Question

Why is alpha $\left( \alpha \right)$ hydrogen of carbonyl compounds acidic in nature?

Answer 1

Hint: The alpha $\left( \alpha \right)$ hydrogen is the hydrogen atom attached to the carbon atom adjacent to the carbon atom of the carbonyl compound. $\text{R}-\overset{*}{{\text{C}}}\,{{\text{H}}_{2}}-\text{CHO}$ , the carbon atom having $*$ has the hydrogen atoms are the alpha $\left( \alpha \right)$ hydrogens of this compound. Acidity means the ability to lose $\left[ {{\text{H}}^{+}} \right]$ ions. If the compound after losing $\left[ {{\text{H}}^{+}} \right]$ ions is stable, then, it is acidic in nature.

Complete step by step answer:
Let us discuss the acidity of alpha-hydrogens in carbonyl compounds. There are two reasons for it.
Reason (1)- In a carbonyl group $\left( \text{C=O} \right)$, the oxygen attached is extremely electronegative. As it attracts the electrons toward itself. This develops a partial positive charge on the $\alpha -$carbon and to reduce this positive charge, $\alpha -$carbon readily loses its hydrogen atom which makes it acidic in nature.

Reason (2)- After the removal of acidic hydrogen, the compound undergoes keto-enol tautomerism (a type of resonance only). This makes the compound stable due to the presence of negative charge on the electronegative oxygen atom. Like

Due to this stability of keto-enol, the carbonyl compounds seem to lose their alpha $\left( \alpha \right)$ hydrogen which is then acidic in nature.

Note: The alpha $\left( \alpha \right)$ hydrogens of aldehydes are more acidic in nature than that of ketones because the electron donating groups like methyl of ketones decreases the polarity of the $\left( \text{C}-\text{H} \right)$ bond. Moreover, after the removal of $\left[ {{\text{H}}^{+}} \right]$ ions, the electron donating groups increases the negative charge on the negatively charged carbon atom. This makes the compound unstable and less prone to removal of alpha $\left( \alpha \right)$ hydrogens. This positive inductive effect is not seen in aldehydes.