See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: The aldol condensation is a reaction between two carbonyl compounds (typically aldehydes or ketones) that proceeds via base (or acid) catalysis to form a beta-hydroxy carbonyl compound (aldol product), which can further dehydrate to give an alpha,beta-unsaturated carbonyl compound. Step 1 - Mechanism under base catalysis: In the base-catalyzed aldol condensation, the base (commonly a hydroxide ion, which is a Bronsted base acting as a Lewis base) abstracts an alpha-hydrogen from one carbonyl compound. This generates an enolate ion, which is a carbanion (negative charge on carbon alpha to the carbonyl). Step 2 - Evaluating option (b): The initial step is the deprotonation at the alpha-carbon to form an enolate carbanion. This carbanion then attacks the carbonyl carbon of the second molecule. So option (b) is correct: the initial step is the formation of a carbanion (enolate). Step 3 - Why other options fail: - Option (a): A Lewis acid is NOT commonly used; the typical catalyst is a base (e.g., NaOH). Acid-catalyzed aldol does exist but proceeds via enol formation, not the most common pathway. Saying 'Lewis acid is commonly used' is incorrect. - Option (c): A Lewis base is indeed sometimes employed, but it induces carbanion (enolate) formation, NOT carbocation formation. Carbocation formation is incorrect for the aldol mechanism. - Option (d): The carbon chain lengthening occurs through the C-C bond formation in the aldol addition step, not through elimination of water. Elimination of water is the condensation step that gives the alpha,beta-unsaturated product, but the chain lengthening itself is due to C-C bond formation, and stating it is 'lengthened through elimination of 1 mole of water' is misleading and incorrect as a description of how the chain grows. Therefore, the correct answer is B.