Organic Chemistry

Almost all organic chemistry starts with crude oil — a mixture of hydrocarbons (compounds of hydrogen and carbon only), formed from ancient sea life. It is useless as a mixture, so it is separated by fractional distillation: heated in a column, the hydrocarbons condense at different heights according to their boiling points.

Short-chain fractions (petrol, LPG) have low boiling points, are runny, ignite easily, and come off high in the column; long-chain fractions (bitumen) are the opposite. That single trend — chain length sets the properties — runs through the whole topic.

Alkanes are hydrocarbons with only single C–C bonds — methane, ethane, propane, butane — with the general formula CₙH₂ₙ₊₂. They are saturated (every carbon holds as many hydrogens as possible) and fairly unreactive, apart from burning.

Their main use is as fuels. Complete combustion (plenty of oxygen) gives carbon dioxide and water. Incomplete combustion (limited oxygen) gives carbon monoxide — a toxic, colourless gas — and soot, which is why faulty heaters are dangerous.

Alkenes contain a carbon–carbon double bond (C=C) — ethene, propene — with the general formula CₙH₂ₙ. They are unsaturated, and that double bond makes them much more reactive than alkanes.

The classic test uses bromine water: an alkene decolourises it (orange to colourless) as bromine adds across the double bond, while an alkane leaves it orange. Alkenes are made by cracking long alkane chains — breaking them into smaller, more useful molecules.

Alkenes are the raw material for polymers. In addition polymerisation, many small alkene molecules (monomers) join into one very long chain: thousands of ethene molecules become poly(ethene), the plastic in bags and bottles. The double bond opens up and links to the next monomer.

Their strength and cheapness are why plastics are everywhere — and their downside is the same chemistry: the strong, unreactive C–C backbone does not break down, so most are non-biodegradable, which is the heart of the plastic-waste problem.

Organic chemistry is tamed by families. A homologous series is a set of compounds with the same general formula and similar chemistry, where each member differs from the next by CH₂ — the alkanes (CₙH₂ₙ₊₂), the alkenes (CₙH₂ₙ), the alcohols and so on. As the chain lengthens, physical properties like boiling point change gradually and predictably.

What gives each family its character is its functional group — the reactive part, such as the C=C of alkenes or the –OH of alcohols. The name tells you both: a stem for the chain length (meth-, eth-, prop-, but-) plus an ending for the functional group (-ane, -ene, -ol).

The C=C double bond makes alkenes far more reactive than alkanes, and it reacts by addition — the double bond opens and atoms add across it. With hydrogen and a nickel catalyst an alkene is hydrogenated to an alkane (this hardens vegetable oils into margarine). With steam and a catalyst it is hydrated to an alcohol. With bromine it adds instantly, decolourising bromine water — the standard test.

Alkene molecules can also add to each other, linking into long chains — that is addition polymerisation, and it is how poly(ethene) and most everyday plastics are made.

Addition polymers such as poly(ethene) are chemically unreactive and non-biodegradable — microbes cannot break their strong carbon backbone — so they persist for centuries in landfill and oceans. Condensation polymers like nylon and polyester join monomers while losing a small molecule (usually water), and some can be broken back down.

Because burning plastics releases carbon dioxide and toxic gases, and landfill wastes both the material and space, recycling and designing biodegradable alternatives are increasingly important.

Alkanes are unreactive, but in ultraviolet light they react with halogens by a free-radical chain reaction in which a hydrogen is substituted by a halogen. It runs in three stages. Initiation: UV light splits a halogen molecule into two radicals, Cl₂ → 2Cl•. Propagation: a radical attacks the alkane and regenerates another radical, so the chain continues — Cl• + CH₄ → •CH₃ + HCl, then •CH₃ + Cl₂ → CH₃Cl + Cl•. Termination: two radicals meet and combine, ending a chain (e.g. Cl• + •CH₃ → CH₃Cl).

Because the reaction is hard to control, it gives a mixture of products — further substitution can replace more hydrogens (CH₂Cl₂, CHCl₃ …) — which limits its usefulness for making a single compound.

The C=C double bond is a region of high electron density, so it attracts electrophiles. When hydrogen bromide adds, the π electrons attack the δ+ hydrogen, forming a positively charged carbocation intermediate; the bromide ion then bonds to it. This step-by-step attack is electrophilic addition.

With an unsymmetrical alkene, Markovnikov's rule predicts the major product: the hydrogen adds to the carbon that already has more hydrogens, because this gives the more stable carbocation (a tertiary carbocation is more stable than a secondary, which beats a primary). So propene + HBr gives mainly 2-bromopropane.

The alcohols are a homologous series with the –OH (hydroxyl) functional group; the most important is ethanol, C₂H₅OH. Ethanol is made two ways: by fermentation, where yeast converts sugars to ethanol and carbon dioxide (C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂) at about 30 °C without air — a slow, batch process from a renewable crop; and by the hydration of ethene with steam and a catalyst — a fast, continuous process from crude oil.

Ethanol burns cleanly (C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O), which makes it a useful fuel and biofuel, and it is also a common solvent. Warmed with an oxidising agent it is oxidised to ethanoic acid — which is what happens, slowly in air, when wine turns to vinegar.

The carboxylic acids contain the –COOH (carboxyl) functional group; ethanoic acid, CH₃COOH, is the acid in vinegar. They are weak acids — only partly ionised — but they show all the typical acid reactions: with reactive metals they give a salt and hydrogen, with carbonates a salt, water and carbon dioxide (the fizz that confirms an acid), and with alkalis a salt and water. The salts are called ethanoates.

Ethanoic acid is made industrially, and in the lab by oxidising ethanol (with acidified potassium dichromate) or by the fermentation of ethanol in air.

Warm a carboxylic acid with an alcohol and a little concentrated sulfuric acid catalyst, and they combine to form a sweet-smelling ester plus water — a reaction called esterification. Ethanoic acid and ethanol, for example, give ethyl ethanoate.

Esters are prized for their pleasant, fruity smells, so they are used in flavourings and perfumes, as well as being useful solvents. Naturally occurring esters give many fruits their scent, and animal and vegetable fats are esters too.