Heats of Formation of Monohalo Carbenes. Stability and Reactivity of CHX.bul.- (X = F, Cl, Br, and I) Radical Anions

Abstract

The heats of formation of mono-ha_logen-substituted singlet carbenes _have been determined to be 157 f 18 kJ mol-' (w 'A' CHF), 317 f 20 kJ mol-' (X 'A' CHCl), 373 f 18 kJ mol-' (X 'A' CHBr), and 428 f 21 kJ mol-'(2 'A' or 5 'A' CHI).These AHof values are based upon the following gas-phase acidities, W a e d , of the *CH2X radicals: 1668 f 6 kJ mol-' (*CH2F), 1610 f 10 kJ mol-' (*CH2Cl), 1593 f 8 kJ mol-' ('CHzBr), and 1566 f 11 kJ mol-' ('CH21).The acidities were determined by observing the wurrence/nonoccurrence of proton transfer in the reactions of the CHX*-carbene radical anions with selected acids in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer.The M 0 & d values reveal that (i) the acidity increases in the series *CH2F, *CH2Cl, *CH2Br, and *CH2I, i.e. the acidity increases as the halogen atom becomes heavier as observed also for the corresponding halogen-substituted methanes, and (ii) a given *CH2X radical is 4 0 kJ mol-' more acidic in the gas phase than the related CH3X molecule.The reaction of a given CHX*-ion with the parent compound yields X-ions in part by a direct S N ~substitution and in part by initial proton transfer leading to a [*CH2X + -CH2X]* complex, which then reacts to form X-. In addition to overall proton transfer, the reaction of the carbene radical anions with aliphatic alcohols yields X-ions by a process which involves initial proton transfer followed by nucleophilic attack of the ROion on the generated *CH2X radical.