Structural effects on the Grunwald-Winstein correlations in the solvolysis of some simple tertiary alkyl chlorides

The rates of solvolysis in various solvents at 25 degrees C were determined for five tertiary alkyl chlorides: 2-chloro-2,4,4-trimethylpentane (4), 2-chloro-2,4-dimethylpentane, 2-chloro-2-methylpentane, 1-chloro-1,3,3-trimethylcyclopentane (7) and 1-chloro-1-methylcyclopentane. The rate data were analysed on the basis of the original and extended Grunwald-Winstein-type equation [log(k/k(0))=mY(Cl)+c and log(k/k(0))=lN(T)+mY(Cl+)c] and the results were compared with those reported for 2-chloro-2-methylpropane (1) and 2-chloro-2,3,3-trimethylbutane (3). The rate data for 4 in 18 solvents give an excellent correlation with l=0.00+/-0.02 and m=0.74+/-0.01. The neopentyl group in 4 more effectively shields the rear-side of the reaction center than the tert-butyl group in 3 that is correlated by l=0.10+/-0.04 and m=0.81+/-0.04. The rate ratio between 4 and 1 at 25 degrees C is 275 in TFE and predicted to increase to 950 in TFA. The previous 4/1 rate ratio of 21 in 80% ethanol evidently underestimates the B-strain effect on the solvolysis rate of 4 by a factor of at least 40, The remote methyl groups in 7 works to increase rear-side shielding without increasing B-strain, The marked difference in the effect of the remote methyl groups between 4 and 7 suggests that the leaving chloride ion in 4 takes a locus that is nearly antiperiplanar to the tert-butyl group. (C) 1997 John Wiley & Sons, Ltd.