J. Phys. Chem. 1994, 98, 303-312
303
Homo- and Heterolytic X-C Bond Energies. 1. Homolytic Bond Energies in Common Unsaturated Organic Compounds Yu-Ran Luo and John L. Holmes' Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada Kl N 6N5 Received: August 24, 1993; In Final Form: October 13, 1993'
It isshown that the homolyticbonddissociationenergy (BDE), DHo(X-R),oftheX-C bond inallylic, propargylic, benzylic, cyanated, and unsaturated compounds can be estimated by the simple equation DHo(X-R) = DHO(X-C(CH3)mH3-m) + E, AVnb: where X is a wide variety of univalent atoms or substituents; R is an allylic, propargylic, benzylic, cyanated, or unsaturated group; DHO (X-C(CH3)mH3-m) are the X - C BDEs in the model saturated compounds; m is the degree of methyl substitution of the group R; E, is the p-?r resonance stabilization energy (RSE) of the given conjugated radical R; AVnb is the steric compression relief and/or the net change of interactions between nonbonding atoms in the compounds RX and in the radical R and in X resulting from the C-X bond cleavage.
+
Introduction Three years ago, an empirical equation (eq 1) for estimating homolytic bond dissociation energiesR-X (BDEs) for R = ethyl, isopropyl, and tert-butyl was introduced by Luo and Benson,' usingobserved values for DHO(X-CH3) as reference points.Here DH0(X-C(CH3),H3-,),t
= DH0(X-CH3)o, 6.33 + 0.36m - V, m (1) 0.67 + 0.21m
X represents F, C1, Br, I, H, CH3, OH, SH, and NH2; m is the degree of methyl substitution at the X-bearing carbon atom; m = 1,2, or 3 for primary, secondary,or tertiary carbons; V,is the covalent potential, or a new scale of electronegativity.2 In the present work, the above compounds, X-C(CH3),,,H3+, and the BDE values will be used respectively as model or reference compounds and the reference points for the new BDE values that we wish to determine. Recently, eq 1 has also been used to determine the propagator of the u inductive effecta3 Two generalized equations for predicting both homolytic and heterolytic BDEs of R-X bonds were given:3b94S DH0(R-X),,
= DHo(CH3-X),,
-
+
6.33 0.36m - V, 0.67 +0.21m +Av,,
(2)
or
and DHO(alky1 cation-)
