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Step-by-Step Solution
Step 1: Identify the Key Reactions
(a) The compound (C8H8O2) undergoes a cross-aldol reaction with acetophenone in the presence of base. (b) The same compound, when treated with concentrated NaOH (a typical condition for the Cannizzaro reaction), furnishes benzyl alcohol (or its substituted form) as one of the products. From (b), we deduce that the compound must be an aromatic aldehyde with no α-hydrogen (so it can undergo Cannizzaro reaction rather than self-aldol).
Step 2: Analyze the Molecular Formula
The molecular formula C8H8O2 suggests: • A benzene ring (which accounts for 6 carbons). • A formyl group (–CHO) adds 1 more carbon and 1 oxygen. • One additional oxygen and 1 extra carbon must be accounted for in a substituent on the ring.
Step 3: Reasoning Out Possible Structures
Because there is no α-hydrogen on the aldehyde (necessary for the Cannizzaro reaction), the compound is most plausibly a ring-substituted benzaldehyde. A common ring substituent that provides one extra carbon and one oxygen is a methoxy group (–OCH3). Placing a methoxy group on the ring para to the aldehyde group satisfies: • C7 from the benzaldehyde skeleton + 1 carbon in –OCH3 = 8 carbons total. • 2 oxygens overall (1 in –CHO and 1 in –OCH3). • Correct hydrogen count of 8. This matches p-anisaldehyde (4-methoxybenzaldehyde).
Step 4: Cross-Aldol Reaction With Acetophenone
In a base-catalyzed cross-aldol reaction, the enolate ion of the ketone (acetophenone) attacks the aldehyde carbonyl. Since the aldehyde (p-anisaldehyde) has no α-hydrogens, it cannot form its own enolate, leading to a single major cross-aldol product.
Step 5: Cannizzaro Reaction With Concentrated NaOH
Aldehydes lacking α-hydrogens undergo the Cannizzaro reaction in strong bases. In this reaction: • One molecule of the aldehyde is oxidized to the carboxylate salt. • Another molecule is reduced to the corresponding benzyl alcohol. Here, 4-methoxybenzaldehyde (p-anisaldehyde) would give the corresponding 4-methoxybenzyl alcohol (often generically called “benzyl alcohol” in many textbook discussions) and the salt of 4-methoxybenzoic acid.
Step 6: Final Structure
Putting all the information together confirms that the compound is p-anisaldehyde (4-methoxybenzaldehyde). The provided correct answer image corresponds exactly to this structure:
Step-by-Step Solution
Step 1: Identify the Key Reactions
(a) The compound (C8H8O2) undergoes a cross-aldol reaction with acetophenone in the presence of base. (b) The same compound, when treated with concentrated NaOH (a typical condition for the Cannizzaro reaction), furnishes benzyl alcohol (or its substituted form) as one of the products. From (b), we deduce that the compound must be an aromatic aldehyde with no α-hydrogen (so it can undergo Cannizzaro reaction rather than self-aldol).
Step 2: Analyze the Molecular Formula
The molecular formula C8H8O2 suggests: • A benzene ring (which accounts for 6 carbons). • A formyl group (–CHO) adds 1 more carbon and 1 oxygen. • One additional oxygen and 1 extra carbon must be accounted for in a substituent on the ring.
Step 3: Reasoning Out Possible Structures
Because there is no α-hydrogen on the aldehyde (necessary for the Cannizzaro reaction), the compound is most plausibly a ring-substituted benzaldehyde. A common ring substituent that provides one extra carbon and one oxygen is a methoxy group (–OCH3). Placing a methoxy group on the ring para to the aldehyde group satisfies: • C7 from the benzaldehyde skeleton + 1 carbon in –OCH3 = 8 carbons total. • 2 oxygens overall (1 in –CHO and 1 in –OCH3). • Correct hydrogen count of 8. This matches p-anisaldehyde (4-methoxybenzaldehyde).
Step 4: Cross-Aldol Reaction With Acetophenone
In a base-catalyzed cross-aldol reaction, the enolate ion of the ketone (acetophenone) attacks the aldehyde carbonyl. Since the aldehyde (p-anisaldehyde) has no α-hydrogens, it cannot form its own enolate, leading to a single major cross-aldol product.
Step 5: Cannizzaro Reaction With Concentrated NaOH
Aldehydes lacking α-hydrogens undergo the Cannizzaro reaction in strong bases. In this reaction: • One molecule of the aldehyde is oxidized to the carboxylate salt. • Another molecule is reduced to the corresponding benzyl alcohol. Here, 4-methoxybenzaldehyde (p-anisaldehyde) would give the corresponding 4-methoxybenzyl alcohol (often generically called “benzyl alcohol” in many textbook discussions) and the salt of 4-methoxybenzoic acid.
Step 6: Final Structure
Putting all the information together confirms that the compound is p-anisaldehyde (4-methoxybenzaldehyde). The provided correct answer image corresponds exactly to this structure:
