Nucleophilic Solvent Participation in the Solvolysis of Tertiary Bromoalkanes

Abstract

The solvolysis of 2-bromo-2-methylpropane (1B), 2-bromo-2-methylbutane (2B), 2-bromo-2,3-dimethylbutane (3B), 2-bromo-2,3,3-trimethylbutane (4B), 3-bromo-3-methylpentane (5B), 3-bromo-2,3-dimethylpentane (6B), 3-bromo-2,2,3-trimethylpentane (7B), 3-bromo-3-ethylpentane (8B), 3-bromo-3-ethyl-2-methylpentane (9B) and 2-bromo-2,4,4-trimethylpentane (11B) in 15 to 21 solvents was studied, and correlation analyses by using the single-and dual-parameter Grunwald-Winstein equations (Eqns 1 and 2) were examined. Substrates 7B, 9B and 11B showed excellent linear relationship (R ³ 0.997) in the logk -Y Br plots and indicated limiting S N 1 mechanism for the solvolysis. On the other hand, bromides 1B-6B and 8B gave linear correlations (R = 0.987-0.996) with the dual-parameter (Y Br and N OTs ) equation (2) only, which indicated the presence of significant nucleophilic solvent participation. Normal trends of reactivity due to the relief of B-strain could be found in the poorly nucleophilic trifluoroethanol. Similar to the corresponding chlorides, the overwhelming influence of nucleophilic solvent assistance results in the observed inverse order of reactivity: k(2B) > k(3B), k(5B) > k(6B) and k(8B) > k(9B).

Use of the Simple and Extended Grunwald-Winstein Equations in the Correlation of the Rates of Solvolysis of Highly Hindered Tertiary Alkyl Derivatives

“…Except for compound 12B , data from a paper by Martins and coworkers [38], the specific rate values are from a publication by Liu and coworkers [42]. They carried out the study using a series of bromides with gradually increasing steric hindrance to nucleophilic attack at the α-carbon.…”

“…As regards the solvent nucleophilicity scale, Takeuchi and coworkers [40] have used the N T scale but Liu and coworkers [33, 42] and Martins and coworkers [38] have both used the N OTs values. The need to arrive at an approximation for the dependence on solvent ionizing power, so as to subtract the mY OTs value from log ( k / k o ) in arriving at N OTs values based on methyl p-toluenesulfonate has been extensively discussed [2, 19] and it has been demonstrated that the 0.3 value employed should have been about 0.55, supported by the observation that revised N’ OTs values are in excellent agreement with N T values [2, 19].…”

“…Since the N T scale is used in this review, we cannot directly compare with the previous two-term analyses using the N OTs scale. However, the two scales do have similar values for most solvents [19, 38, 42] and approximate comparisons can be made.…”

“…42. b Number of solvents. c From equation 1, using Y Br values and with associated standard errors. d Correlation coefficient. e F -test value. f From Table 1 of ref. 42.…”