Isomerism: Cahn–Ingold–Prelog (CIP) priority rules.

The idea

CIP rules decide which group on each end of a double bond takes priority, and therefore whether the isomer is E or Z. Look at the atom directly attached to each doubly bonded carbon and compare atomic numbers: the higher atomic number wins, so Br beats Cl, which beats O, then N, then C, then H. If the two attached atoms are the same element, move one atom further out along each branch and compare the sets of atoms you find, taking the first point of difference; that is how CH₂CH₃ outranks CH₃, because the set C, H, H beats H, H, H. An atom joined by a double bond is counted twice. Z means both higher priority groups on the same side and E means on opposite sides.

What earns the marks

• Assigning priority at each carbon of the double bond by the atomic number of the attached atom.
• Resolving a tie by moving outwards to the next atoms and using the first point of difference.
• The correct E or Z label deduced from the positions of the two higher priority groups.
• The full name including the E or Z prefix and the correct locants.

Common mistakes and how to put them right

• Assigning priority by the size or mass of the whole group rather than by the atomic number of the attached atom.

• Using cis and trans labels when all four groups differ and CIP priorities are required.

• Comparing only the first atom and stopping, so failing to separate CH₃ from CH₂CH₃.

• Assigning Z because two groups look similar rather than because both are higher priority.

Exam tips

• Circle the two atoms directly attached at each end before comparing anything else.
• Write the priority you assigned next to each group so the examiner can follow the reasoning.
• Remember that Z means together on the same side and E means opposite sides.