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The rates of solvolysis in various solvents at 25°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°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.

The methodology of changing ring flexibility to detect the π-conjugative stabilization of bridgehead carbocations has been applied to eight 2-oxo (X = O) bridgehead carbocations. On the basis of the solvolytic behavior observed in kinetics and product analyses, the eight 2-oxo bridgehead substrates were classified into three categories:  three substrates solvolyzing without ion-pair return that leads to primary isomers (class A), three substrates that form primary isomers by ion-pair return during solvolysis (class B), and two substrates that undergo solvent addition to the carbonyl group to form hemiacetals during solvolysis (class C). It was concluded that the substrates of class C could not be used for the present purpose. Essentially constant ethanolysis rate ratios, k(X = O)/k(X = H2), of 10-8.2−10-8.7 at 25 °C were obtained between four 2-oxo substrates in classes A and B and the corresponding parent unsubstituted ones. The result was interpreted to suggest that the π-conjugative stabilization of tertiary α-carbonyl carbocations is negligibly small, if present. Slightly more negative k(X = O)/k(X = H2) values of 10-9.7 and 10-9.2 for highly flexible bicyclo[4.2.2]dec-1-yl and bicyclo[4.3.1]dec-1-yl systems, respectively, were attributed to complex conformations in the ground and incipient carbocations. PM3 calculations on some 2-methylene and 2-oxo bridgehead carbocations supported the experimental results. Comparison of the solvolysis rates of 1,1,3,3-tetramethyl-2-oxobutyl mesylate with those of 1,1,3,3-tetramethylbutyl mesylate estimated from the rates of the corresponding chloride also failed to support the π-conjugative stabilization of α-carbonyl carbocations.

The solvolysis rate ratios of 2-methylene bicyclic bridgehead compounds relative to the parent compounds increase with flexibility of the ring system, whereas the corresponding rate ratios related to 2-oxo homologues are essentially constant, suggesting the unimportance of a-conjugative stabilization in tertiary a-keto cations.In recent years, a number of solvolytic rate data have been interpreted to support the notion that the a-keto and a-cyano carbocations (1 and 2, respectively) are stabilized by *-conjugation to an extent which partly offsets the destabilizing inductive effect of the substituent.'