Compare the structures and chemical formulas of the two essential fatty acids linoleic acid and linolenic acid.

Similarities:

Award [3 max] for any three similarities of:

both contain 18 carbons/same number of carbons;

both unsaturated/contain C=C/carbon-carbon double bonds;

Do not allow just double bond.

both contain carboxyl/COOH/\({\text{C}}{{\text{O}}_{\text{2}}}{\text{H}}\);

Allow “both carboxylic acids”.

both have first (carbon to carbon) double bond/C=C on C9;

both have cis-configuration of (all) C=C (fragments);

Differences:

Award [1 max] for any one difference of:

linoleic acid (omega-6) contains 2 C=C and linolenic acid (omega-3) contains 3 C=C / linolenic acid has one more C=C;

Allow linolenic acid more unsaturated.

closest C=C on linoleic acid is on sixth carbon from methyl end and closest C=C on linolenic acid is on third carbon from methyl end / OWTTE;

Accept linoleic acid is omega-6/\(\omega \)-6 and linolenic acid is omega-3/\(\omega \)-3.

Award similarity mark M2 automatically as long as either difference mark is scored.

Candidates must remember that they are describing carbon-carbon double bonds; just “double bonds” does not score. Few candidates scored all four marks because some obvious similarities, such as same number of carbons and both contain COOH group were omitted. Some candidates found it helpful to tabulate the answer.