In a
chemical reaction that yields one product (X) from one conformational
isomer (A') and a different product (Y) from another conformational
isomer (A'') (and provided these two isomers are rapidly interconvertible relative to the
rate of product formation, whereas the products do not undergo interconversion) the
product composition is not in direct proportion to the relative concentrations of
the conformational isomers in the
substrate ; it is controlled only by the difference in standard free energies
(
)
of the respective
transition states.
It is also true that the product composition is formally related to the relative concentrations
of the conformational isomers A' and A'' (i.e. the conformational
equilibrium constant) and the respective rate constants of their reactions; these parameters are generally
— though not invariably — unknown. The diagram below represents the energetic situation
for
transformation of interconverting isomers A and A' into products X and Y.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1102
