Isotope Effect Quiz

1
Isotope Effect Quiz
Consider the following reactions and
associated rate constants measured at 298.2 K (J.
Phys. Chem., A 101, 3125, 1997) CH4 (g) OH
(g) --gt CH3 (g) H2O (g) k1 3.8 106 L
mol-1 s-1 CD4 (g) OH (g) --gt CD3 (g) DOH
(g) k2 5.2 105 L mol-1 s-1 CH4 (g) OD
(g) --gt CH3 (g) DOH (g) k3 4.2 106 L
mol-1 s-1 a. Label the 2nd and 3rd reactions
as either exhibiting a primary or secondary
isotope effect, when compared to the 1st
reaction Since an isotopically substituted C-D
bond in CD4 is broken in the second reaction,
this reaction is exhibiting a primary isotope
effect. Since the isotopically substituted O-D
bond in OD is not broken in the third reaction,
this reaction is exhibiting a secondary isotope
effect.
2

• b. Why is the 2nd reaction in the list an order
  of magnitude slower than either the 1st or 3rd
  reaction?
• The effect of isotopic substitution on the
  activation energy, Ea, will be much greater where
  a primary isotopic substitution has occurred. In
  the case of a primary isotope effect substitution
  of the heavier deuterium atom will increase Ea
  and decrease the rate of the reaction.
• In comparing the 1st and 3rd reactions are the
  reactants or the activated complex more affected
  by the isotopic substitution? Explain your
  answer.
• Since the 3rd reaction is slightly faster than
  the 1st reaction, isotopic substitution of
  deuterium must have lowered the energy of
  activated complex more than that of reactants
  resulting in a slight lowering of the activation
  energy, as compared to the 1st reaction, and a
  consequent slight increase in the reaction rate.