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Stability of Alkenes
Concepts tested here
- Substitution and steric strain
All Questions
2000 AIPMT 1 question
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Correct order of stability is :More substituted alkenes are more stable, because of hyperconjugation by the alkyl groups. 2-Butene (disubstituted) is more stable than 1-butene (monosubstituted).
Between the two 2-butenes, the trans isomer is more stable than the cis isomer, because in the cis form the two methyl groups are on the same side and repel each other (steric strain).
This is confirmed by heats of hydrogenation: 1-butene 127, cis-2-butene 120 and trans-2-butene 116 kJ/mol (less heat means more stable).
So the order is trans-2-butene $\gt$ cis-2-butene $\gt$ 1-butene.