4aIntroduction

Organic chemistry has more vocabulary than any other part of the course, and most of the marks come from using it precisely.

Hydrocarbons

A hydrocarbon is a compound of hydrogen and carbon only.

The word only is doing all the work. Ethanol, C₂H₅OH, contains carbon and hydrogen — and also oxygen, so it is not a hydrocarbon.

When deciding, scan the formula for a third element. If there is one, the answer is no.

Homologous series

A homologous series is a family of compounds with the same general formula.

Its members share one functional group, so they react in similar ways, and each differs from the next by CH₂.

The families you meet are alkanes, alkenes, alcohols, carboxylic acids and esters.

Functional group

The functional group is the atom or group of atoms that gives a compound its characteristic reactions.

It is why members of a series behave alike: the functional group is where the chemistry happens, and the carbon chain is largely along for the ride.

Chemical alike, physical different

Within a series:

• Chemical properties stay similar, because the functional group is unchanged.
• Physical properties change steadily, because the molecules get longer. Boiling point rises with relative molecular mass, as the intermolecular forces get stronger.

A question about similar reactions and a question about a trend can both be about one series and want opposite answers. Read which one is being asked.

Isomers

Isomers have the same molecular formula but different structural formulae.

Butane and methylpropane are both C₄H₁₀. Same atoms, different arrangement, genuinely different substances with different boiling points.

Check the molecular formulae match first. If they differ, the compounds are not isomers at all — and consecutive members of a homologous series can never be isomers, because they differ by CH₂.

Four kinds of formula

Read which one a question asks for. Giving a molecular formula when the structural one was wanted answers a different question.

Using a general formula

CₙH₂ₙ₊₂ for alkanes. For six carbons: (2 × 6) + 2 = 14, so C₆H₁₄.

Double the carbons then add two — not add two to n first.

Naming: stem plus ending

Every organic name has two parts.

The stem gives the number of carbon atoms:

These first four do not follow the usual number prefixes, so they simply have to be learned.

The ending gives the family:

So propene has three carbons and a C=C double bond. Propane has three carbons and no double bond. One letter, a different compound.

Counting carbons carefully

In a carboxylic acid, the carbon of the −COOH group counts towards the total. Ethanoic acid, CH₃COOH, has two carbons altogether.

Why a molecular formula is not enough

C₄H₁₀ describes two different compounds. C₅H₁₂ describes three.

So a molecular formula alone cannot identify a substance — you need the structural or displayed formula to say which isomer it is. That is exactly why those formulae exist.

Three reaction types to recognise, and the easiest way to tell them apart is to count the products.

Addition

Two molecules join to form one product. Nothing is left over.

ethene + bromine → dibromoethane

All the atoms of both reactants end up in the single product. Add the two formulae together and you have it.

Only unsaturated compounds can do this, because a double bond must open to accept the incoming atoms.

Substitution

An atom is replaced, and there are two products.

methane + chlorine → chloromethane + hydrogen chloride

One hydrogen is swapped for a chlorine; the displaced hydrogen leaves with the other chlorine atom. That second, smaller product is the giveaway.

Saturated compounds react this way, because they have no double bond to open — replacing an atom is the only option available.

Combustion

Reaction with oxygen, releasing energy.

Complete combustion of a hydrocarbon gives carbon dioxide and water — and only those two, because a hydrocarbon contains only carbon and hydrogen, so their oxides are all that can form.

Telling addition from substitution

Track where the second halogen atom goes. In addition it joins the molecule; in substitution it leaves as HBr or HCl.

That single question settles most classification questions in this topic.