See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: Aldol condensation of alpha,beta-unsaturated aldehydes under basic conditions followed by heating (dehydration). Step 1: Identify the starting material. CH3CH=CHCHO is crotonaldehyde (but-2-enal). It has alpha hydrogens on the CH3 group (at the gamma position relative to C=O, i.e., the methyl group alpha to the C=C). Step 2: Under OH^- (base), the alpha carbon (the methyl group CH3 adjacent to the conjugated system) is deprotonated to form an enolate. In crotonaldehyde, the active methylene/methyl is the CH3 at C1 (position alpha to the conjugated double bond system). The base abstracts a proton from the terminal CH3 group. Step 3: The enolate of one molecule of crotonaldehyde (nucleophile at the terminal carbon) attacks the carbonyl carbon (C=O) of another molecule of crotonaldehyde (electrophile). This gives an aldol addition product: a beta-hydroxy aldehyde with the combined 8-carbon chain. Step 4: Upon heating (Delta), dehydration occurs (elimination of water from the beta-hydroxy aldehyde), giving an extended conjugated alpha,beta-unsaturated aldehyde. Step 5: The product after aldol condensation (addition + dehydration) of two molecules of CH3CH=CHCHO: - Two C4 units join: the alpha carbon of one molecule (terminal CH3, which becomes CH2-) attacks the carbonyl carbon of the second molecule. - After dehydration: CH3-CH=CH-CH=CH-CH=CH-CHO, which is CH3(CH=CH)3CHO. This is an 8-carbon fully conjugated dienal: 2,4,6-octatrienal, written as CH3(CH=CH)3CHO. Why other options fail: - Option (b) CH3CH2CH2(CH=CH)2CHO has a propyl group and only two conjugated double bonds - this would require a different starting material or different reaction. - Option (c) CH3(CH2CH2)3CH=CH-CHO is a saturated chain with only one double bond - inconsistent with conjugation from dehydration of an unsaturated aldehyde condensation product. - Option (d) 'none is correct' is wrong because option (a) is indeed the correct product. Therefore, the correct answer is A.