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Q8.16(iii):
Describe the following: (iii) Cross aldol condensation

Solution :

Theoretical Foundation of Cross Aldol Condensation

Cross Aldol Condensation is a nucleophilic addition reaction occurring between two different carbonyl compounds (aldehydes or ketones), at least one of which must contain $\alpha$-hydrogen atoms. In the presence of a dilute base (e.g., $NaOH$, $Ba(OH)_2$), the process involves the generation of an enolate ion from one carbonyl component, which then attacks the carbonyl carbon of the second component [Per the Mechanism of Nucleophilic Addition to Carbonyls].

Step 1: Mechanism of Enolate Ion Formation

Consider two carbonyl compounds: $R-CH_2-CHO$ (Aldehyde A) and $R'-CHO$ (Aldehyde B). The base abstracts an acidic $\alpha$-hydrogen from the compound that possesses it, forming a resonance-stabilized enolate ion.

$\text{R-CH}_2\text{-CHO} + \text{OH}^- \rightleftharpoons [\text{R-CH-CHO}]^- + \text{H}_2\text{O}$

[Justification: The acidity of $\alpha$-hydrogens is due to the electron-withdrawing effect of the carbonyl group and the resonance stability of the resulting enolate ion].

Step 2: Nucleophilic Attack and Adduct Formation

The enolate ion acts as a nucleophile and attacks the electrophilic carbonyl carbon of the second aldehyde. This forms a $\beta$-hydroxy aldehyde (aldol).

$[\text{R-CH-CHO}]^- + \text{R'-CHO} \rightarrow \text{R'-CH(O}^-\text{)-CH(R)-CHO}$

Following protonation by water, we obtain the $\beta$-hydroxy product:

$\text{R'-CH(OH)-CH(R)-CHO}$

Step 3: Dehydration and Final Product

Upon heating, the $\beta$-hydroxy aldehyde undergoes dehydration via an $E1cB$ (Elimination Unimolecular conjugate base) mechanism to form an $\alpha,\beta$-unsaturated aldehyde.

$\text{R'-CH(OH)-CH(R)-CHO} \xrightarrow{\Delta, -\text{H}_2\text{O}} \text{R'-CH=C(R)-CHO}$

Visual Representation of the Reaction

Carbonyl 1 + Carbonyl 2 $\beta$-hydroxy aldehyde $\alpha,\beta$-unsaturated aldehyde dil. NaOH, Heat

Analytical Summary of Outcomes

In a cross-aldol condensation between two different carbonyls, four possible products can theoretically be formed if both possess $\alpha$-hydrogens:

Reaction Type Components
Self-Condensation 1 Aldehyde A + Aldehyde A
Self-Condensation 2 Aldehyde B + Aldehyde B
Cross-Condensation 1 Enolate A + Electrophile B
Cross-Condensation 2 Enolate B + Electrophile A

Final Solution: Cross aldol condensation is the base-catalyzed reaction between two different carbonyl compounds, resulting in a mixture of four products unless one reactant lacks $\alpha$-hydrogens (e.g., formaldehyde or benzaldehyde), which limits the reaction to specific cross-products.


More Questions from Class 12 Chemistry Aldehydes Exercises


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