Question

Which of the following does not undergo Cannizzaro reaction?

• A. \((CH_3)_3CCHO\)
• B. \(C_6H_5CHO\)
• C. \(C_6H_5COC_6H_5\)
• D. \(CH_3CHO\)
• E. \(HCHO\)

Hint:

The Cannizzaro reaction is specific to aldehydes without \(\alpha\)-hydrogens. Aldehydes with \(\alpha\)-hydrogens typically undergo aldol reactions instead.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The Cannizzaro reaction is a base-induced disproportionation reaction in which two molecules of a non-enolizable aldehyde (an aldehyde with no alpha-hydrogen atoms) are converted into a primary alcohol and a carboxylate salt. The key requirement for a compound to undergo the Cannizzaro reaction is that it must be an aldehyde that lacks alpha-hydrogens.
Step 2: Key Formula or Approach:
General reaction:
\[ 2 \text{R-CHO} \xrightarrow{\text{conc. base}} \text{R-CH}_2\text{OH} + \text{R-COO}^-\text{Na}^+ \] (where R-CHO has no \( \alpha \)-H)
We must check each compound for two things: 1) Is it an aldehyde? 2) Does it have alpha-hydrogens?
Step 3: Detailed Explanation:
Let's analyze the given compounds:
(A) (CH\(_3\))\(_3\)CCHO (Pivaldehyde or 2,2-Dimethylpropanal): This is an aldehyde. The carbon atom adjacent to the -CHO group (the alpha-carbon) is a quaternary carbon, bonded to three methyl groups. It has no alpha-hydrogen atoms. Therefore, it will undergo the Cannizzaro reaction.
(B) C\(_6\)H\(_5\)CHO (Benzaldehyde): This is an aldehyde. The alpha-carbon is part of the benzene ring and has no hydrogen atoms attached (it's bonded to another ring carbon and the -CHO group). Therefore, it will undergo the Cannizzaro reaction.
(C) C\(_6\)H\(_5\)COC\(_6\)H\(_5\) (Benzophenone): This is a ketone, not an aldehyde. The Cannizzaro reaction is specific to aldehydes. Therefore, it will not undergo the Cannizzaro reaction. However, the question asks which one does not undergo the reaction, and there might be a better answer among the aldehydes. Let's keep checking.
(D) CH\(_3\)CHO (Acetaldehyde): This is an aldehyde. The carbon atom adjacent to the -CHO group (the alpha-carbon) is a methyl group (-CH\(_3\)). It has three alpha-hydrogen atoms. Aldehydes with alpha-hydrogens undergo the Aldol condensation reaction in the presence of base, not the Cannizzaro reaction. Therefore, acetaldehyde does not undergo the Cannizzaro reaction.
(E) HCHO (Formaldehyde): This is an aldehyde. It has no alpha-carbon atom, and thus no alpha-hydrogens. It readily undergoes the Cannizzaro reaction. In fact, it's a classic example.
Conclusion:
Both benzophenone (C) and acetaldehyde (D) do not undergo the Cannizzaro reaction. However, benzophenone is a ketone and is thus not even a candidate for the reaction class. Acetaldehyde is an aldehyde but fails the specific requirement of having no alpha-hydrogens. In the context of aldehyde chemistry, acetaldehyde is the standard example of a compound that undergoes aldol condensation instead of the Cannizzaro reaction. Therefore, it is the intended correct answer.
Step 4: Final Answer:
Acetaldehyde (CH\(_3\)CHO) does not undergo the Cannizzaro reaction because it possesses alpha-hydrogens and will undergo aldol condensation instead. This corresponds to option (D).