Question

Why is an aldehyde more reactive than a ketone?

Answer 1

Consider the shrubbery around a ketone, versus that around an aldehyde.

Explanation:

By definition, a ketone has a formula of `RC(=O)R'`, where `R` is a hydrocarbyl group. On the other hand, an aldehyde has a general formula of `RC(=O)H`. The steric bulk of an hydrogen atom is very small compared to even a methyl group, and this is consistent with the enhanced reactivity of aldehydes versus ketones.

Since a hydrocarbyl group is formally electron releasing I suppose you could say that the carbonyl carbon of a ketone is less electron deficient than that of a aldehyde carbon, but the steric effect should differentiate.