Give the general structural formula for a fat or oil and describe the difference in structure between a saturated and an unsaturated fatty acid.

Explain why unsaturated fats have a lower melting point than saturated fats.

Oils can be hydrogenated. One possible problem is that partial hydrogenation may occur which produces an oil containing trans fatty acids. Explain the structural difference between a cis fatty acid and a trans fatty acid and state one disadvantage of ingesting oils containing trans fatty acids.

Difference:

Disadvantage:

 ;

saturated fatty acids contain only single bonds between carbon atoms/C–C whereas unsaturated fatty acids contain at least one double bond between carbon atoms/C=C / OWTTE;

the double (C=C) bond in unsaturated fats causes a “kink” so the molecules cannot pack so closely / OWTTE;

the weaker van der Waals/intermolecular forces between the molecules cause unsaturated fats to have lower melting points;

Difference

in cis the R groups on either side of the C=C point in the same direction and in trans the R groups point in opposite directions / OWTTE /

 ;

Disadvantage

hard to metabolize;

accumulate in fatty tissue;

difficult to excrete;

increase levels of LDL;

not a good source of energy;

Award [1] for any one of the above points.

Few candidates knew the basic structure of a fat or oil in part (a), with the ester linkage frequently missing from the structures drawn. Describing the difference between a saturated and an unsaturated fatty acid was reasonably well answered, although often the strength of the van der Waals‟ forces was not mentioned.

In part (b) most candidates could explain why the melting point of unsaturated fats is lower than that of saturated fats.

Many candidates struggled to adequately describe the structural difference between cis and trans fatty acids in part (c). A simple diagram would have been sufficient. The disadvantage of consuming oils containing trans fatty acids was generally answered well, although some weaker candidates resorted to stating that they were bad for our health. Again, this reflects the difficulties some candidates experience in providing responses with sufficient detail.