Page 440 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Table 4.11. Solvolysis Rates of -Chloro 421
Alcohols a
SECTION 4.3
-Chloro alcohol Approximate relative rate
Neighboring-Group
Cl CH 2 2 OH 2000 Participation
Cl CH 2 3 OH 1
Cl CH 2 4 OH 5700
Cl CH 2 5 OH 20

a. B. Capon, Q. Rev. Chem. Soc., 18, 45 (1964);
W. H. Richardson, C. M. Golino, R. H. Wachs, and
M. B. Yelvington, J. Org. Chem., 36, 943 (1971).

Like the un-ionized hydroxyl group, an alkoxy group is a weak nucleophile, but
it can function as a neighboring nucleophile. For example, solvolysis of the isomeric
p-bromobenzenesulfonate esters 1 and 2 leads to identical product mixtures, indicating
the involvement of a common intermediate. This can occur by formation of a cyclic
oxonium ion by intramolecular participation. 58

CH 3 OCH 2 CH 2 CH 2 CHCH 3
ArSO 3 CH 2 CH 2 CH 2 CHCH 3
O 3 SAr
1 2 OCH 3

O +
CH 3
ROH CH 3 ROH

ROCH 2 CH 2 CH 2 CHCH 3 CH 3 OCH 2 CH 2 CH 2 CHCH 3
OR
OCH 3
The occurrence of nucleophilic participation is also indicated by a rate enhancement.
The maximum rate enhancement is observed when participation of a methoxy group
occurs via a five-membered ring (see Table 4.12).
Table 4.13 provides data on two series of intramolecular nucleophilic substitution.
One data set pertains to cyclization of -bromoalkylmalonate anions and the other

Table 4.12. Relative Solvolysis Rates of Some
-Methoxyalkyl p-Bromobenzenesulfonates
in Acetic Acid a
CH 3 CH 2 2 OSO 2 Ar 1.00
CH 3 O CH 2 2 OSO 2 Ar 0.28
CH 3 O CH 2 3 OSO 2 Ar 0.67
CH 3 O CH 2 4 OSO 2 Ar 657
CH 3 O CH 2 5 OSO 2 Ar 123
CH 3 O CH 2 6 OSO 2 Ar 1.16

a. S. Winstein, E. Allred, R. Heck, and R. Glick, Tetrahedron,
3, 1 (1958).
58
E. L. Allred and S. Winstein, J. Am. Chem. Soc., 89, 3991 (1967).