Curtin-Hammett
Principle
- A and B are conformational isomers.
- Under elimination conditions A leads to trans-stilbene
while B leads to cis-stilbene.
- The compound exists as a mixture of the two
conformational isomers A and B, among others and therefore elimination
leads to a mixture of the two stereoisomers P1and P2.
- The question is which is the major product?
- Normally the more stable comformer should lead
to the major product.
But according to Curtin
Hammett principle,
“The product proportion does
not depend on the ground state population of the two conformational
isomers, but depends on the transition state energies of the two
processes.”
- That is the more stable conformational isomer
does not necessarily lead to the major product.
- The major product is the one that has less
activation energy barrier.
- Conformation B has more steric problems. Due
to the interactions of the two phenyl rings has a higher energy
barrier and thus leads to the minor product.
- Mathematically the Principle can be explained
as follows.
P1 / P2 is the product proprtion,
k1and k2 are the rate constants of the two
processes and K is the equilibrium constant for the equilibrium
between the two isomers. [x] is the concentration of all other species. |
