Organic Chemistry

The organic toolkit

Carbon forms millions of compounds, but they organise into a few families. Each family shares a functional group, follows a general formula, and behaves in predictable ways — so the whole subject rests on recognising a handful of patterns.

Families and general formulae

Consecutive members differ by a single –CH₂– unit, so each series follows one general formula. Knowing the formula lets you write any member: an alkene with five carbons is C₅H₁₀.

ExtendedDisplaying a molecule

A displayed formula shows every atom and every bond (Core). A structural formula — such as CH₃CH₂OH — is a shorthand that still describes the arrangement unambiguously (Supplement).

ExtendedSaturated, unsaturated & isomers

A saturated compound has only single C–C bonds; an unsaturated one has at least one C=C. Two molecules can share a molecular formula yet differ in arrangement — structural isomers. Position matters: moving the double bond gives but-1-ene (C=C at the end) or but-2-ene (C=C in the middle).

Fuels & fractional distillation

The fossil fuels — coal, natural gas (mainly methane) and petroleum — are the raw material of organic chemistry. Petroleum is a mixture, and almost nothing useful comes out of it until that mixture is separated.

Separating the mixture

Because petroleum is a mixture of hydrocarbons with different chain lengths, its components have different boiling points and can be separated by fractional distillation. The crude oil is heated; vapours rise up a column that is hot at the base and cool at the top, and each fraction condenses at the height matching its boiling range.

Reading the column

Everything about a fraction follows from its chain length. Going up the column — from bitumen to refinery gas — chains get shorter, so the fractions become more volatile, boil at lower temperatures and flow more easily (lower viscosity). Going down, the opposite: long chains, thick and hard to ignite, ending in solid bitumen.

Alkanes & alkenes

Alkanes are the saturated hydrocarbons — carbon skeletons carrying the maximum number of hydrogens. They are generally unreactive, with two exceptions: they burn, and they react with chlorine in ultraviolet light.

Alkanes: combustion and substitution

In a plentiful supply of oxygen, alkanes undergo complete combustion to carbon dioxide and water only — for example 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O (too little oxygen gives carbon monoxide and soot instead).

In ultraviolet light, an alkane reacts with chlorine by substitution: a hydrogen atom is replaced by a chlorine atom. At IGCSE only monosubstitution is required — one hydrogen of methane replaced by chlorine, giving chloromethane and hydrogen chloride.

Making alkenes: cracking

Long-chain fractions are in surplus; short chains and alkenes are in demand. Cracking solves both — a high temperature and a catalyst break long alkanes into a shorter alkane plus an alkene (and often hydrogen), e.g. C₁₀H₂₂ → C₈H₁₈ + C₂H₄.

Testing for unsaturation

Aqueous bromine distinguishes alkenes from alkanes. Shake it with an alkene and the orange colour vanishes as bromine adds across the C=C; an alkane leaves it unchanged.

ExtendedAddition reactions of ethene

Because the double bond can open, an alkene undergoes addition — one product only. Three reactions of ethene are required: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane); CH₂=CH₂ + H₂ → CH₃CH₃ (ethane, over a nickel catalyst); CH₂=CH₂ + H₂O → CH₃CH₂OH (ethanol, with an acid catalyst).

Alcohols

Ethanol is the alcohol of the syllabus — a solvent, a fuel, and the meeting point of the two great feedstocks of chemistry: living crops and crude oil. It can be made from either.

Two routes to ethanol

Combustion & uses

Ethanol burns cleanly in a good supply of air, which is why it is used as a fuel; it is also a common solvent. Complete combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

Carboxylic acids & esters

Ethanoic acid behaves as a typical (if weak) acid, and it sits one oxidation step beyond ethanol. Combine an acid and an alcohol and you get something new — an ester.

Reactions of ethanoic acid

Ethanoic acid shows the characteristic acid reactions, always forming an ethanoate salt: with reactive metals → ethanoate + hydrogen; with bases → ethanoate + water; with carbonates → ethanoate + water + carbon dioxide.

ExtendedOxidation of ethanol

Ethanol is oxidised to ethanoic acid by warming with acidified potassium manganate(VII), whose purple colour fades to colourless, or by bacterial oxidation in air — how wine turns to vinegar.

ExtendedEsterification

Heating a carboxylic acid with an alcohol and an acid catalyst forms an ester and water. The –OH of the acid and the –H of the alcohol leave together as H₂O, joining the fragments through the ester linkage: ethanoic acid + ethanol → ethyl ethanoate + water.

Polymers

A polymer is a molecule made by joining thousands of monomers. How they join — by addition or by condensation — decides everything about the product and how it can be recycled.

Addition polymerisation

Unsaturated monomers such as ethene add together: the C=C opens and the units link into a long chain with nothing else formed. Poly(ethene) is the classic example — n ethene monomers open their double bonds to give the poly(ethene) repeat unit, with the continuation bonds running through the brackets.

ExtendedCondensation polymerisation

Monomers carrying two functional groups link with loss of water. A diacid + diamine gives a polyamide (nylon); a diacid + diol gives a polyester (PET). Proteins are natural polyamides, built from amino-acid monomers (H₂N–CHR–COOH) joined by the same amide link. PET can be broken back into monomers and re-polymerised. The two condensation linkages are the amide link –CO–NH– (nylon, proteins) and the ester link –CO–O– (PET); a water molecule leaves as each one forms.