HOMOLYTIC AND HETEROLYTIC X-C BOND-ENERGIES .2. HETEROLYTIC BOND-ENERGIES IN UNSATURATED ORGANIC CHAIN COMPOUNDS

The heterolytic bond dissociation energies (BDEs), DH degrees(X(-)-R()), of X-C bonds at the P-position, relative to the pi bond in unsatur ated chain compounds, can be estimated by the equation DH degrees(X(-) -R(+)) DH degrees(X(-)-C+(CH3)(m)H-3-m) - 1.4 gamma(+) + Delta V-nb E(s)(+) kcal/mol, where R(+) represents the unsaturated chain cation ( allylic, propargylic, cyanated, or others) and X is a wide variety of univalent atoms or substituents. The first three terms on the right si de of the above expression are the same as those used previously for s aturated compounds; tn is the degree of methyl substitution with m 1, 2, or 3 for primary, secondary, or tertiary cations; DH degrees(X(-)-C +(CH3)(m)H-3-m) are the heterolytic BDEs of model compounds; gamma(+) is the total number of framework atoms at and beyond the gamma-positio n, relative to the formal charge center; Delta V-nb is the steric comp ression relief due to the bond fission. The last term, E(s)(+), is a n ew parameter, the interaction energy between the empty orbital (at the formal charge site) and the pi orbital in the unsaturated cations. E( s)(+) has the value -13.4, 3.5, and 33.4 kcal mol(-1) for allylic, pro pargylic, and cyanated cations, respectively. The negative and positiv e E,+ values respectively describe the stabilization and destabilizati on of the given unsaturated cation, relative to its saturated analogue . Agreement between experimental and estimated values is generally ver y good; cases of significant disagreement are critically evaluated.