fats and oils

monosaccharides.

Liver is a source of arachidonic acid, \({\text{C}}{{\text{H}}_{\text{3}}}{{\text{(C}}{{\text{H}}_{\text{2}}}{\text{)}}_{\text{4}}}{{\text{(CH=CHC}}{{\text{H}}_{\text{2}}}{\text{)}}_{\text{4}}}{{\text{(C}}{{\text{H}}_{\text{2}}}{\text{)}}_{\text{2}}}{\text{COOH}}\), and fish oils are a source of linolenic acid. With reference to the structure of linolenic acid in Table 22 of the Data Booklet, explain why arachidonic acid has a much lower melting point compared to linolenic acid, even though it contains two more carbon atoms.

(tri)esters/contains COO group/(tri)glycerides;

(three) fatty acid chains joined to glycerol/propan-123-triol / OWTTE;

Accept long-chain carboxylic acid and glycerine.

empirical formula is \({\text{C}}{{\text{H}}_{\text{2}}}{\text{O}}\) / general formula is \({{\text{C}}_n}{{\text{H}}_{{\text{2}}n}}{{\text{O}}_n}\);

contains one carbonyl/C=O group and at least two/several hydroxyl/OH groups;

four C=C bonds in arachidonic acid and three C=C bonds in linolenic acid / greater unsaturation/number of C=C bonds in arachidonic acid;

presence of double bonds prevents close-packing/kinks in structure / extra double bond decreases ability of arachidonic acid molecules to align themselves together / OWTTE;

(so) van der Waals’/London/dispersion/intermolecular forces weaker in arachidonic acid;

The descriptions of structure in (b) often lacked the essential points - perhaps some candidates did not know how to interpret “structural composition”.

the descriptions of structure in (b) often lacked the essential points - perhaps some candidates did not know how to interpret “structural composition”.

The effect of double bonds on the intermolecular forces in unsaturated fats was well known in (c), and thankfully few candidates referred to the breaking of covalent bonds.