Substituent Effects on the C-C Bond Strength, 15. Geminal Substituent Effects, 7. Thermochemistry and Thermal Decomposition of Alkyl-substituted Tricyanomethyl Compounds

Rakus, Klaus,Verevkin, Sergej P.,Beckhaus, Hans-Dieter,Ruechardt, Christoph

, p. 2225 - 2234 (2007/10/02)

The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile (10a) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile (10b) decomposed heterolytically into carbenium ions and (CN)3C(-) anions, while 9-methyl-9-(tricyanomethyl)fluorene (10c) underwent about 11percent homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals.The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics.From the temperature effect on the rate constants the activation parameters were determined -1, ΔS(excit.) (10c) = 58 +/- 5 J mol-1 K-1>.Standard enthalpies of formation ΔHf0 (g) were determined for 2,2-dicyanopropionitrile (2) (422.45 kJ mol-1), 2,2-dicyanohexanenitrile (3) (349.74 kJ mol-1), 2,2-dicyano-3-phenylpropionitrile (4) (540.75 kJ mol-1), 2-butyl-2-methylhexanenitrile (5) (-133.20 kJ mol-1), 2,2-dimethylpentanenitrile (6) (-45.78 kJ mol-1), and 2-methylbutyronitrile (7) (2.44 kJ mol-1) from the enthalpies of combustion and enthalpies of sublimation/vaporization.From these data and known ΔHf0 (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔHf0 (g) were derived for alkyl groups with one, two, or three cyano groups attached.The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔHf0 (g) of 10c and ΔH(excit.) of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1 RSE (1) = -18 kJ mol-1 was determined.Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanoalkyl (23 kJ mol-1) and α,α-dicyanoalkyl (27 kJ mol-1) radicals, which were recalculated from bond homolysis measurements and the new thermochemical data.This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion.The amount of the dipolar energies was estimated by molecular mechanics (MM2). - Key Words: Enthalpies of formation / Geminal substituents, energetic interaction of / Bond cleavage, C-C, kinetics of / Radicals, stability of / Increments, thermochemical, of cyanides / Bond strength, substituent effects on