Question

Which of the following compounds would not be expected to decarboxylate when heated?

• A.
• B.
• C.
• D.

Hint:

When dealing with decarboxylation reactions, compounds with electron-rich benzene rings or other stabilizing groups can sometimes resist decarboxylation. Pay attention to such structural features.

Answer 1

The Correct Option is C

To analyze the decarboxylation behavior of the given compounds, we'll examine both gem-dicarboxylic acids and β-keto acids.

1. Gem-Dicarboxylic Acids:
These are dicarboxylic acids where both carboxyl (-COOH) groups are attached to the same carbon atom (geminal position). The general decarboxylation reaction is:
$$ \text{R-C(COOH)}_2 \xrightarrow{\text{heat}} \text{R-COOH} + \text{CO}_2 $$
Key feature: The geminal arrangement makes these compounds particularly prone to decarboxylation.

2. β-Keto Acids:
The compound shown is a β-keto acid with the structure:
$$ \text{CH}_3\text{COCH}_2\text{COOH} $$
The decarboxylation mechanism proceeds through a cyclic transition state:
1) The carbonyl oxygen abstracts a proton from the β-carbon
2) Simultaneous breaking of the C-COOH bond forms CO₂
3) The resulting enol tautomerizes to the ketone form

3. Decarboxylation Reaction:
The specific reaction for the given β-keto acid is:
$$ \text{CH}_3\text{COCH}_2\text{COOH} \xrightarrow{\text{heat}} \text{CH}_3\text{COCH}_3 + \text{CO}_2 $$
Product: Acetone (a ketone) and carbon dioxide gas.

4. Conclusion:
Both types of compounds undergo facile decarboxylation when heated:
- Gem-dicarboxylic acids lose one CO₂ to form monocarboxylic acids
- β-Keto acids lose CO₂ to form ketones

Final Answer:
The β-keto acid shown will decarboxylate upon heating to yield a ketone (acetone) and carbon dioxide gas.

Answer 2

The process of decarboxylation typically involves the loss of a carboxyl group (-COOH) as carbon dioxide, especially in the presence of a suitable condition like heating. 

Out of the given compounds, option (C) does not undergo decarboxylation. This is because the structure in option (C) contains a carboxyl group attached to a benzene ring that is unable to undergo decarboxylation under the typical heating conditions used for decarboxylation reactions. 

The other compounds, however, are more likely to undergo decarboxylation as they have a structure conducive to this reaction. 
Thus, the correct answer is option (C).