Isomerization of Neopentyl Chloride and Neopentyl Bromide by a 1,2-Interchange of a Halogen Atom and a Methyl Group

Abstract

The recombination of chloromethyl and t-butyl radicals at room temperature was used to generate neopentyl chloride molecules with 89 kcal mol(-1) of internal energy. The observed unimolecular reactions, which give 2-methyl-2-butene and 2-methyl-1-butene plus HCl, as products, are explained by a mechanism that involves the interchange of a methyl group and the chlorine atom to yield 2-chloro-2-methylbutane, which subsequently eliminates hydrogen chloride by the usual four-centered mechanism to give the observed products. The interchange isomerization process is the rate-limiting step. Similar experiments were done with CD(2)Cl and C(CH(3))(3) radicals to measure the kinetic-isotope effect to help corroborate the proposed mechanism. Density functional theory was employed at the B3PW91/6-31G(d',p') level to verify the Cl/CH(3) interchange mechanism and to characterize the interchange transition state. These calculations, which provide vibrational frequencies and moments of inertia of the molecule and transition state, were used to evaluate the statistical unimolecular rate constants. Matching the calculated and experimental rate constants, gave 62 ± 2 kcal mol(-1) as the threshold energy for interchange of the Cl atom and a methyl group. The calculated models also were used to reinterpret the thermal unimolecular reactions of neopentyl chloride and neopentyl bromide. The previously assumed Wagner-Meerwein rearrangement mechanism for these reactions can be replaced by a mechanism that involves the interchange of the halogen atom and a methyl group followed by HCl or HBr elimination from 2-chloro-2-methylbutane and 2-bromo-2-methylbutane. Electronic structure calculations also were done to find threshold energies for several related molecules, including 2-chloro-3,3-dimethylbutane, 1-chloro-2-methyl-2-phenylpropane, and 1-chloro-2-methyl-2-vinylpropane, to demonstrate the generality of the interchange reaction involving a methyl, or other hydrocarbon groups, and a chlorine atom. The interchange of a halogen atom and a methyl group located on adjacent carbon atoms can be viewed as an extension of the halogen atom interchange mechanisms that is common in 1,2-dihaloalkanes.

Computational study of the threshold energy for the 1,2-interchange of X and R (X, R = halogens, pseudohalogens, and monovalent hydrocarbon groups) on CH2XCH2R

“…[1][2][3][4][5][7][8][9][10][11] Scanning Tables 1 and 2 suggests that the interchange of Cl or Br might be observed with all of the groups in Table 1 results confirm E 0 (Cl-CH 3 ) = 259 kJ/mol for the CH 3 -Cl interchange reaction for 2,2-dimethyl-1chloropropane; i.e., two methyl groups replace two hydrogen atoms on C-2 of 1-chloropropane. 20 In addition, reactions with E 0 (interchange) > 290 kJ/mol might be observed for photochemical and thermal activation methods that prepare reactants with energy greater than 400 kJ/mol.…”

“…It was also shown that a Br-CH 3 interchange mechanism was more likely than the Wagner-Meerwein mechanism for (CH 3 ) 3 CCH 2 Br. 20 Finally, the interchange of Cl with CH 3 was suggested to explain the decomposition of 1,1-dichloroacetone. 21 The experimental observation of the halogen-halogen and halogen-methyl interchange reactions leads to the question of whether or not pseudohalogens 22…”

Studies of 1,2-Dihalo Shifts in Carbon–Carbon, Carbon–Silicon, and Silicon–Silicon Systems: A Computational Study

“…Similar transition-state structures and energetics have been found for other systems by Holmes and co-workers as well as others; these include not only doubly bridged species with halogen, 4,6,7,[9][10][11][12][13]15 but ones with chlorine and methyl. 5,8 Turning to examples containing silicon, there also are cases of dihalogen exchange across a carbon−silicon single bond. 2,16 These typically are first-order reactions, but the nature of their "concertedness" often is not addressed.…”

“…Holmes and co-workers have studied a large variety of 1,2-rearrangement reactions occurring across a carbon–carbon single bond. − Such studies emphasize understanding the dynamics and product distributions, particularly in systems interchanging halogen atoms through doubly bridged transition states. Unimolecular processes in these systems lead to rearranged as well as elimination products.…”

“…Unimolecular processes in these systems lead to rearranged as well as elimination products. The focus and importance of such studies have been in elucidating the complex nature of the processes, facilitated by their amenability to study using a variety of experimental and computational methods, − as well as their relationship to chlorofluorocarbons (CFCs) . A 2011 study by Enstice, Duncan, Setser, and Holmes examined fluorine–chlorine exchange that had been initiated by the combination of CF 2 Cl and CH 2 F radicals to produce chemically activated [CF 2 ClCH 2 F]* (∼93 kcal/mol of vibrational energy) .…”