moderately concentrated acid is used and the mixture is heated under reflux. A carboxylic acid and an alkylammonium ion is produced.
Alkali hydrolysis: Moderately concentrated alkali is used and the mixture is heated under reflux. A carboxylate ion is produced as well as an amine.
Polyesters and polyamides are both produced in condensation reactions in which a molecule of water is lost when the two monomers join together. At room temperature most polymers have crystalline regions (highly ordered chains) and amorphous regions (randomly arranged chains).
Glass Transition Temperature(Tg) - The temperature below which the polymer becomes brittle or glass-like.
Melting Temperature(Tm) - The temperature above which the polymer becomes liquid. Chains are able to slide over one another as the polymer warms up allowing it to become more flexible.
The longer the polymer chains, the stronger the polymer as more intermolecular bonds are able to form. Hydrogen bonds are able to hold polyamide chains together, whereas permanent dipole-permanent dipole bonds hold polyester chains together.
Co-polymerisation - Introducing monomers with larger side groups.
Plasticisers - Molecules that sit between polymer chains allowing them to slide freely.
Green Chemistry and Recycling
Rather than sending them to landfill, plastics can be recycled, however this means that they must first be sorted, and many thermoplastics are made from more than one monomer so can only be remoulded.
Polymers can instead be returned to their original monomers by means of cracking. The monomers can then be turned into new plastics or used elsewhere in chemical industry.
Further to this, degradable plastics can be made. In future we could use more biopolymers (molecules made by living organisms such as plants or bacteria), synthetic biodegradable plastics or photodegradable plastics. All of these materials are able to break down in the environment, although it may still be a slow process.
Plastics can also be burned to release energy, which can then be used to generate electricity or to heat houses, but this is a less favourable option.
Life-cycle assessments are being put in place, during which the carbon emissions of a material are measured over its entire life cycle.