Alkenes and geometric isomerismEdexcel International A Level Chemistry: Revision notes

Alkenes are hydrocarbons containing a carbon–carbon double bond, C=C. They have the general formula $C_nH_{2n}$, for example ethene $C_2H_4$ and but-2-ene $C_4H_8$.

They are unsaturated: they contain fewer hydrogen atoms than the alkane with the same number of carbons, because a C=C can take part in addition reactions. Cycloalkenes are also unsaturated; a cycloalkene with one C=C has the formula $C_nH_{2n-2}$, e.g. cyclohexene $C_6H_{10}$.

A C=C double bond has two parts.

• A sigma (σ) bond: end-on (head-on) overlap of orbitals along the line joining the nuclei. The electron density lies between the nuclei.
• A pi (π) bond: sideways overlap of p orbitals, with electron density above and below the plane of the carbon atoms.

The pi bond is weaker than the sigma bond, and its exposed electron density makes the C=C the reactive site of an alkene.

Rotation about a C–C single bond is free, but rotation about a C=C is restricted: turning one end would break the sideways overlap of the pi bond. The groups on each carbon are therefore fixed in space.

Geometric isomers (a type of stereoisomer) have the same structural formula but a different arrangement of groups in space. They occur when:

• there is a C=C bond (restricted rotation), and
• each carbon of the C=C has two different atoms or groups.

But-2-ene shows this; but-1-ene does not, because C1 carries two H atoms.

If each carbon of the C=C carries one atom or group matching one on the other carbon, the isomers are named cis (matching groups on the same side) and trans (opposite sides). For example, cis-but-2-ene has both CH₃ groups on the same side.

This breaks down when the four groups are all different, for example $CH_3CBr=CHCl$, because there is no pair to compare.

The E-Z system works for any alkene with two different groups on each carbon. It uses priority rules:

1. On each carbon, rank the two groups by the atomic number of the atom attached to the double bond (higher atomic number = higher priority): Br > Cl > C > H.
2. If the two higher-priority groups are on the same side, the isomer is Z (from German zusammen, together).
3. If they are on opposite sides, it is E (entgegen, opposite).

Worked example: $CH_3CBr=CHCl$. On one carbon Br beats CH₃; on the other Cl beats H. If Br and Cl are on the same side, the isomer is Z-2-bromo-1-chloroprop-1-ene.