Journal of the American Chemical Society p. 567 - 571 (1988)
Update date:2022-07-29
Topics:
Schade, Christian
Mayr, Herbert
Arnett, Edward M.
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 (ΔHi) 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 (ΔHa) of the Lewis acid catalyzed additions of the corresponding para-substituted diarylmethyl chlorides to 2-methyl-1-pentene are independent of the para substituents (ΔHa=-86.5 +/-2.7 kJ/mol).Similar values of ΔHa were obtained for the addition of p-anisylphenylmethyl chloride to trimethylethylene, styrene, β-methylstyrene, and isoprene. ΔHa 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 ΔHb=(9.5 +/- 6.3 pKR+) kJ/mol.
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