GCSE Chemistry Organic families revision

Alcohols are a homologous series — a family with the same general formula and similar chemistry — whose molecules carry the –OH (hydroxyl) functional group: methanol CH₃OH, ethanol C₂H₅OH, propanol C₃H₇OH. The functional group is what reacts, so all alcohols burn, and all can be oxidised to carboxylic acids. Ethanol is the star: alcoholic drinks, hand sanitiser, biofuel.

Alcohols burn completely in oxygen: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O, releasing useful energy (spirit burners, ethanol biofuel). Exposed to air, ethanol is slowly oxidised — microbes add oxygen to give ethanoic acid, CH₃COOH, the acid in vinegar. In the lab the same oxidation is done with acidified potassium manganate(VII): C₂H₅OH + 2[O] → CH₃COOH + H₂O.

Carboxylic acids carry the –COOH functional group: ethanoic acid CH₃COOH (vinegar), propanoic acid, butanoic acid (rancid-butter smell). In water they ionise only partially: CH₃COOH ⇌ CH₃COO⁻ + H⁺. That makes them weak acids — at the same concentration a weak acid has a higher pH and reacts more slowly than a strong acid like hydrochloric, because fewer H⁺ ions are swimming around at any moment. They still do classic acid chemistry, just gently: fizzing with carbonates (CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂), forming salts with metals and bases.

Two household consequences worth knowing: vinegars taste sharp but do not corrode like strong acids, and carboxylic acids dissolve in water to give acidic solutions with pH around 3–4 (weakly acidic, not neutral). Their salts are named from the acid: ethanoic acid gives ethanoates.

An alkene’s C=C double bond can open up and link to neighbouring monomers: thousands of ethene molecules zip into poly(ethene). Because the monomers simply add together with no other product, this is addition polymerisation. The repeat unit shows the polymer’s backbone — the two carbons that were double-bonded, drawn with continuation bonds either side — with any side groups (like the chlorine in poly(chloroethene), PVC, or the phenyl group in polystyrene) hanging off unchanged.

(Higher) Polymer properties follow from the chains: strong covalent bonds run along each chain, but chains are held to each other only by intermolecular forces. Long, straight chains pack closely, so those forces are strong — the polymer is dense, stiff and higher-melting. Branches or bulky side groups keep chains apart, weakening the forces — the polymer is softer, more flexible, lower-melting. Cross-links change everything again, making thermosets that will not melt.

Monomers with two functional groups (one at each end) can react head to tail, joining and releasing a small molecule — usually water. That is condensation polymerisation. A dicarboxylic acid plus a diol make a polyester (like PET); each new ester link comes with one H₂O out. (Higher) Every condensation link needs two functional groups reacting and two covalent bonds forming, so condensation polymers often need two different monomers — unlike addition polymerisation, which needs just the one alkene and produces nothing else.

Amino acids are nature’s double-ended monomers: each has an amine group –NH₂ and a carboxylic acid group –COOH. They condense into polypeptides (amide links, water out), and long polypeptides fold into proteins — enzymes, muscle, hair. DNA takes the same idea further: nucleotide monomers polymerise into two strands that cross-link through paired bases (A–T, C–G) and coil into a double helix — the base sequence storing genetic information.