Is There a Need to Discuss Atomic Orbital Overlap When Teaching Hydrogen–Halide Bond Strength and Acidity Trends in Organic Chemistry?

Abstract

Undergraduate organic chemistry textbooks and Internet websites use a variety of approaches for presenting and explaining the impact of halogen atom size on trends in bond strengths and/or acidity of hydrogen halides. In particular, several textbooks and Internet websites explain these trends by invoking decreasing orbital overlap between the hydrogen 1s atomic orbital and successively larger group 17 halogen atomic orbitals. A similar orbital overlap rationalization is often extended to the trends in alkyl halide bond strengths. We examined this orbital overlap explanation using quantum mechanical calculations. Calculations reveal that orbital overlap increases rather than decreases with successively larger group 17 halogen atomic orbitals. This suggests that an orbital overlap explanation is physically incorrect and unneeded. Alternative to orbital overlap, we briefly discuss physically correct models for rationalizing halogen bond strength and acidity based on quantum mechanical valence bond theory and molecular orbital theory.