Question 1

Which of the following statements best explains why the acetone enolate is more stabilized than an alkoxide anion?

Accepted Answer

The negative charge in the acetone enolate is delocalized by resonance between the oxygen and the alpha carbon.

Question 2

Which of the following best describes the difference between the keto and enol tautomers?

Accepted Answer

The keto tautomer has a $$\ce{C=O}$$ and an additional $$\ce{C-H}$$ bond, while the enol tautomer has an $$\ce{O-H}$$ group bonded to a $$\ce{C=C}.$$

Question 3

Why is the enol form of a simple carbonyl compound usually present at less than 1% at equilibrium?

Accepted Answer

The keto form is more thermodynamically stable due to stronger $$\ce{C=O}$$ bond energy.

Question 4

Which factor(s) contribute to the increased acidity of the alpha hydrogen in a $$\beta-$$dicarbonyl compound?

Accepted Answer

Resonance stabilization and intramolecular hydrogen bonding.

Question 5

Which base would be most effective for complete enolate formation from acetone ($$pK_a \approx 20$$)?

Accepted Answer

Lithium diisopropylamide (LDA), $$pK_a of $$conjugate acid $$\approx 40$$

Question 6

Which conditions favor the formation of the kinetic enolate in an unsymmetrical ketone?

Accepted Answer

Strong, non-nucleophilic base (LDA), polar aprotic solvent, low temperature ($$-78^\circC).$$

Question 7

What is the outcome when a methyl ketone is treated with excess $$\ce{I2}$$ and $$\ce{OH^-}$$?

Accepted Answer

Cleavage of a $$\ce{C-C}$$ bond to form a carboxylate anion and iodoform ($$\ce{CHI3}).$$

Question 8

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

Accepted Answer

It allows for the preparation of carboxylic acids with one or two alkyl groups on the alpha carbon.

Question 9

Why does halogenation of a methyl ketone in base often result in multiple halogenations at the alpha carbon?

Accepted Answer

Each halogen atom added increases the acidity of the remaining alpha hydrogens, making further halogenation easier.

Question 10

Which product is formed when 2-bromocyclohexanone reacts with ethanethiol ($$\ce{C2H5SH}) $$under $$\ce{S_N2}$$ conditions?

Accepted Answer

2-(Ethylthio)cyclohexanone

Question 11

Which of the following is a diagnostic test for methyl ketones?

Accepted Answer

Formation of a yellow precipitate of iodoform ($$\ce{CHI3}) $$upon treatment with $$\ce{I2}$$ and $$\ce{OH^-}.$$

Question 12

Which of the following statements about enolate alkylation is correct?

Accepted Answer

The reaction proceeds via an $$\ce{S_N2}$$ mechanism and is most efficient with unhindered alkyl halides.

Question 13

Which of the following best explains why $$\beta-$$keto acids readily undergo decarboxylation upon heating?

Accepted Answer

The carboxyl group is bonded to the alpha carbon of a carbonyl, allowing for a six-membered cyclic transition state.

Question 14

Which of the following is a key advantage of the acetoacetic ester synthesis over direct enolate alkylation?

Accepted Answer

It uses less expensive bases such as $$\ce{NaOEt}$$ instead of LDA.

Question 15

Which of the following statements about racemization at the alpha carbon of a carbonyl compound is correct?

Accepted Answer

Treatment with aqueous base leads to racemization via enolate formation and reprotonation from either face.

Question 16

Which of the following best describes the product of an intramolecular malonic ester synthesis using a 1,4-dihalide?

Accepted Answer

A five-membered ring carboxylic acid

Question 17

Which of the following is true regarding the reactivity of enolates?

Accepted Answer

Enolates react with electrophiles at the alpha carbon due to its increased nucleophilicity.

Enolate and Enol Chemistry: Reactions at the α-Carbon of Carbonyl Compounds

Study Guide - Practice Questions