Page 122 - Mechanism and Theory in Organic Chemistry

Problems 111

Substitution of deuterium at the /3 position leads to the /3 deuterium isotope
effect in reactions like 2.79.

Here the C--H bond is weakened and the frequencies lowered by delocalization
of electron density toward the positively charged center (hyperconjugation; see
Section 10.2 for further discussion). A bending mode is probably again the most
important k,/k, is greater than 1, values ranging up to about 1.4 for
favorably situated hydrogens,59 but more typically on the order of 1.1.

Solvent Isotope Effects
Isotope effects are frequently observed when reactions are carried out in solvents
with 0-H (0-D) groups. The reader is referred to the literature for further
inf~rmation.~~

PROBLEMS

1. Cyanohydrin formation, shown below, may involve rate-determining attack of
either H+ or -CN. From the p value for the formation of cyanohydrins from substituted
benzaldehydes (Table 2.3), which step do you think is rate-determining?

0 OH
II I
RC-H + HCN + RC-H
I

2. Derive a rate equation for formation of C in the following mechanism, assuming
the stationary state for B:

3. Derive a rate equation for formation of C in the following mechanism, assuming
the stationary state for B and constant concentrations of D and E:

4. Derive the rate equation for rate of formation of E in terms of concentrations of
reactants A and B in the following mechanism, assuming that the rates of steps k, and

See note 50, p. 106.
V. J. Shiner, Jr. and J. G. Jewett, J. Amer. Chem. Soc., 86, 945 (1964).
80 (a) R. L. Schowen, Prog. Phys. Org. Chem., 9, 275 (1972); see also (b) P. M. Laughton and R. E.
Robertson, in Solute-Solvent Interactions, J. F. Coetzee and C. D. Ritchie, Eds., Marcel Dekker, New
York, 1969, p. 399.