Page 436 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

t-pentyl system, although use of good nucleophiles in polar aprotic solvents permits 417
direct displacement to occur. 44
SECTION 4.2
Structural and Solvation
NaCN Effects on Reactivity
(CH ) CCH OTs (CH ) CCH CN
3 3
2
3 3
2
HMPA
90%
Steric effects of another kind become important in highly branched substrates,
and ionization can be facilitated by relief of steric crowding in going from the tetra-
hedral ground state to the TS for ionization. 45 The relative hydrolysis rates in 80%
aqueous acetone of t-butyl p-nitrobenzoate and 2,3,3-trimethyl-2-butyl p-nitrobenzoate
are 1:4.4.
R
R = t-butyl
CH 3 C OPNB k rel = 4.4
R = methyl
CH 3

This effect has been called B-strain (back strain), and in this example only a modest
rate enhancement is observed. As the size of the groups is increased, the effect on rate
becomes larger. When all three of the groups in the above example are t-butyl, the
solvolysis occurs 13,500 times faster than in t-butyl p-nitrobenzoate. 46

4.2.5. Effects of Conjugation on Reactivity

In addition to steric effects, there are other important substituent effects that
influence both the rate and mechanism of nucleophilic substitution reactions. As we
discussed on p. 302, the benzylic and allylic cations are stabilized by electron delocal-
ization. It is therefore easy to understand why substitution reactions of the ionization
type proceed more rapidly in these systems than in alkyl systems. Direct displacement
reactions also take place particularly rapidly in benzylic and allylic systems; for
example, allyl chloride is 33 times more reactive than ethyl chloride toward iodide
ion in acetone. 47 These enhanced rates reflect stabilization of the S 2 TS through
N
overlap of the p-type orbital that develops at carbon. 48 The systems of the allylic
and benzylic groups provide extended conjugation. This conjugation can stabilize the
TS, whether the substitution site has carbocation character and is electron poor or is
electron rich as a result of a concerted S 2 mechanism.
N

2 2 2
interaction of sp interaction of empty sp interaction of empty sp
hybridized substitution orbital with π HOMO orbital of benzyl cation with
center with π LUMO HOMO aromatic π system
44 B. Stephenson, G. Solladie, and H. S. Mosher, J. Am. Chem. Soc., 94, 4184 (1972).
45
H. C. Brown, Science, 103, 385 (1946); E. N. Peters and H. C. Brown, J. Am. Chem. Soc., 97, 2892
(1975).
46
P. D. Bartlett and T. T. Tidwell, J. Am. Chem. Soc., 90, 4421 (1968).
47 J. B. Conant and R. E. Hussey, J. Am. Chem. Soc., 47, 476 (1925).
48
A. Streitwieser, Jr., Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962, p. 13; F.Carrion
and M. J. S. Dewar, J. Am. Chem. Soc., 106, 3531 (1984).