Why does an unsaturated fatty acid with cis double bonds generally have a lower melting point than a saturated fatty acid of the same chain length?
Answer and explanation
Correct answer: A. The kinks in the chain stop the molecules packing closely
- Step 1: A cis double bond puts a bend in the hydrocarbon chain.
- Step 2: Bent chains cannot pack as closely, so there is less contact for dispersion forces.
- Step 3: Weaker intermolecular forces mean less energy is needed to melt the fat.
Answer
kinks reduce packing and dispersion forces.
Why not the others: Chain length is the same; hydrogen bonding would raise, not lower, melting point; the chains are not charged.
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