SUBSTITUENT EFFECTS .5. VINYL AND ETHYNYL DERIVATIVES - AN EXAMINATION OF THE INTERACTION OF AMINO AND HYDROXY-GROUPS WITH C-C DOUBLE AND TRIPLE BONDS

The effect of substituents on the energies of vinyl derivatives was examined with the use of group separation reactions with ethane to give propene and a substituted methane. The energy changes for these reactions were much smaller than those found for a corresponding set of acetyl derivatives. The bond dissociation energies (BDEs) for the vinyl derivatives were found to be linearly related to those of the related methyl derivatives with a slope close to unity. The average difference between BDEs in the two series was 12.1 +/- 1.5 kcal/mol, with the larger values found in the vinyl series. This is in accord with the difference in hybridization between vinyl and methyl. The results for the vinyl series stand in marked contrast to the acetyl series previously studied. Here, the effect of electronegativity on the BDEs was much larger than for the methyl compounds, and the correlation for changes in electronegativity was different from that for changes in hybridization in the substituent. The results of the present study confirm our conclusion that the effects in the acetyl series are dominated by the strong polarization of the carbonyl group and the resultant Coulombic interactions. The effect of substituents on acetylene also was studied. An unusual trend in group separation reactions was found to result from the atypically high C-C bond dissociation energy of propyne combined with the smaller effect of electronegative substituents on bond dissociation energies for ethynyl derivatives as compared to methyl derivatives.