Question 7 of 13: Melting Points of Elaidic, Oleic, and Stearic Acids

Question 7: Melting Points of Elaidic, Oleic, and Stearic Acids (equal value)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Melting point in this C18 fatty-acid series is governed entirely by how efficiently the chains can pack into an ordered solid lattice, which in turn depends on chain shape:

• Stearic acid (fully saturated, C18:0) is a straight, flexible zig-zag chain that packs very efficiently — highest melting point (≈69–70 °C).
• Oleic acid (C18:1, cis/Z double bond) has a rigid ~30° kink at the double bond that prevents the chain from packing closely against its neighbours — lowest melting point (≈13–16 °C, liquid at room temperature).
• Elaidic acid (C18:1, trans/E double bond) keeps the same nearly-linear, extended shape as a saturated chain right through the double bond (the two alkyl substituents point in opposite/anti directions, just like a saturated C–C–C–C dihedral), so it packs almost as efficiently as stearic acid.

So the predicted order is stearic acid > elaidic acid > oleic acid, with elaidic acid's melting point far closer to the saturated acid than to its own cis isomer — the reason trans fats behave like saturated fats (solid/semi-solid at room temperature, "heart-healthy" concerns) even though they are technically unsaturated.