Which of the following statement is incorrect about aldol condensation

They carbonyl compound must possess an \(\beta -H\) atom for undergoing aldol condensation.

The correct answer is option 3. They carbonyl compound must possess an \(\beta -H\) atom for undergoing aldol condensation.

Let us take a closer look at aldol condensation, a key reaction in organic chemistry, to understand why statement 3 is incorrect.

Overview of Aldol Condensation

Aldol condensation is a reaction in which two molecules of an aldehyde or ketone, each containing at least one \(\alpha\)-hydrogen atom, react in the presence of a base or acid catalyst. The reaction involves the following steps:

Formation of the Enolate Ion:

The base abstracts an \(\alpha\)-hydrogen from the carbonyl compound, forming an enolate ion. The enolate ion is stabilized by resonance, where the negative charge is delocalized between the \(\alpha\)-carbon and the oxygen of the carbonyl group.

\(\text{R-CH}_2\text{-CO-R} \overset{\text{Base}}{\longrightarrow} \text{R-CH}^-\text{-CO-R} \rightleftharpoons \text{R-C}^=\text{C-OH-R}\)

Nucleophilic Attack:

The enolate ion then acts as a nucleophile and attacks the carbonyl carbon of another molecule of the aldehyde or ketone. This results in the formation of a new carbon-carbon bond, creating a β-hydroxy aldehyde or ketone, commonly referred to as an "aldol."

\(\text{R-CH}^- \text{-CO-R} + \text{R-CHO} \rightarrow \text{R-CH(OH)-CH}_2\text{-CO-R}\)

Dehydration:

The aldol product can lose a molecule of water (dehydration) under the reaction conditions to form an \(\alpha,\beta\)-unsaturated carbonyl compound.

\(\text{R-CH(OH)-CH}_2\text{-CO-R} \rightarrow \text{R-CH=CH-CO-R} + \text{H}_2\text{O}\)

Evaluation of the Statements

1. C-C Bond Formation: This reaction indeed involves the formation of a carbon-carbon bond between the \(\alpha\)-carbon of one molecule and the carbonyl carbon of another. This statement is correct.

2. Presence of \(\alpha\)-Hydrogen: The reaction requires the presence of an \(\alpha\)-hydrogen because it is necessary to form the enolate ion, which is the nucleophile in the reaction. This statement is correct.

3. Presence of \(\beta\)-Hydrogen: This is the incorrect statement. The reaction does not require the presence of a \(\beta\)-hydrogen atom. The \(\beta\)-hydrogen is not involved in the reaction mechanism at all. The focus is on the \(\alpha\)-hydrogens, which are essential for forming the enolate ion.

4. \(\alpha,\beta\)-Unsaturated Carbonyl Product: After the aldol addition, the aldol product often undergoes dehydration to form an \(\alpha,\beta\)-unsaturated carbonyl compound. This is a common outcome of the reaction. This statement is correct.

Summary: The presence of \(\beta\)-hydrogen is not required for aldol condensation. Instead, the reaction depends on the presence of \(\alpha\)-hydrogens. Therefore, statement 3 is incorrect because it incorrectly states that \(\beta\)-hydrogens are necessary for the reaction. The correct statements focus on the role of \(\alpha\)-hydrogens and the formation of an \(\alpha,\beta\)-unsaturated carbonyl product.