The cis–trans isomerization of but-2-ene in the gas phase can be represented as
cis-CH₃CH=CHCH₃ ⇌ trans-CH₃CH=CHCH₃
Isomerization requires rotation about the C=C bond, which temporarily disrupts π-bond overlap.
(a) Describe at the molecular level which bonds are partially broken or weakened as the molecule approaches the transition state for rotation.
(b) Sketch a qualitative reaction energy profile for the forward (cis → trans) reaction. Label reactants, transition state, products, forward and reverse activation energies, and overall energy change.
(c) Explain why raising the temperature accelerates both the forward and the reverse isomerization even though ΔE for the reaction is approximately zero.
(d) Predict how replacing each CH₃ group with a bulkier tert-butyl group would alter the shape or height of the energy profile, and justify your prediction in terms of steric and bonding considerations.