See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: When a dicarboxylic acid (alkanedioic acid) is heated, the product depends on the chain length (value of n, where n = number of CH2 groups between the two carboxylic acid groups). The compound shown is HOOC-(CH2)n-COOH, a dicarboxylic acid. Step 1: Identify the behavior of dicarboxylic acids upon heating. - Very short-chain dicarboxylic acids (oxalic acid, n=0; malonic acid, n=1) undergo decarboxylation upon heating, losing CO2 to give a simpler carboxylic acid or CO2 + H2O. - Succinic acid (n=2) and glutaric acid (n=3) undergo intramolecular dehydration (not decarboxylation) to form cyclic anhydrides (5-membered or 6-membered ring anhydrides), releasing H2O instead of CO2. - Longer chain dicarboxylic acids (n=4, adipic acid; n=5, pimelic acid) at higher temperatures can form cyclic ketones via decarboxylation and loss of water, or cyclic anhydrides that are less favored. Step 2: Analyze each option. - n = 1: Malonic acid (HOOC-CH2-COOH). Upon heating, it readily undergoes decarboxylation to give acetic acid + CO2. CO2 is evolved. - n = 4: Adipic acid (HOOC-(CH2)4-COOH). Upon heating, it can form cyclohexanone via decarboxylation and loss of water (pyrolysis over metal oxide catalysts), evolving CO2. - n = 5: Pimelic acid (HOOC-(CH2)5-COOH). Similarly can evolve CO2 upon heating. - n = 2: Succinic acid (HOOC-CH2-CH2-COOH). Upon heating, it undergoes intramolecular dehydration to form succinic anhydride (a 5-membered cyclic anhydride), releasing only H2O. No CO2 is evolved. Step 3: Why n=2 does not evolve CO2. Succinic acid (n=2) forms a stable 5-membered ring anhydride (succinic anhydride) upon heating. The reaction is simply a dehydration: HOOC-CH2-CH2-COOH → succinic anhydride + H2O. No decarboxylation occurs, so no CO2 is produced. Step 4: Why other options fail. - n=1 (malonic acid): classic decarboxylation gives CO2. - n=4 (adipic acid): pyrolysis gives cyclopentanone + CO2 + H2O. - n=5 (pimelic acid): pyrolysis gives cyclohexanone + CO2 + H2O. Therefore, the correct answer is C.