Module 15: Chemistry

The "boring" molecules — chemically unreactive (hence "inert") but excellent fuels because burning them releases a lot of energy. The length of the chain determines the state: methane (1C) = gas, octane (8C) = liquid, paraffin (20+ C) = solid wax.

The –OH group forms hydrogen bonds with water — which is why ethanol and water mix completely. Ethanol is polar enough to dissolve in water but also has a nonpolar carbon chain — so it can dissolve some oils too. This dual nature makes it a versatile solvent (perfumes, cleaning agents). The difference between drinkable and poisonous alcohol is a single carbon: ethanol (C₂) is metabolised to acetaldehyde then to acetic acid (harmless); methanol (C₁) is metabolised to formaldehyde — toxic and fatal even in small doses.

Aldehydes are often responsible for strong, distinctive smells. Many flavour compounds (vanilla, cinnamon, almond) are aldehydes. The smell of fresh-cut grass is partly due to short-chain aldehydes. Formaldehyde — the simplest aldehyde — is toxic and used in preservation (embalming fluid). Alcohol oxidises to aldehyde: when you drink ethanol, your liver converts it to acetaldehyde — the molecule largely responsible for the hangover.

Ketones are superb solvents because their polarity is intermediate — they dissolve many things water won't. Acetone (nail polish remover) is the simplest ketone, miscible with water but also dissolves plastics, paints, and oils. The body produces ketones (like acetoacetate) when burning fat instead of glucose — this is the basis of the "keto diet." Breath smelling of nail polish remover can indicate diabetic ketoacidosis — a medical emergency.

The sharp smell of vinegar is pure acetic acid (ethanoic acid). The soreness you feel after intense exercise comes partly from lactic acid accumulating in muscle. Long-chain carboxylic acids are called fatty acids — the building blocks of fats and oils. When you react a carboxylic acid with an alcohol, you get an ester (see below) — which is how many flavours and fragrances are made.

Esters are responsible for most fruit and flower smells. "Artificial banana" flavour is almost pure isoamyl acetate — a single ester molecule. All fats and vegetable oils are triglycerides: three fatty acid chains attached to a glycerol backbone via ester bonds. When you digest fat, enzymes break those ester bonds. Polyester fabric is also an ester polymer — long chains of ester-linked monomers.

Amines are biochemically critical — all amino acids contain an amine group, and neurotransmitters (dopamine, serotonin, adrenaline) are amines. The fishy smell of old fish is trimethylamine — an amine produced as bacteria break down proteins. Lemon juice on fish works because the citric acid (an acid) reacts with the amine (a base), neutralising it and reducing the smell. This is real chemistry on your plate.

Diethyl ether was the first widely used surgical anaesthetic (1846). Its pleasant smell and rapid onset made it transformative for surgery — though highly flammable. The smell of old-fashioned hospitals sometimes involved ether. THF is one of the most common solvents in organic chemistry labs — it dissolves almost everything. Note: "ether" in everyday speech often refers to diethyl ether specifically, but chemically it names any R–O–R structure.

Every protein in your body — muscle, enzymes, hair, antibodies — is built from amino acids joined by amide (peptide) bonds. Nylon is a synthetic polyamide: long chains of amide-linked monomers that mimic the structural properties of proteins. Paracetamol contains an amide group, which is why it behaves differently from aspirin (which is an ester). The amide bond is exceptionally stable — which is why proteins can persist for thousands of years in fossils.