Question 1

Which of the following best describes the difference between keto–enol tautomerism and resonance?

Accepted Answer

Keto–enol tautomerism involves the interconversion of constitutional isomers, while resonance involves different electron arrangements of the same atomic connectivity.

Question 2

Which of the following compounds would have the highest percentage of enol tautomer at equilibrium?

Accepted Answer

2,4-Pentanedione

Question 3

What is the role of acid in the acid-catalyzed halogenation of ketones?

Accepted Answer

It protonates the carbonyl oxygen, increasing the acidity of the alpha hydrogens and facilitating enol formation.

Question 4

Which of the following statements about enolate ions is TRUE?

Accepted Answer

Enolate ions can react with electrophiles at either the oxygen or the alpha carbon.

Question 5

Why is the alpha hydrogen of a carbonyl compound more acidic than the hydrogens on a typical alkane?

Accepted Answer

Because the resulting anion is stabilized by resonance with the carbonyl group.

Question 6

Which reagent is commonly used to generate enolate ions quantitatively from ketones?

Accepted Answer

Lithium diisopropylamide (LDA)

Question 7

In the Hell–Volhard–Zelinskii (HVZ) reaction, what is the function of PBr$$_3$$?

Accepted Answer

It converts the carboxylic acid into an acid bromide, which can then enolize and react with Br$$_2.$$

Question 8

Which of the following best explains why base-promoted halogenation of methyl ketones often leads to complete halogenation of the methyl group?

Accepted Answer

The halogenated product is more acidic, so it forms enolate ions more readily and reacts further.

Question 9

Which of the following alkyl halides would be most suitable for SN2 alkylation of an enolate ion?

Accepted Answer

Benzyl bromide

Question 10

What is the overall transformation achieved by the malonic ester synthesis?

Accepted Answer

Conversion of an alkyl halide into a carboxylic acid with two additional carbon atoms.

Question 11

Which of the following compounds would NOT undergo the haloform reaction?

Accepted Answer

Propionaldehyde

Question 12

Which of the following best explains why 1,3-diketones are more acidic than simple ketones?

Accepted Answer

Their enolate ions are stabilized by resonance with two carbonyl groups.

Question 13

Which of the following is a key step in the laboratory synthesis of barbiturates?

Accepted Answer

Malonic ester alkylation followed by nucleophilic acyl substitution with urea

Question 14

Which of the following statements about the acetoacetic ester synthesis is correct?

Accepted Answer

It converts an alkyl halide into a methyl ketone with three additional carbons.

Question 15

Which of the following best describes the mechanism of alpha-halogenation of aldehydes and ketones in acidic solution?

Accepted Answer

Formation of an enol intermediate, which then reacts with the halogen

Question 16

Which of the following is a reason why direct alkylation of aldehydes is less common than for ketones or esters?

Accepted Answer

Aldehyde enolate ions tend to undergo condensation reactions instead of alkylation.

Question 17

Which of the following best explains why the enol content of most monocarbonyl compounds is very low at equilibrium?

Accepted Answer

The enol is less stabilized by resonance than the keto form.

Question 18

Which of the following is a practical application of alpha-bromo ketones in organic synthesis?

Accepted Answer

They can be dehydrohalogenated to yield alpha,beta-unsaturated ketones.

Question 19

Which of the following is a requirement for decarboxylation to occur in the malonic ester synthesis?

Accepted Answer

A second carbonyl group must be two atoms away from the carboxyl group.

Question 20

Which of the following best describes the role of enolate ions in biological alkylation reactions?

Accepted Answer

Proton removal and alkylation occur essentially simultaneously, so the enolate ion is not isolated.

Study Notes: Carbonyl Alpha-Substitution Reactions

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