SolutionsCatalytic Hydrogenation

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1. Catalytic hydrogenation

Predict the result of the catalytic hydrogenation of the following compounds:

Six alkenes: cis- and trans-but-2-ene, 1,2-dimethylcyclohexene, two bicyclic alkenes with a dimethylated bridge (the second optically pure) and octahydronaphthalene with the double bond at the ring fusion

Both but-2-enes give butane; 1,2-dimethylcyclohexene gives cis-1,2-dimethylcyclohexane (meso); the bicyclic alkenes are hydrogenated from the face opposite the dimethylated bridge, and the optically pure one gives an optically pure product; octahydronaphthalene gives cis-decalin

Both hydrogens add to the same face (syn addition) and, in rigid molecules, to the less hindered face.

2. Heats of hydrogenation

Why can heats of hydrogenation be used to compare the stability of isomeric alkenes?

Because isomeric alkenes that give the same alkane start from different energies and end at the same one: the difference between their heats of hydrogenation is the energy difference between them.

3. How many double bonds?

A compound C₁₀H₁₆ takes up 2 mol of H₂ and gives C₁₀H₂₀. How many rings and how many double bonds does it have?

C₁₀H₁₆ has 3 degrees of unsaturation. Two are double bonds (2 mol of H₂) and the third is a ring. C₁₀H₂₀ keeps that ring. Limonene is an example.

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