Question

Show how to prepare each \(\alpha, \beta\)-unsaturated aldehyde by an aldol reaction followed by dehydration of the aldol product. (a) O=CC=Cc1ccccc1 (b) CCCCCCCC=C(C=O)CCCCCC

Short answer

Answer: The key steps involved in the formation of α, β-unsaturated aldehydes are: 1. Identify the key functional groups in the target molecules. 2. Determine the starting materials for the aldol reaction, including an electrophilic carbonyl compound and a nucleophilic carbonyl compound with an α-hydrogen. 3. Perform the aldol reaction, where the nucleophile attacks the electrophile, forming a new C-C bond and an aldol product. 4. Perform dehydration of the aldol products in the presence of an acid or a base catalyst, leading to the formation of α, β-unsaturated aldehydes.

Step-by-step solution

Identify the key functional groups in the given molecules

In both cases, the target molecules are α, β-unsaturated aldehydes. In molecule (a), there is a phenyl ring connected to the unsaturated portion. In molecule (b), there is a long alkyl chain containing a double bond close to the carbonyl.

Determine the starting materials for the aldol reaction

In order to perform an aldol reaction, we need an aldehyde or ketone that can act as an electrophile (with a carbonyl group) and another carbonyl compound that can act as a nucleophile (with an α-hydrogen). For molecule (a): Electrophile: benzaldehyde (\((C_6H_5CHO)\)) Nucleophile: acetaldehyde (\((CH_3CHO)\)) For molecule (b): Electrophile: nonanal (\((CH_3(CH_2)_7CHO)\)) Nucleophile: butanal (\((CH_3(CH_2)_2CHO)\))

Perform the aldol reaction

In this step, the nucleophile (with an α-hydrogen) will attack the electrophile (carbonyl carbon), forming a new C-C bond and creating an aldol product. For molecule (a): (a1) Mix benzaldehyde and acetaldehyde in the presence of a suitable base (like hydroxide ion or an alkoxide ion). (a2) The α-hydrogen of acetaldehyde gets deprotonated, and the resulting enolate ion attacks the carbonyl carbon of benzaldehyde. (a3) After the formation of a new C-C bond, the aldol product is obtained: \((C_6H_5CH(OH)CH_2CHO)\) For molecule (b): (b1) Mix nonanal and butanal in the presence of a suitable base (like hydroxide ion or an alkoxide ion). (b2) The α-hydrogen of butanal gets deprotonated, and the resulting enolate ion attacks the carbonyl carbon of nonanal. (b3) After the formation of a new C-C bond, the aldol product is obtained: \((CH_3(CH_2)_3CH(OH)CH_2(CH_2)_7CHO)\)

Perform dehydration of the aldol products

To generate an α, β-unsaturated aldehyde from the aldol products, we need to remove a water molecule (dehydration) in the presence of an acid or a base. The hydroxyl group and an adjacent hydrogen will be eliminated, leading to the formation of a double bond. For molecule (a): Heat the aldol product \((C_6H_5CH(OH)CH_2CHO)\) in the presence of an acid or a base catalyst. This dehydration reaction will lead to the α, β-unsaturated aldehyde: \((C_6H_5CH=CHCHO)\) For molecule (b): Heat the aldol product \((CH_3(CH_2)_3CH(OH)CH_2(CH_2)_7CHO)\) in the presence of an acid or a base catalyst. This dehydration reaction will lead to the α, β-unsaturated aldehyde: \((CH_3(CH_2)_3CH=CH(CH_2)_7CHO)\)