Organic Chemistry

Section 11.7 A Kinetic Isotope Effect 421

11.7 A Kinetic Isotope Effect

A mechanismis a model that accounts for all the experimental evidence that is known

about a reaction. For example, the mechanisms of the E1, and E2 reactions

are based on a knowledge of the rate law of the reaction, the relative reactivities of the

reactants, and the structures of the products.

Another piece of experimental evidence that is helpful in determining the mecha-

nism of a reaction is the deuterium kinetic isotope effect—the ratio of the rate con-

stant observed for a compound containing hydrogen to the rate constant observed for

an identical compound in which one or more of the hydrogens has been replaced by

deuterium, an isotope of hydrogen. Recall that the nucleus of a deuterium has one pro-

ton and one neutron, whereas the nucleus of a hydrogen contains only a proton

(Section 1.1).

The chemical properties of deuterium and hydrogen are similar; however, a

bond is about stronger than a bond. Therefore, it is

more difficult to break a bond than a corresponding bond.

When the rate constant for elimination of HBr from 1-bromo-2-phenylethane

is compared with the rate constant for elimination of DBr from 2-bromo-

1,1-dideuterio-1-phenylethane (determined under identical conditions), is found to

be 7.1 times greater than The deuterium kinetic isotope effect, therefore, is 7.1.

The difference in the reaction rates is due to the difference in energy required to break

a bond compared with a bond.

Because the deuterium kinetic isotope effect is greater than unity for this reaction, we

know that the (or ) bond must be broken in the rate-determining step—a

fact that is consistent with the mechanism proposed for an E2 reaction.

PROBLEM 15

List the following compounds in order of decreasing reactivity in an E2 reaction:

PROBLEM 16

If the two reactions described in this section were E1 elimination reactions, what value

would you expect to obtain for the deuterium kinetic isotope effect?

CH 3 C

CH 3

CH 3

Cl CH 3 C

CH 3

CH 3

F CH 3 C

CH 3

CH 3

CD 3 C Br

CD 3

CD 3

F CH 3 C

CD 3

CD 3

F

C¬H C¬D

C¬H C¬D

kD.

kH

(kD)

(kH)

C¬D C¬H

1.2 kcal>mol(5 kJ>mol) C¬H

C¬D

kH

kD

deuterium kinetic isotope effect = = rate constant for H-containing reactant

rate constant for D-containing reactant

SN1,SN2,

CH 2 CH 2 Br CH 3 CH 2 O−

kH

CH 3 CH 2 OH

1-bromo-2-phenylethane

+ CH CH 2 ++Br− CH 3 CH 2 OH

CD 2 CH 2 Br CH 3 CH 2 O−

kD

CH 3 CH 2 OH

2-bromo-1,1-dideuterio-

1-phenylethane

+ CD CH 2 ++Br− CH 3 CH 2 OD

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