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Q8.7(ii):
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. (ii) 2-Methylpentanal

Solution :

Step 1: Structural Analysis of 2-Methylpentanal

The chemical structure of 2-Methylpentanal is $CH_3CH_2CH_2CH(CH_3)CHO$. To determine its reactivity toward Aldol condensation and the Cannizzaro reaction, we must identify the presence of $\alpha$-hydrogens.

  • The functional group is an aldehyde ($-CHO$).
  • The $\alpha$-carbon is the carbon atom attached directly to the carbonyl carbon. In 2-Methylpentanal, the carbon at position 2 is the $\alpha$-carbon.
  • This $\alpha$-carbon is bonded to one hydrogen atom ($CH$ group).

[Justification: Per the definition of $\alpha$-hydrogen, any aldehyde or ketone containing at least one $\alpha$-hydrogen atom undergoes aldol condensation in the presence of dilute alkali.]

Step 2: Determining Reaction Classification

Reaction Type Applicability to 2-Methylpentanal Reasoning
Aldol Condensation Yes Presence of one $\alpha$-hydrogen atom.
Cannizzaro Reaction No Requires the absence of $\alpha$-hydrogens.

Step 3: Product of Aldol Condensation

When 2-Methylpentanal ($CH_3CH_2CH_2CH(CH_3)CHO$) undergoes aldol condensation, two molecules participate. The $\alpha$-hydrogen of one molecule is abstracted by a base to form an enolate ion, which then attacks the carbonyl carbon of the second molecule.

Reaction:

$2 CH_3CH_2CH_2CH(CH_3)CHO \xrightarrow{Dil. NaOH} CH_3CH_2CH_2CH(CH_3)CH(OH)C(CH_3)(CH_2CH_2CH_3)CHO$

[Justification: The nucleophilic attack occurs at the carbonyl group of the second molecule, resulting in a $\beta$-hydroxy aldehyde.]

Step 4: Visualizing the Structural Mechanism

Aldehyde: CH3-CH2-CH2-CH(CH3)-CHO Alpha-carbon highlighted: The CH at position 2 has one H available. Product (Aldol): 3-hydroxy-2,4-dimethylheptanal derivative Reaction: Aldol Condensation (Proceeds due to alpha-H)

Step 5: Cannizzaro Reaction Assessment

As established, 2-Methylpentanal contains an $\alpha$-hydrogen atom. According to the reaction requirements for Cannizzaro [Per the Cannizzaro mechanism], the substrate must lack $\alpha$-hydrogens to undergo disproportionation in concentrated alkali. Therefore, no Cannizzaro product is expected.

Final Solution: 2-Methylpentanal undergoes Aldol condensation because it contains one $\alpha$-hydrogen atom. It does not undergo the Cannizzaro reaction. The Aldol product is 3-hydroxy-2,4-dimethylheptanal (or the corresponding isomer based on alkyl chain branching).


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