Substituent Effects on the C‐C Bond Strength, 15^[1] Geminal Substituent Effects, 7^[2]. Thermochemistry and Thermal Decomposition of Alkyl‐substituted Tricyanomethyl Compounds

Abstract

Abstract 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) 3 C − anions, while 9‐methyl‐9‐(tricyanomethyl)fluorene ( 10c ) underwent about 11% 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 [Δ H ( 10c ) = 155· 2 kJ mol −1 , Δ S ( 10c ) = 58· 5 J mol −1 K −1 ]. Standard enthalpies of formation Δ H (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‐methylhexanentrile ( 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 Δ (g) values for alkanenitriles and ‐dinitriles, thermochemical increments for Δ H (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 Δ H (g) of 10c and Δ H 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 RSE s of tertiary α‐cyanalkyl (23 kJ mol −1 ) and α,α‐dicyanoalkyl (27 kJ mol −1 ) radicals, which were recalculated from bond homolysis measurements [4] 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).