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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

To determine the reactivity of 2,2-Dimethylbutanal, we must first derive its condensed structural formula based on IUPAC nomenclature: $CH_3-CH_2-C(CH_3)_2-CHO$.

Analyzing the alpha ($\alpha$) carbon: The carbon atom directly attached to the carbonyl group ($-CHO$) is the $\alpha$-carbon. In 2,2-dimethylbutanal, the $\alpha$-carbon is the quaternary carbon atom (bearing two methyl groups and one ethyl group). Consequently, this $\alpha$-carbon possesses zero $\alpha$-hydrogen atoms.

Step 2: Determining Reactivity Criteria

  • Aldol Condensation: Requires at least one $\alpha$-hydrogen atom to form an enolate ion via deprotonation. [Per the mechanism of Aldol Addition].
  • Cannizzaro Reaction: Applicable to aldehydes lacking $\alpha$-hydrogen atoms in the presence of concentrated alkali. [Per the Cannizzaro Disproportionation mechanism].
  • Neither: Applies to compounds that fail both criteria.

Step 3: Evaluation of the Compound

Since 2,2-Dimethylbutanal possesses no $\alpha$-hydrogen atoms, it is incapable of undergoing aldol condensation. However, it is an aldehyde lacking $\alpha$-hydrogens, satisfying the fundamental requirement for the Cannizzaro reaction.

CHO CH3-CH2-C(CH3)2 Structure: 2,2-Dimethylbutanal (No alpha-H)

Step 4: Predicting the Cannizzaro Product

In a Cannizzaro reaction, the aldehyde undergoes self-oxidation and reduction (disproportionation). The reactants are two molecules of 2,2-dimethylbutanal in the presence of concentrated $NaOH$.

Reaction Scheme:

$2 \ CH_3CH_2C(CH_3)_2CHO \xrightarrow{conc. \ NaOH} CH_3CH_2C(CH_3)_2CH_2OH + CH_3CH_2C(CH_3)_2COO^-Na^+$

Products:

  • Alcohol: 2,2-Dimethylbutan-1-ol
  • Carboxylic acid salt: Sodium 2,2-dimethylbutanoate

Step 5: Summary Table of Reactivity

Reaction Type Applicability Reasoning
Aldol Condensation No Absence of $\alpha$-hydrogen atoms.
Cannizzaro Reaction Yes Presence of aldehyde group without $\alpha$-hydrogens.

Final Solution: 2,2-Dimethylbutanal does not undergo aldol condensation. It undergoes the Cannizzaro reaction to produce 2,2-dimethylbutan-1-ol and sodium 2,2-dimethylbutanoate.


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