Atom Hybridization and Acid/Base Strength

Acidic X–H bonds are always covalent single bonds (σ bonds) arising from overlap of an atomic orbital on the X atom and the 1s orbital of H. The most common X atoms in organic molecules will be C, N, or O atoms involved in single, double or triple bonds. The σ-bonding orbitals associated with individual C, N, and O atoms are frequently envisioned as hybrid orbitals with varying degrees of s and p character, depending on the type of hybridization (sp, sp², or sp³). Experimentally, it is observed that as hybridization of the orbital involved in an X–H bond changes from sp³ → sp² → sp, the X–H bond becomes more acidic. This effect is shown for a series of similar molecules with acidic N–H and O–H bonds in the table (Table 1.1).

Table 1.1 Effect of X Atom Hybridization on Acidities of C–H, N–H, and O–H Acids (R = Methyl).

The explanation for this trend is that as the hybridization of an atom changes from sp³ → sp² → sp, the percentage of “s” character increases from 25% to 33% to 50%. Recall that s orbitals are spherical in shape such that the electrons surround and overlap with the positively charged nucleus. In contrast, p orbitals have a bilobed “dumbbell” shape with a node at the nucleus such that the electrons are distributed further away from the nucleus. Thus, with increasing s character of the bonding orbital on the X atom (C, N, O), electrons in the X–H σ bond interact more strongly with the positively charged nucleus of X and less so with the bound proton. This results in a weakening of the X–H bond strength and an increased acidity (lower pK_a).