Question

Why are $\mathrm{\alpha}$-hydrogen atoms of aldehydes and ketones acidic in nature ?

Hint:

Loss of α-H gives a resonance-stabilised enolate, so the α-H is acidic.

Answer 1

Concept: Whether a hydrogen is acidic depends on how stable the leftover particle is after that hydrogen leaves as $\mathrm{H^+}$. The more stable the leftover, the more acidic the hydrogen.

Step 1: What is an alpha-hydrogen
The alpha-hydrogen is a hydrogen on the carbon right next to the $\mathrm{C{=}O}$ group. We want to know why this hydrogen comes off fairly easily.

Step 2: What happens when it leaves
When the alpha-hydrogen leaves, the carbon next door is left with a negative charge (a carbanion). On its own this would be unstable.

Step 3: Why the leftover is stable
But that negative charge does not stay stuck on the carbon. It can shift onto the carbonyl oxygen, because oxygen is happy to hold a negative charge. This sharing of charge between carbon and oxygen is resonance, and it gives the stable enolate ion. Because the leftover ion is stabilised this way, the alpha-hydrogen is removed easily, which is what we mean by acidic.

Answer: Losing the alpha-hydrogen gives a carbanion whose negative charge spreads onto the $\mathrm{C{=}O}$ oxygen by resonance (the enolate), and this stability makes the alpha-hydrogen acidic.