Thermochemical Study of the Addition of Carbenium Ions to Alkenes

Article abstract of DOI:10.1021/ja00210a042

Scales of carbenium ion stabilities have been obtained from 1H NMR equilibrium studies of mixtures of partly ionized para-substituted diarylmethyl chlorides in CD2Cl2/BCl3 at -70 deg C and via calorimetric determination of the heats of ionization (ΔH_i) of the alkyl chlorides (Ar2CHCl + BCl3 -> Ar2CH⁺BCl4^-) in CH2Cl2/BCl3 at -70 deg C.The heats of the reactions of diarylmethyl tetrachloroborates with 2-methyl-1-pentene (Ar2CH⁺BCl4^- + CH2=C(CH3)C3H7 -> Ar2CHCH2C(CH3)ClC3H7 + BCl3), which were determined by low-temperature calorimetry, increase from -53.1 kJ/mol for (H3CC6H4)2CH⁺BCl4^- to -33.0 kJ/mol for the better stabilized (H3COC6H4)(H3CC6H4)CH⁺BCl4^-.In contrast, the heats (ΔH_a) of the Lewis acid catalyzed additions of the corresponding para-substituted diarylmethyl chlorides to 2-methyl-1-pentene are independent of the para substituents (ΔH_a=-86.5 +/-2.7 kJ/mol).Similar values of ΔH^a were obtained for the addition of p-anisylphenylmethyl chloride to trimethylethylene, styrene, β-methylstyrene, and isoprene. ΔH_a is predominantly determined by the conversion of a ?(C=C) into a ?(C-C) bond, and the heats of addition of diarylmethyl tetrachloroborates to alkenes in CH2Cl2 are given by the equation ΔH_b=(9.5 +/- 6.3 pK_R+) kJ/mol.