M. Born et al., HEATS OF FORMATION OF MONO-HALOGEN-SUBSTITUTED CARBENES - STABILITY AND REACTIVITY OF CHX(CENTER-DOT-) (X=F, CL, PR, AND I) RADICAL-ANIONS, Journal of the American Chemical Society, 116(16), 1994, pp. 7210-7217
The heats of formation of mono-halogen-substituted singlet carbenes ha
ve been determined to be 157 +/- 18 kJ mol(-1) ((X) over tilde (1)A' C
HF), 317 +/- 20 kJ mol-1 ((X) over tilde (1)A' CHCl), 373 +/- 18 kJ mo
l-1 ((X) over tilde (1)A' CHBr), and 428 +/- 21 kJ mol(-1) ((X) over t
ilde (1)A' or (a) over tilde (1)A' CHI). These Delta H degrees(f) valu
es are based upon the following gas-phase acidities, Delta H degrees(a
cid), of the (.)CH(2)X radicals: 1668 +/- kJ mol(-1) ((CH2F)-C-.), 161
0 +/- kJ mol(-1) ((CH2Cl)-C-.), 1593 +/- 8 kJ mol(-1) ((CH2Br)-C-.), a
nd 1566 +/- 11 kJ mol(-1) ((CH2I)-C-.). The acidities were determined
by observing the occurrence/nonoccurrence of proton transfer in the re
actions of the CHX(.-) carbene radical anions with selected acids in a
Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer.
The Delta H degrees(acid) values reveal that (i) the acidity increase
s in the series (CH2F)-C-., (CH2Cl)-C-., (CH2Br)-C-., and (CH2I)-C-.,
i.e. the acidity increases as the halogen atom becomes heavier as obse
rved also for the corresponding halogen-substituted methanes, and (ii)
a given (.)CH(2)X radical is approximate to 50 kJ mol(-1) more acidic
in the gas phase than the related CH(3)X molecule. The reaction of a
given CHX(.-) ion with the parent compound yields X(-) ions in part by
a direct S(N)2 substitution and in part by initial proton transfer le
ading to a [(.)CH(2)X + (-)CH(2)X] complex, which then reacts to form
X(-). In addition to overall proton transfer, the reaction of the car
bene radical anions with aliphatic alcohols yields X(-) ions by a proc
ess which involves initial proton transfer followed by nucleophilic at
tack of the RO(-) ion on the generated (.)CH(2)X radical.