Isomerism: structural isomerism and stereoisomerism — common mistakes

A common mistake is to assume any molecule with a C=C bond will show E/Z isomerism. Always check the second condition! For example, propene (CH₃CH=CH₂) does not show E/Z isomerism because one of the double-bonded carbons is attached to two identical hydrogen atoms.

To quickly identify a chiral centre, scan the molecule for carbon atoms with four single bonds. Then, for each of these carbons, carefully list the four groups attached. Remember that a 'group' can be a single atom (like -H or -Br) or an entire chain (like -CH₂CH₃). If all four are different, you've found a chiral centre. Star (*) the chiral carbon in your diagrams to make it clear.

Cis/trans is an older, simpler system used when there is a common atom or group (e.g., hydrogen) on each carbon of the C=C bond. 'Cis' means the common groups are on the same side, and 'trans' means they are on opposite sides. E/Z notation is a more rigorous and universal system based on Cahn-Ingold-Prelog (CIP) priority rules. It can be used in all cases, even when there are four different groups attached to the double bond. 'Z' (zusammen) means the highest priority groups are on the same side, and 'E' (entgegen) means they are on opposite sides. For A-Level, you should be familiar with and able to use the E/Z system.

A carbon-carbon double bond consists of one sigma (σ) bond and one pi (π) bond. The σ-bond is formed by the head-on overlap of orbitals and lies along the axis between the two carbon nuclei. The π-bond is formed by the sideways overlap of p-orbitals, creating regions of electron density above and below the plane of the σ-bond. To rotate the bond, this π-bond would have to be broken, which requires a significant amount of energy (approx. 270 kJ mol⁻¹). This energy is not available at room temperature, so rotation is restricted.

Yes, larger molecules, especially those found in biology like sugars and amino acids, can have multiple chiral centres. The number of possible stereoisomers increases with the number of chiral centres. A molecule with 'n' chiral centres can have a maximum of 2ⁿ stereoisomers. This is explored in more detail in the A2 part of the course.