Sec 4 Chemistry Alcohols, Acids & Polymers notes

Alcohols carry the hydroxyl group (–OH) replacing one hydrogen of an alkane: family formula CₙH₂ₙ₊₁OH. Methanol (CH₃OH) is industrial solvent; ethanol (C₂H₅OH) is the alcohol of drinks, biofuel and hand sanitiser.

Fermentation is biology doing chemistry: yeast enzymes split glucose without air at about 35 °C — C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Warm enough for the enzymes, cool enough not to kill the yeast; air would oxidise the ethanol to vinegar instead. Hydration is the industrial shortcut: C₂H₄ + H₂O → C₂H₅OH over a catalyst — fast and continuous, but built on fossil ethene. Fermentation is slow and batch-wise but renewable.

Warm ethanol with acidified potassium manganate(VII) and it oxidises: C₂H₅OH + 2[O] → CH₃COOH + H₂O. The manganate(VII) shows the change by losing its purple colour — it is reduced while ethanol is oxidised (redox, Chapter 1). The product, ethanoic acid CH₃COOH, is the sour acid of vinegar.

The same oxidation happens biologically: leave an open bottle of wine in the kitchen and bacteria (acetobacter) oxidise the ethanol with air — wine sours into vinegar. Both routes deliver the same [O] story: two oxygen atoms in, one water out.

Ethanoic acid is a carboxylic acid — its –COOH group ionises only partially in water, making it a weak acid (like the ethanoic acid of Chapter 6: same pH vs rate story). Yet it performs every acid duty: with metals it fizzes hydrogen — 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂; with carbonates it fizzes CO₂ — 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂; with bases it neutralises to a sodium ethanoate salt.

The salts are named ethanoates — swap the "ic" for "ate". Because the acid is weak, its reactions are slower than hydrochloric acid's at the same concentration, and its pH is higher (less acidic) at the same molarity.

Warm ethanol with ethanoic acid and a few drops of concentrated sulfuric acid (catalyst and dehydrating agent): C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O. The product, ethyl ethanoate, is an ester — sweet, fruity, and the reason pear drops smell the way they do. Esters scent perfumes and flavourings and dissolve nail-varnish (ethyl ethanoate as solvent).

The naming recipe: alcohol gives the alkyl part (ethyl…), acid gives the alkanoate part (…ethanoate). Methanol + ethanoic acid → methyl ethanoate; propanol + ethanoic acid → propyl ethanoate. The reversible arrow matters — the reaction never goes to completion, which is why the sulfuric acid (removing water) pushes the yield up.

Addition polymerisation: thousands of C=C monomers open their double bonds and link into one giant saturated chain. Ethene → poly(ethene); chloroethene → PVC. No atoms are lost — the polymer is just the monomer repeated: n CH₂=CH₂ → –(CH₂–CH₂)ₙ–. Name the repeat unit and draw it inside brackets with an n.

Condensation polymerisation instead joins monomers that each carry two reactive groups, releasing a small molecule (usually water) at every link. Nylon links diacid + diamine units through amide bonds (used in ropes, clothing); Terylene (polyester) links diol + diacid through ester bonds. Identify them by the link, not the look.

The price of convenience: addition plastics are non-biodegradable — bacteria have no enzyme for them, so they fill landfills for centuries and choke oceans as microplastics. Burning is no better across the board: PVC combustion releases toxic hydrogen chloride on top of CO₂. Partial fixes: recycling codes, biodegradable polymers, and cracking plastics back to feedstock.