6 Analysis of H – AHn and CH 3 – AHn bonds

Along the series of H–AHn bonds, with AHn = CH3, NH2, OH, and F, the bond dissociation energies show a steady increase as can be expected from the increasing difference in electronegativity along this series. However, in the same series for CH3– AHn the bond strength first decreases from CH3–CH3 to CH3–NH2 and only thereafter increases again along CH3–NH2, CH3–OH and CH3–F. To understand the origin of the apparent anomaly occurring for the trend in C–A bond strengths, we have analyzed the bonding mechanism in H–AHn, CH3–AHn and other model systems. We recover that increasing electronegativity difference across a bond causes an increasing stability. But we also find that the nature of the bond changes qualitatively for AHn = CH3 due to the saturation of A with hydrogens. The need of the methyl group to adopt an umbrella shaped geometry plays a key role in the difference between bonds with CH3 and other second period AHn radicals.