Now let's look at the hybridization effect. The definition of the hybridization effect is this: the higher the s character in an acid, the more stable the conjugate base becomes. We discussed s character when we talked about hybridization. Recall that 25% s character comes from sp3 hybridization, where you have one s and three p orbitals; therefore, 25% of the hybrid is s character. The s orbital is the closest to the nucleus and is the smallest, which means the more s character present, the closer the lone pairs are held to the nucleus. This proximity makes the conjugate base more stable by holding the electrons tighter to the positive, enhancing stability.
Now, considering the acidity trend, it would emerge that sp hybridization has 50% s character because it consists of one part s and one part p. Sp2 would have 33% s character, and sp3 would have 25% s character. Let's examine the pKa values to understand how this trend affects acidity. The pKa for an sp hybridized CH is 25. For sp2, characteristic of alkyne, the pKa is 44. And for sp3, typically found in alkane, it is 50. As the s character increases, so does the acidity. Does that make sense? When the s character proportion increases, the entirety of the hybrid orbital being s leads to greater acidity.
