Page 401 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

382 Several mechanisms, all of which involve a hydride transfer as a key step, have
been postulated. On the basis of the following information, formulate one or more
CHAPTER 3 mechanisms that would be consistent with all the data provided. Indicate the
Structural Effects on significance of each observation with respect to the mechanism(s) you postulate.
Stability and Reactivity
1. When the reaction is carried out in D O, the benzyl alcohol contains no
2
deuterium in the methylene group.
2. When the reaction is carried out in H 2 18 O, both the benzyl alcohol and sodium
benzoate contain 18 O.
3. The overall reaction rate is given by the expression
2 −
Rate = k PhCH=O OH
obs
4. The rates of substituted benzaldehydes are correlated by a Hammet LFER
with =+3 76.
5. The solvent isotope effect k /k = 1 90
D2O H2O
3.12. A mechanism for alkene arylation catalyzed by Pd(II) is outlined below. The
isotope effect k /k D was found to be 5 when benzene-d was used. There
H
6
was no isotope effect when styrene- -d was used. Which steps in the reaction
2
mechanism could be rate determining, given this information on isotope effects?
(1) + Pd(O CCH ) Ph PdO CCH 3 + HO CCH 3
2
3 2
2
2
Ph
PdO CCH 3
2
(2) PhCH CH 2 + Ph PdO CCH 3 PhCH CH 2
2
Ph
(3)
PdO CCH 3 PhCH CHPh + Pd(II) + CH 3 CO 2 H
2
PhCH CH 2

3.13. Comparison of the gas phase acidity of substituted benzoic acids with pK values
a
in aqueous solutions reveals some interesting comparisons.
1. The trend in acidity as a function of substituent is the same for both gas phase and
aqueous solution, but the substituent effects are much stronger in the gas phase.
(The G for any give substituent is about 10 times larger in the gas phase.)
2. Whereas acetic and benzoic acid are of comparable acidity in water, benzoic
acid is considerably more acidic in the gas phase. (pK values are 4.75 and
a
4.19, respectively; and G of ionization is 8.6 kcal/mol more positive for
acetic acid.)
3. While the substituent effect in the gas phase is nearly entirely an enthalpy
effect, it is found that in solution, the substituent effect is largely due to
changes in S.

Discuss how difference between the gas phase and solution can cause these effects.
13
3.14. It has been found that the C chemical shift of aromatic ring carbons are a good
indicator of the intrinsic electron-releasing or electron-withdrawing capacity of
substituents, without any perturbation from approaching reagents. Such pertur-
bation is always present when substituent effects are measured on the basis of