Amphetamine exists as optical isomers. Describe how chiral auxiliaries can be used to synthesize only the desired enantiomeric form of a drug from a non-chiral starting compound.

The structures of morphine and heroin are shown in Table 20 of the Data Booklet. Explain the increased potency of heroin compared to morphine.

a chiral auxiliary is itself an enantiomer/optically active;

it is bonded to the reacting molecule so that reaction forms one product;

(remove chiral auxillary) to give one enantiomer;

polar hydroxyl groups in morphine are replaced by less/non-polar ester groups;

Accept morphine more polar/heroin less polar without mentioning hydroxyl and ester groups for M1.

allows transport into the non-polar central nervous system / more soluble in nonpolar lipids / penetrates/crosses blood-brain barrier / OWTTE;

Reference to difference in polarity needed for first mark.

Candidates found it more difficult to explain the use of chiral auxiliaries.

Discussion of polarity with regard to morphine and heroin was often omitted in (e), again suggesting poor chemical understanding by many candidates.