Sec 4 Combined Science (Chemistry) Organic Chemistry notes

Crude oil and natural gas formed from dead marine organisms buried and compressed over millions of years. We burn them millions of times faster than they form, so they are non-renewable: stocks only run down. Crude oil itself is a mixture of hydrocarbons — compounds of carbon and hydrogen only — with different-sized molecules and therefore different boiling points, which is exactly what makes it separable.

In fractional distillation, crude oil is heated so most of it vaporises, then the vapours rise up a tower that is hot at the bottom and cooler towards the top. Each fraction — a group of similar-sized hydrocarbons — condenses at the height where the column temperature matches its boiling range. Small molecules (few carbons) have weak forces between molecules, low boiling points, so they rise to the top; large molecules condense near the hot bottom.

Down the column, molecules get bigger, less runny (more viscous), darker, and harder to light — all consequences of the growing intermolecular attraction. Biofuels such as bioethanol are the renewable alternative: made by fermenting sugars from crops, which can be regrown each season. Burning bioethanol still releases CO₂, but the crop absorbed CO₂ while growing (photosynthesis), so the fuel can be roughly carbon-neutral over its growth cycle. The trade-offs are real: land used for fuel crops is not growing food, and farming, fertiliser and processing all cost energy.

A homologous series is a family of compounds with (1) the same general formula, (2) similar chemical properties (same reactive group), and (3) a smooth trend in physical properties, each member differing by CH₂. The alkanes are the simplest series: CₙH₂ₙ₊₂, hydrocarbons with only single bonds. Saturated means no double bonds — the molecule carries the maximum possible hydrogen.

Combustion. With plenty of oxygen, alkanes burn completely to carbon dioxide and water: methane gives CH₄ + 2O₂ → CO₂ + 2H₂O. With limited oxygen, incomplete combustion produces the toxic gas carbon monoxide (CO) and/or soot (carbon) instead of CO₂ — the reason a yellow, sooty flame signals danger and poor oxygen supply.

Substitution. Alkanes are unreactive towards most reagents — the single C–C and C–H bonds are hard to break — but in ultraviolet light an alkane reacts with chlorine by substitution: a chlorine atom replaces a hydrogen atom. Methane: CH₄ + Cl₂ → CH₃Cl + HCl. One H of the alkane is swapped for one Cl, and the displaced H pairs with the second Cl to form hydrogen chloride. No UV, (almost) no reaction.

The alkenes have general formula CₙH₂ₙ and contain one carbon–carbon double bond (C=C). With fewer hydrogens than the alkane of the same carbon count, they are unsaturated. Ethene is C₂H₄, propene is C₃H₆. The C=C is the reactive site: it can open up and let other atoms attach — that is an addition reaction.

The bromine test. Aqueous bromine (orange/brown) is the examiner's detector for unsaturation: with an alkene the bromine adds across the C=C and the colour disappears (ethene: C₂H₄ + Br₂ → C₂H₄Br₂, a colourless dibromo compound). With a saturated alkane the orange colour stays — alkanes need UV light to react with halogens, and a test tube in daylight provides none. Orange → colourless fast test unsaturated.

More addition chemistry. With hydrogen (over a nickel catalyst, warmed), the C=C gains two H atoms and becomes the corresponding alkane — ethene + H₂ → ethane. This hydrogenation is how liquid vegetable oils are hardened into margarine: adding some hydrogen across some of the double bonds makes the fat more solid and spreadable. Oils with several C=C bonds are polyunsaturated — promoted in foods because unsaturated fats are linked to healthier blood cholesterol. Alkenes also burn, but their higher carbon-to-hydrogen ratio makes the flame smokier than an alkane's.

Cracking. Fractional distillation delivers more large molecules than the market wants and fewer small ones than it needs. Catalytic cracking heats a large alkane over a hot catalyst (or with steam) and splits it: one alkane → one smaller alkane + one alkene. Decane: C₁₀H₂₂ → C₈H₁₈ + C₂H₄. The alkene product (here ethene) is a vital feedstock for polymers. Cracking is how supply is matched to demand — petrol-sized molecules and alkene feedstock from unwanted heavy fractions.

The alcohols carry the hydroxyl group, –OH, on a carbon chain: general formula CₙH₂ₙ₊₁OH. Methanol CH₃OH, ethanol C₂H₅OH, propanol C₃H₇OH — names end in -ol. Ethanol is the drinking and fuel alcohol; both alcohols burn cleanly in oxygen (ethanol: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O), which is why ethanol-blend biofuels work in car engines.

Fermentation makes ethanol renewably: yeast enzymes convert glucose (sugar) into ethanol and carbon dioxide under warm conditions (about 35 °C) and absence of air: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Too cold and the yeast works slowly; too hot (above about 40–45 °C) and the enzymes denature, killing the process. The anaerobic condition matters because oxygen would push the chemistry onward — to oxidation.

And onward it goes: oxidation turns ethanol into ethanoic acid, CH₃COOH — the acid in vinegar. Leave wine open to the air and oxygen (assisted by microbes) does the job slowly; in the lab, warm acidified potassium manganate(VII) does it fast. The equation in symbolic shorthand: C₂H₅OH + 2[O] → CH₃COOH + H₂O. Ethanoic acid belongs to the carboxylic acids, whose carboxyl group –COOH gives the family its name and its weak acidity: methanoic HCOOH, ethanoic CH₃COOH, propanoic C₂H₅COOH — names end in -oic acid.

An alkene molecule can do one more thing with its double bond: share it. In addition polymerisation, the C=C of each monomer opens and the monomers link into one long polymer chain — many (poly) units (mers). Ethene makes poly(ethene): n CH₂=CH₂ → (–CH₂–CH₂–)ₙ. Only unsaturated monomers can do this; an alkane has nothing to open. Poly(ethene)'s cheapness, flexibility and water resistance make it the workhorse plastic: carrier bags, bottles, cling film, pipes.

Reading the chain. The repeat unit is the monomer with its C=C opened to C–C, drawn with open bonds at both ends and the letter n outside the bracket: monomer CH₂=CH₂ ↔ repeat unit –CH₂–CH₂–. Going backwards, take the repeat unit, close the bond between the two carbons back into a double bond, and you have the monomer. The n means very many — a single poly(ethene) molecule carries thousands of units.

The problem. Addition polymers are chemically inert: the strong, non-polar C–C backbone gives microbes nothing to digest, so poly(ethene) is non-biodegradable — it does not rot. Discarded plastic fills landfills, litters oceans, breaks into microplastics that enter food chains, and burning it wastefully releases CO₂ (and worse with some plastics).

What can be done. Physical recycling sorts plastics by type, melts and remoulds them into new products: cheap-ish and proven, but sorting is laborious and repeated melting degrades quality. Chemical recycling breaks the polymer back into small molecules/monomers that can be re-polymerised like fresh feedstock: higher-quality output, but it costs more energy and infrastructure. Around both: reduce (less packaging), reuse (refillables), and design for recycling. Any sustainability answer should name the chemical reason (inert backbone, non-biodegradable), then a trade-off (cost, energy, land, quality) — one-sided answers cap your marks.