What is Organic Chemistry?

Organic chemistry is the chemistry of carbon-containing compounds.

Carbon is special because each carbon atom can form four covalent bonds — to other carbons, to hydrogen, and to a wide range of other elements like oxygen, nitrogen and the halogens.

This produces a near-infinite variety of stable molecules, from methane (CH₄) up to enormous biological molecules like DNA and proteins.

All organic chemistry at A Level is built on the same foundation: a chain or ring of carbon atoms, with hydrogens or other atoms attached. Once you can name and draw these molecules and understand a handful of recurring reaction types, organic chemistry becomes a logical, predictable subject.

Naming Organic Compounds (IUPAC Nomenclature)

Every organic compound has a systematic IUPAC name that uniquely describes its structure.

The system has three parts: a stem, a suffix, and any prefixes.

The stem tells you the number of carbons in the longest chain.

1 carbon: meth-. 2: eth-. 3: prop-. 4: but-. 5: pent-. 6: hex-. 7: hept-. 8: oct-. 9: non-. 10: dec-.

The suffix tells you the main functional group.

-ane for alkanes (C–C single bonds only), -ene for alkenes (C=C double bond), -ol for alcohols, -al for aldehydes, -one for ketones, -oic acid for carboxylic acids, -amine for amines.

Prefixes describe side groups (substituents) attached to the main chain.

Halogens become fluoro-, chloro-, bromo- and iodo-.

Alkyl side groups become methyl-, ethyl-, propyl- and so on.

Numbering rules ensure each compound has only one valid name.

Number the carbons in the main chain from the end that gives the lowest set of locants for substituents and functional groups.

If two substituents could give the same set, alphabetical order decides.

Example: CH₃CH(Br)CH₂CH₃ is 2-bromobutane, not 3-bromobutane, because numbering from the right gives the lower locant.

Ways of Representing Organic Molecules

There are several common ways of writing the structure of an organic molecule, each giving a different level of detail.

The empirical formula is the simplest whole-number ratio of atoms. For ethane, C₂H₆, the empirical formula is CH₃.

The molecular formula gives the actual number of each atom. C₂H₆ for ethane.

The structural formula shows how atoms are connected in a condensed line, like CH₃CH₂OH for ethanol.

The displayed formula shows every bond and every atom drawn out — the most explicit version.

The skeletal formula simplifies things by hiding all hydrogens on carbon and showing only the bonds between non-hydrogen atoms.

Each end of a line and each intersection represents a carbon atom with enough hydrogens to make up four bonds.

Skeletal formulae are the standard for larger or more complex molecules because they’re quick to draw and easy to read.

The general formula gives the pattern for a whole family. For alkanes it’s CₙH₂ₙ₊₂. For alkenes it’s CₙH₂ₙ.

Functional Groups and Homologous Series

A functional group is a particular atom, bond or group of atoms that gives the molecule its characteristic chemical behaviour.

The C=C bond is the functional group of alkenes. The –OH group is the functional group of alcohols. The –COOH group is the functional group of carboxylic acids.

A homologous series is a family of organic compounds with the same functional group and the same general formula.

Each member differs from the next by a CH₂ unit.

Within a homologous series, the chemistry is similar but the physical properties change in a predictable way as the molecules get larger — boiling points rise, density changes, and so on.

Structural Isomerism

Structural isomers are molecules with the same molecular formula but different structural arrangements of atoms.

AQA recognises three types of structural isomerism.

Chain isomerism: same molecular formula, different carbon skeleton.

Pentane (a straight chain) and 2,2-dimethylpropane (a branched chain) both have molecular formula C₅H₁₂ but very different shapes.

Position isomerism: same molecular formula, same functional group, but the group sits at a different position on the chain.

Propan-1-ol and propan-2-ol are both C₃H₈O, but the OH is on a different carbon.

Functional group isomerism: same molecular formula, but a different functional group entirely.

Ethanol (CH₃CH₂OH, an alcohol) and methoxymethane (CH₃OCH₃, an ether) both have formula C₂H₆O — but they’re completely different compounds with different chemistry.

Stereoisomerism

Stereoisomers have the same structural formula — the same atoms connected in the same order — but a different arrangement of atoms in three-dimensional space.

At AS level, the relevant type is E/Z (cis-trans) isomerism, which occurs around a C=C double bond.

Because the C=C bond cannot rotate, two groups on each carbon are locked in their positions.

If the two highest-priority groups are on the same side of the double bond, the isomer is Z (German zusammen, ‘together’).

If they are on opposite sides, it is E (entgegen, ‘opposite’).

More detail comes in the alkenes section (3.3.4) and in optical isomerism (A2 topic 3.3.7).

Reaction Mechanisms: Curly Arrows and Bond Fission

Reaction mechanisms show the step-by-step movement of electrons during a reaction.

Curly arrows are used to indicate this movement.

A full curly arrow shows the movement of a pair of electrons.

A half-headed arrow (sometimes called a ‘fish-hook’) shows the movement of a single electron.

Every arrow starts from a region of electron density — a lone pair, a bond, or a negative charge — and ends where the electrons end up.

Bonds can break in two ways.

Homolytic fission: the bond breaks evenly, with one electron going to each atom.

This produces two species each with a single unpaired electron, called free radicals.

Homolytic fission is favoured by non-polar bonds and high-energy conditions like UV light or heat.

Heterolytic fission: the bond breaks unevenly, with both electrons going to one atom.

This produces one positive ion and one negative ion.

Heterolytic fission is favoured by polar bonds, and it underlies the majority of organic reaction mechanisms you’ll meet.

Two key types of attacking species appear repeatedly in heterolytic mechanisms.

A nucleophile is an electron-pair donor — something with a lone pair that attacks regions of positive charge. Common nucleophiles include OH⁻, CN⁻, NH₃ and Cl⁻.

An electrophile is an electron-pair acceptor — something that attacks regions of negative charge. Common electrophiles include H⁺, NO₂⁺ and the partially-positive carbon of a polar bond.