Kinetic Isotopic Effect

kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes

1. KINETIC ISOTOPIC EFFECT

2. A n e l e m e n t is i d en t ifi e d b y i t s s y m bo l , ma s s nu mbe r , and atomic number. T h e a t o mic nu mber is t h e nu mber o f p r o t o n s in t h e nucleus while the mass number is the total number of protons and neutrons in the nucleus.

3. Isotopes are two atoms of the same element that have the same number of protons but different numbers of neutrons. Isotopes are specified by the mass number.

4. Why kinetic Isotopic effect? Traditional kinetics studies do not provide information as to what bonds are broken/formed and changes in hybridization that occur during the rate limiting step of a reaction. Isotope effects can provide this information Substituting one iosotope for another at or near an atom at which bonds are breaking or re-hybridizing typically leads to a change in the rate of the reaction.

5. The kinetic isotope effect (KIE) is a phenomenon associated with isotopically substituted molecules exhibiting different reaction rates. KINETIC ISOTOPIC EFFECT

6. Differences in the properties which arise from the difference in mass are called as isotope effect. Rates of reactions are measurable different for the process in which E-H & E-D bonds are broken, made or rearranged (E – another element). The detection of this kinetic isotope effect help to support a proposed reaction mechanism of many chemical reactions.

7. The isotope effect is expressed as a ratio of rate constants: The rate constant for the reaction with the natural abundance isotope over the rate constant for the reaction with the altered isotope. For H/D substitutions: kH/kD

8. Primary kinetic isotopic effect 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.

9. Origin of primary kinetic isotopic effect The origins of isotope effects is the difference in the frequencies of various Vibrational modes of a molecule, arising when one isotope is substituted for another. Different type of bond in a molecule have different frequency.

10. Vibrational frequancy of a molecule is calculated by using the formula: Where Mr = Reduced mass The VF is directly proportional to FORCE constant ‘k’ and inversely proportional to the mass of the atom.

11. As deuterium is heavier than hydrogen the VF of C-H bond will be more as compared to C-D bond. Energy for bond breaking is directly proportional to VF Thus energy for C-H bond will be less as compared to C-D bond.

12. Secondary Kinetic Isotope Effects When the IE is attributed to a REHYBRIDIZATION or when IE arises from substitution remote/away from the bond undergoinig reaction it is reffered to as Secondary IE. α or β secondary isotope effects: based on whether the isotope is on a position α or β to the bond that is changing.

13. In Secondary IE we consider change in Hybridization of Carbon atom. As the hybridization state of carbon changes the VF of C- H and C-D bond will change When C-H bond involving an sp3 hybridized carbon is changing to a bond involving an sp2 hybridized carbon the vibrational modes changes. C-H bond strengths decrease in the order sp > sp2 > sp3 Hybridization Changes