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Q8.7(ix):
Which of the following compounds would undergo aldol condensation, which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction. (ix) 2,2-Dimethylbutanal

Solution :

Step 1: Structural Analysis of 2,2-Dimethylbutanal

First, we determine the molecular structure of 2,2-Dimethylbutanal to identify the presence of $\alpha$-hydrogens. The structure is as follows:

$CH_3-CH_2-C(CH_3)_2-CHO$

  • The functional group is the aldehyde ($-CHO$) attached to the first carbon (C1).
  • The carbon adjacent to the carbonyl group is the $\alpha$-carbon (C2).
  • In 2,2-Dimethylbutanal, the C2 carbon is bonded to:
    • One ethyl group ($-CH_2CH_3$)
    • Two methyl groups ($-CH_3$)
    • One formyl group ($-CHO$)

[Per the definition of $\alpha$-hydrogen: An $\alpha$-hydrogen is a hydrogen atom attached to the $\alpha$-carbon, which is the carbon atom adjacent to the carbonyl group.]

As the C2 carbon is bonded to four other carbon atoms, it has zero $\alpha$-hydrogens ($0 \alpha$-$H$).

Step 2: Evaluating Potential Reactions

Based on the structural analysis, we apply the following chemical criteria:

Reaction Type Condition Applicability to 2,2-Dimethylbutanal
Aldol Condensation Requires at least one $\alpha$-hydrogen. Does not occur.
Cannizzaro Reaction Requires absence of $\alpha$-hydrogen. Occurs.

[Justification: Aldol condensation proceeds via the formation of an enolate ion, which requires the abstraction of an acidic $\alpha$-hydrogen. Cannizzaro reaction proceeds via nucleophilic acyl substitution on aldehydes lacking $\alpha$-hydrogens, resulting in self-oxidation-reduction (disproportionation).]

Step 3: Product Prediction (Cannizzaro Reaction)

The Cannizzaro reaction involves the disproportionation of two molecules of the aldehyde in the presence of concentrated alkali ($NaOH$ or $KOH$). One molecule is oxidized to a carboxylic acid salt, and the other is reduced to an alcohol.

Reaction: $2 \, C_2H_5-C(CH_3)_2-CHO \xrightarrow{conc. NaOH} C_2H_5-C(CH_3)_2-COO^-Na^+ + C_2H_5-C(CH_3)_2-CH_2OH$

Reactant: 2,2-Dimethylbutanal Product A: Sodium 2,2-dimethylbutanoate Product B: 2,2-Dimethylbutan-1-ol

Step 4: Conclusion Summary

2,2-Dimethylbutanal lacks $\alpha$-hydrogens, rendering it incapable of undergoing aldol condensation. However, this exact structural feature fulfills the fundamental requirement for the Cannizzaro reaction.

Final Solution: The compound 2,2-Dimethylbutanal undergoes the Cannizzaro reaction. It does not undergo aldol condensation. The products of the Cannizzaro reaction are Sodium 2,2-dimethylbutanoate ($C_2H_5C(CH_3)_2COONa$) and 2,2-dimethylbutan-1-ol ($C_2H_5C(CH_3)_2CH_2OH$).


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