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Chapter 19: Problem 3

Draw the product of the base-catalyzed crossed aldol reaction between benzaldehyde and 3-pentanone and the product formed by its dehydration.

Short Answer

Expert verified
Answer: The product formed in the base-catalyzed crossed aldol reaction between benzaldehyde and 3-pentanone is CH3CH2C(OH)CH2CH2CH2C6H5. Upon dehydration, this aldol product forms an α,β-unsaturated ketone, CH3CH2CH=CHCH2C6H5.

Step by step solution

01

Identifying the reactants

In the given exercise, we have two reactants: benzaldehyde and 3-pentanone. Remember that in a base-catalyzed aldol reaction, one carbonyl compound acts as a nucleophile and the other as an electrophile. Benzaldehyde: C6H5CHO 3-Pentanone: CH3CH2CH2COCH3
02

Identifying the enolate and carbonyl

In a crossed aldol reaction, we must determine which of the reactants will form the enolate ion and which will act as the carbonyl compound. In this case, the enolate will be formed from 3-pentanone, and benzaldehyde will act as the carbonyl compound, since aldehydes are more reactive than ketones.
03

Formation of the enolate ion

Upon addition of a base, the 3-pentanone reacts to form the enolate ion, which will act as a nucleophile in the subsequent aldol reaction. The enolate ion will be formed as follows: CH3CH2CH2COCH3 + Base -> CH3CH2C(O-)CH2CH3 + Base-H
04

Nucleophilic attack on the carbonyl

The enolate ion acts as a nucleophile and will attack the electrophilic carbonyl carbon of benzaldehyde. This will lead to the formation of an alkoxide intermediate. CH3CH2C(O-)CH2CH3 + C6H5CHO -> CH3CH2C(OH)CH2CH2CH2C6H5
05

Protonation of the alkoxide

In the final step of the aldol reaction, the alkoxide formed in the previous step will undergo protonation to form a new hydroxyl (OH) group. CH3CH2C(OH-)CH2CH2CH2C6H5 + Acid -> CH3CH2C(OH)CH2CH2CH2C6H5
06

Dehydration of the aldol product

The aldol product, CH3CH2C(OH)CH2CH2CH2C6H5, can undergo dehydration to form an α,β-unsaturated carbonyl compound. This is facilitated by an elimination reaction involving the hydroxyl group and an adjacent hydrogen atom. CH3CH2C(OH)CH2CH2CH2C6H5 -> CH3CH2CH=CHCH2C6H5 + H2O The final product after dehydration is CH3CH2CH=CHCH2C6H5, which is an α,β-unsaturated ketone.

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