Question 1

Which of the following best explains why the α-hydrogen of a carbonyl compound is more acidic than hydrogens bonded to other sp³ carbons?

Accepted Answer

The electrons left behind after deprotonation are delocalized onto an oxygen atom, which is more electronegative and stabilizes the negative charge through resonance.

Question 2

Given the following pKa values, which compound has the most acidic α-hydrogen?

Accepted Answer

$$\mathrm{CH_3COCH_2COCH_3} (2$$,4-pentanedione) with $$pK_a = 8.9$$

Question 3

Which of the following statements about keto-enol tautomerism is correct?

Accepted Answer

Keto-enol interconversion can be catalyzed by either acid or base.

Question 4

In the base-catalyzed mechanism for keto-enol interconversion, what is the first step?

Accepted Answer

Removal of a proton from the α-carbon by a base.

Question 5

Which product is favored under kinetic control in the alkylation of an asymmetrical ketone?

Accepted Answer

The product formed from the less substituted enolate ion, which forms faster.

Question 6

Which of the following best describes the difference between an aldol addition and a Claisen condensation?

Accepted Answer

Aldol addition forms a β-hydroxyaldehyde or β-hydroxyketone, while Claisen condensation forms a β-keto ester.

Question 7

Why are esters less acidic at the α-hydrogen than aldehydes or ketones?

Accepted Answer

Because the resonance stabilization of the enolate ion is less effective due to competition with delocalization of the lone pair on the ester oxygen.

Question 8

Which of the following is a necessary condition for a successful Claisen condensation?

Accepted Answer

The ester must have at least two α-hydrogens.

Question 9

What is the major product when two molecules of acetaldehyde ($$\mathrm{CH_3CHO}) $$undergo an aldol addition followed by dehydration?

Accepted Answer

$$\mathrm{CH_3CH=CHCHO} (an$$ \alpha,\beta$-unsaturated aldehyde)

Question 10

Which of the following statements about crossed aldol additions is correct?

Accepted Answer

They can produce up to four different products if both reactants have α-hydrogens.

Question 11

In the mechanism for the Claisen condensation, why must the base used correspond to the leaving group of the ester?

Accepted Answer

To prevent transesterification and ensure that the reactant does not change if the base adds to the carbonyl group.

Question 12

Which of the following best describes the thermodynamic enolate ion?

Accepted Answer

It is the enolate ion that is more stable due to greater alkyl substitution at the double bond.

Question 13

Which of the following is true about the dehydration of aldol addition products?

Accepted Answer

Dehydration forms an $$\alpha,\beta-$$unsaturated carbonyl compound, which is stabilized by conjugation.

Question 14

Which of the following is a key mechanistic difference between the base-catalyzed and acid-catalyzed keto-enol interconversion?

Accepted Answer

In base-catalyzed interconversion, the α-proton is removed first; in acid-catalyzed, the carbonyl oxygen is protonated first.

Question 15

Which of the following best explains why the product of a Claisen condensation is a β-keto ester?

Accepted Answer

Because the new C–C bond forms between the α-carbon of one ester and the carbonyl carbon of another, followed by elimination of an alkoxide ion.

Question 16

Which of the following is a reason why β-diketones have much lower pKa values than simple ketones?

Accepted Answer

The negative charge after deprotonation can be delocalized onto two oxygen atoms, greatly stabilizing the anion.

Reactions at the α-Carbon of Carbonyl Compounds: Mechanisms and Applications

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