Solvolysis of 2-aryl-2-chloro-4,4-dimethylpentanes. Confirmation of validity of Brown-Okamoto sigma(+) constants

Specific rate constants of solvolysis in 90% aqueous acetone have been dete rmined at 25 degrees C for 10 2-aryl-2-chloro-4,4-dimethylpentanes (3) havi ng a substituent p-cycPr, p-Me, m-Me, p-F, (H), p-Cl, m-Cl, m-CF3, p-CF3, o r p-NO2. The Hammett-Brown relation is satisfactorily Linear (r = 0.999) wi th pi of -4.51, showing that Brown-Okamoto sigma(+) values based on the sol volysis of the cumyl chloride system 1 are widely usable for the substrates solvolyzing without appreciable nucleophilic solvent participation. Solvol ysis of 3 having 3,5-Cl-2 or 3,5-(CF3)(2) substituents suggests the use of sigma(+) values of +0.740 and +0.982 instead of respective 2 sigma(m)(+) va lues of +0.798 and +1.040. The large negative rho(+) values in TFE of -5.91 and -5.63 for 1 and 3, respectively, reflect decreased nucleophilic (or Br phi nsted base type) solvation. Comparison of solvolysis rates of 1 with 3 , both having m-Cl, p-CFB, or 3,5-(CF3)(2) substituents, in aqueous ethanol , aqueous acetone, TFE, and TFE-EtOH suggests that nucleophilic interventio n by solvent in the cumyl system increases as the intermediate carbocation becomes more electron-demanding.