1-Phenylethyl carbocation

Tertiary aliphatic carbocations 85 Ring-substituted 1-phenylethyl carbocations 86 Cumyl and di-orf/io-methylcumyl carbocations 91 a,a-Diphenyl carbocations 95 Oxocarbenium ions 95... 

The rate and equilibrium constants for the reactions of ring-substituted 1-phenylethyl carbocations (X-[6+]) in 50/50 (v/v) trifluoroethanol/water (Table 2 and Scheme 8),13 14 17 43, and for interconversion of ring-substituted 1-phenyl-... 

Fig. 5 Logarithmic plots of rate-equilibrium data for the formation and reaction of ring-substituted 1-phenylethyl carbocations X-[6+] in 50/50 (v/v) trifluoroethanol/water at 25°C (data from Table 2). Correlation of first-order rate constants hoh for the addition of water to X-[6+] (Y) and second-order rate constants ( h)so1v for the microscopic reverse specific-acid-catalyzed cleavage of X-[6]-OH to form X-[6+] ( ) with the equilibrium constants KR for nucleophilic addition of water to X-[6+]. Correlation of first-order rate constants kp for deprotonation of X-[6+] ( ) and second-order rate constants ( hW for the microscopic reverse protonation of X-[7] by hydronium ion ( ) with the equilibrium constants Xaik for deprotonation of X-[6+]. The points at which equal rate constants are observed for reaction in the forward and reverse directions (log ATeq = 0) are indicated by arrows.

Table 5. Rate and equilibrium constants for the formation and reaction of cyclic benzylic carbocations [18 + ] and [20+ ] and analogous ring-substituted 1-phenylethyl carbocations (Scheme 15)°... 

The partitioning of simple tertiary carbocations, ring-substituted 1-phenylethyl carbocations, and cumyl carbocations between deprotonation and nucleophilic addition of solvent strongly favors formation of the solvent adduct. The more favorable partitioning of these carbocations to form the solvent adduct is due, in part, to the greater thermodynamic stability of the solvent... 

The intrinsic barrier for the addition of solvent to an a-alkoxy benzyl carbocation is several kcal mol-1 smaller than that for the corresponding reaction of ring-substituted 1-phenylethyl carbocations. This result is consistent with the conclusion that these nucleophile addition reactions become intrinsically easier as stabilizing resonance electron donation from an a-phenyl group to the cationic center is replaced by electron donation from an a-alkoxy group. 

In retrospect, it should have been clear to me - as I am sure it was to Bill Jencks -that the rate and equilibrium constants for addition of solvent to 1-phenylethyl carbocation intermediates of solvolysis of 1-phenylethyl derivatives would serve as the first step in the characterization of the dynamics of the reactions of their ion pair intermediates. Therefore, this earlier work has served as a point of departure for our experiments to determine relative and absolute barriers to the reactions of ion pair intermediates of solvolysis. 

The carbocation intermediate must exist in water long enough to allow for racemization of the carbocation-anion or ion dipole pair to occur, so that k < 1 X 10 s for addition of solvent (Scheme 15). In the case of solvolysis of (R)-l-phenylethanol in H2 0, which proceeds by a stepwise mechanism through a 1-phenylethyl carbocation intermediate that is captured by water... 

The benzylic carbocation formed from the reaction of styrene oxide with H +, like the 1-phenylethyl carbocation and simple tertiary carbocations, most likely reacts with water molecules from within the inner solvent shell that are present when the carbocation is formed.30 The lifetime of the carbocation formed in the acid-catalyzed hydrolysis of styrene oxide must be similar to the time required for solvent relaxation and rotation about the Ca-Cy3 bond. 

Figure 2. Estimated rate constants for reactions of nucleophiles with substituted 1-phenylethyl carbocations, plotted against the effective Hammett constant of the ring substituent with r+ = 2.1 (+) trifluoroethanol (A) methanol acetate anion ( ) trifluoroethoxide anion propanethiol (o) azide. (Reproduced from reference 17.

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