Difference between primary and secondary kinetic isotope effect
Answers
Answer:Secondary kinetic isotope effects are rate changes due to isotopic substitutions at a site other than the bond breaking site in the rate determining step of the reaction. These come in three forms: α , β , and γ effects.β secondary isotope effects occur when the isotope is substituted at a position next to the bond being broken.
(CH3)2CHBr+H2O−→kH(CH3)2CHOH
(CD3)2CHBr+H2O−→kD(CD3)2CHOH
This is thought to be due to hyperconjugation in the transition state. Hyperconjugation involves a transfer of electron density from a sigma bond to an empty p orbital
Primary kinetic isotope effects are rate changes due to isotopic substitution at a site of bond breaking in the rate determining step of a reaction.
EXAMPLE
Consider the bromination of acetone: kinetic studies have been performed that show the rate of this reaction is independent of the concentration of bromine. To determine the rate determining step and mechanism of this reaction the substitution of a deuterium for a hydrogen can be made.
When hydrogen was replaced with deuterium in this reaction a kHkD of 7 was found. Therefore the rate determining step is the tautomerization of acetone and involves the breaking of a C-H bond. Since the breaking of a C-H bond is involved, a substantial isotope effect is expected.
Answer:
Primary kinetic isotope effects are observed if the isotopic bond itself is being broken or formed during the reaction.
Secondary kinetic isotope effects are observed if an isotopic label is located adjacent to or remote from the bond that is being broken or formed during the reaction
Explanation: