When can the molecule 2-butene, #C_4H_8#, undergo a geometric change called cis-trans isomerization?
But-2-ene can undergo cis-trans isomerization
- By acid catalysis
- By free radical catalysis
The cis-trans isomerization of but-2-ene requires breaking of the π bond.
The minimum energy required for isomerization is 267 kJ/mol.
At temperatures of about 500 °C the molecules have enough kinetic, vibrational, and rotational energy that the π bond breaks and isomerization can take place.
A quantum of light can excite a π electron to a π* orbital and allow free rotation about the central C-C bond.
In the excited π* state, the double bond is effectively reduced to a single bond, and one methyl group can rotate relative to the other.
The isomerization can be accomplished by providing another pathway with a lower activation energy (catalysis).
In the presence of strong acids, which act as catalysts, the isomerization can take place at room temperature.
The proton adds reversibly to the π bond forming a singly-bonded carbocation.
There is a temporary free rotation about the single bond before the proton leaves to regenerate the π bond.
Free-Radical Catalyzed Isomerization
If we add iodine, the temperature required for the isomerization drops drastically.
Iodine sets up an equilibrium with iodine atoms.
Then the mechanism proceeds as below.