Great question! First, let’s review what led you to this question. In the unimolecular substitution reaction we looked at, an intermediate tert–butyl cation was formed, but in the bimolecular substitution reaction, the cation (this time a methyl cation, CH₃⁺) was too high in energy. In this case the hydroxide ion directly displaced the chloride ion.

Generally, the more highly substituted a carbocation is, the more stable it is. There are a number of ways to explain why this is true. The first is that carbon substituents are more electron-donating than hydrogen atoms. Electrons on neighboring carbon atoms can help stabilize the cationic center. Simply put, the order of carbocation stability matches the number of carbons bonded to the cationic carbon, with higher numbers leading to greater stability.