Discussion
For the following three alkenes, 1, 2 and 3, the rates of hydrogenation using Wilkinson's catalyst at 25 °C vary in the order
Answer: B
The hydrogenation rates vary as 1 \(\gt\) 2 \(\gt\) 3.
Wilkinson's catalyst, RhCl(PPh3)3, is bulky, and the alkene must coordinate to rhodium crowded by triphenylphosphine ligands, so the rate falls steeply as the alkene becomes more substituted and hindered.
- Cyclohexene (1) is a cis-disubstituted alkene, the least hindered of the three, and is hydrogenated fastest.
- trans-2-Pentene (2) is also disubstituted, but trans alkenes bind and react more slowly than cis ones.
- 1-Methylcyclohexene (3) is trisubstituted and reacts slowest of all.
Any order that puts the trisubstituted alkene ahead of a disubstituted one, or the trans alkene ahead of cyclohexene, runs against this steric control.
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