Question

\(\alpha\)-hydrogen of carbonyl compound is acidic due to (A) Electron withdrawing effect of carbonyl (B) Resonance stabilisation of conjugate base (C) (+) Inductive effect of alkyl group (D) Both (A) and (B)

Short answer

The α-hydrogen of carbonyl compounds is acidic due to both (A) the electron-withdrawing effect of the carbonyl group and (B) resonance stabilization of the conjugate base, as stated in option (D).

Step-by-step solution

Understanding the carbonyl compound and α-hydrogen

A carbonyl compound consists of a carbon atom double-bonded to an oxygen atom (C=O). The α-hydrogen is the hydrogen atom that is directly attached to the carbon atom adjacent to the carbonyl group.

Checking option (A)

The electron-withdrawing effect of the carbonyl group is due to the electronegativity difference between carbon and oxygen. The oxygen atom attracts electrons from the carbon and the α-hydrogen, making the α-hydrogen more acidic. This option seems to be part of the correct answer.

Checking option (B)

When the α-hydrogen is removed as a proton (H+), the resulting conjugate base is stabilized through resonance. This resonance stabilization makes the α-hydrogen more acidic as well. So, this option also seems to be part of the correct answer.

Checking option (C)

The (+) inductive effect of alkyl groups is an electron-donating effect, which would make the hydrogen less acidic as it reduces the electron-withdrawing effect. Thus, this option is not the correct reason for the acidity of the α-hydrogen.

Checking option (D)

Option (D) suggests that both options (A) and (B) are responsible for the acidity of the α-hydrogen, which is consistent with our analysis. #Conclusion# The correct answer explaining why the α-hydrogen of carbonyl compounds is acidic is (D) both (A) the electron-withdrawing effect of the carbonyl group and (B) resonance stabilization of the conjugate base.

Key concepts

Electron Withdrawing Effect

The electron withdrawing effect plays a crucial role in determining the acidity of hydrogens in organic compounds. In the context of carbonyl compounds, the carbon atom of the carbonyl group, being double-bonded to an oxygen atom, is highly electron-deficient. Oxygen is more electronegative than carbon, and thus, it pulls electron density towards itself. This leaves the carbon atom positively polarized.

As a consequence, the alpha-hydrogens, which are the hydrogens on the carbon adjacent to the carbonyl, experience a drop in electron density. When these hydrogens are less electron-rich, it becomes easier for them to be removed as protons (H+ ions). In essence, the electron withdrawing effect of the carbonyl enhances the acidity of alpha-hydrogens, making them more likely to participate in acid-base reactions.

Resonance Stabilization

Resonance stabilization is another key concept that influences the acidity of hydrogens in carbonyl compounds. When an alpha-hydrogen is removed, it forms a conjugate base. The stability of this conjugate base is paramount in determining the ease with which the hydrogen atom leaves.

The conjugate base benefits from resonance stabilization because the negative charge that results from the loss of a proton can be delocalized over the entire structure. This is possible due to the overlap of p-orbitals in the carbonyl group with p-orbitals in the adjacent carbon atom. Delocalization of charge through resonance is energetically favorable and reduces the electronic strain on any one atom, thus stabilizing the conjugate base and, by extension, increasing the acidity of the alpha-hydrogen.

Acyl Hydrogen Acidity

Acyl hydrogen acidity specifically refers to the acidity of the hydrogen atoms attached to the acyl group, which is part of the larger carbonyl compound family. The acyl group is characterized by the formula RCO-, where R is an organic substituent, and C=O is the carbonyl group.

Though the concept applies broadly to carbonyl compounds, acyl hydrogen acidity is often discussed with regard to the reactivity of acyl chlorides, anhydrides, esters, and amides. In these molecules, the electron withdrawing effect of the carbonyl and any additional electronegative substituents, like a chlorine atom in acyl chlorides, enhances the acidity of the hydrogen atoms. This results in the 'activating' of these hydrogens, making them ripe for various organic reactions, such as nucleophilic acyl substitution.

Inductive Effect

The inductive effect refers to the transmission of charge through a chain of atoms in a molecule, which can have significant implications on the molecule’s reactivity. These effects are oftentimes relayed through sigma bonds and are dependent on the electronegativity of the atoms involved.

In the context of carbonyl compounds, the electron-withdrawing inductive effect of the carbonyl group reinforces the acidity of alpha-hydrogens. In contrast, the (+) inductive effect of alkyl groups, which are electron-donating, would diminish this acidity. Since alkyl groups push electron density toward the carbonyl and alpha-hydrogens, they counteract the electron-pulling inductive effect of the carbonyl, making the alpha-hydrogens less acidic. Thus, the balance between electron-donating and electron-withdrawing inductive effects is a subtle dance that modulates the acidity of the hydrogens in these complex organic molecules.