Named Reactions: Dieckmann condensation MCQs With Answer

Named Reactions: Dieckmann condensation MCQs With Answer

Introduction: Dieckmann condensation is a vital intramolecular variant of the Claisen condensation used widely in M.Pharm organic synthesis to build five- and six-membered cyclic β-keto esters from diesters. This blog presents clear, focused multiple-choice questions tailored for M.Pharm students pursuing MPC 102T Advanced Organic Chemistry I. Questions cover mechanism steps, choice of base and solvent, regiochemical and ring-size factors, common side reactions (transesterification, intermolecular reactions), and synthetic applications including decarboxylation to cyclic ketones. Each MCQ includes plausible distractors and concise answers to strengthen conceptual understanding and exam readiness for advanced organic synthesis topics.

Q1. What best describes the Dieckmann condensation?

• An intermolecular aldol condensation between two different esters
• An intramolecular Claisen condensation of a diester to form a cyclic β-keto ester
• A nucleophilic aromatic substitution on an ester
• An oxidation of esters to diketones

Correct Answer: An intramolecular Claisen condensation of a diester to form a cyclic β-keto ester

Q2. What is the first mechanistic step in the Dieckmann condensation?

• Protonation of the ester carbonyl
• Nucleophilic attack of alkoxide on the carbonyl
• Deprotonation at the α-carbon to form an enolate
• Elimination of an alcohol molecule to form an alkene

Correct Answer: Deprotonation at the α-carbon to form an enolate

Q3. Which base is generally preferred for a Dieckmann condensation of ethyl esters?

• Sodium ethoxide (NaOEt)
• Sodium hydroxide (NaOH)
• Pyridine
• Hydrochloric acid (HCl)

Correct Answer: Sodium ethoxide (NaOEt)

Q4. Which ring sizes are most commonly and favorably formed via Dieckmann condensation?

• 3- and 4-membered rings
• 4- and 5-membered rings
• 5- and 6-membered rings
• 7- and 8-membered rings

Correct Answer: 5- and 6-membered rings

Q5. In the Dieckmann mechanism, which species serves as the leaving group during cyclization?

• Hydride (H-)
• Alkoxide (RO-)
• Carbon dioxide (CO2)
• Halide ion (X-)

Correct Answer: Alkoxide (RO-)

Q6. What is the primary organic product type obtained directly after Dieckmann condensation and acidic workup?

• Cyclic β-keto ester
• α,β-Unsaturated ester
• Diol
• Cyclic lactone

Correct Answer: Cyclic β-keto ester

Q7. Which sequence is commonly used to convert the cyclic β-keto ester product of Dieckmann condensation into a cyclic ketone?

• Reduction followed by oxidation
• Hydrolysis of the ester followed by decarboxylation (heat)
• Direct hydrogenolysis of the β-keto ester
• Radical bromination and elimination

Correct Answer: Hydrolysis of the ester followed by decarboxylation (heat)

Q8. Why is it recommended to use an alkoxide base that corresponds to the ester alkyl group (e.g., NaOEt with ethyl esters)?

• To increase the acidity of the α-protons
• To avoid transesterification and side reactions
• To make the reaction irreversible
• Because other alkoxides cannot deprotonate esters

Correct Answer: To avoid transesterification and side reactions

Q9. Which experimental precaution helps minimize competing intermolecular Claisen condensations when performing a Dieckmann condensation?

• Using a large excess of strong acid
• Maintaining a very high reaction concentration
• Using the appropriate alkoxide base and controlling concentration (dilution if needed)
• Adding water to the reaction mixture

Correct Answer: Using the appropriate alkoxide base and controlling concentration (dilution if needed)

Q10. How does using a bulky, non-nucleophilic base like LDA affect Dieckmann condensations compared with alkoxide bases?

• LDA prevents enolate formation and stops the reaction
• LDA always gives higher yields of cyclic β-keto esters without side products
• LDA can generate kinetic enolates and may cause side reactions, so it is less commonly used than matching alkoxides
• LDA converts esters to alcohols directly

Correct Answer: LDA can generate kinetic enolates and may cause side reactions, so it is less commonly used than matching alkoxides

Q11. Which statement differentiates Dieckmann condensation from the intermolecular Claisen condensation?

• Dieckmann is intermolecular and Claisen is intramolecular
• Dieckmann requires acid catalysis while Claisen requires base
• Dieckmann is an intramolecular cyclization of a diester; Claisen is generally intermolecular between two esters
• Dieckmann produces lactones whereas Claisen produces ethers

Correct Answer: Dieckmann is an intramolecular cyclization of a diester; Claisen is generally intermolecular between two esters

Q12. Which side reaction is especially relevant during Dieckmann condensations if the alkoxide base does not match the ester?

• Radical polymerization
• Transesterification leading to mixed esters
• Nucleophilic aromatic substitution
• Hydrogenation of the ester

Correct Answer: Transesterification leading to mixed esters

Q13. What determines the regioselectivity of enolate formation in asymmetrically substituted diesters undergoing Dieckmann condensation?

• The solvent dielectric constant only
• The more acidic α-protons and steric hindrance influence which enolate forms
• The color of the reagent
• The presence of light

Correct Answer: The more acidic α-protons and steric hindrance influence which enolate forms

Q14. Which of the following is a common synthetic application of Dieckmann condensation in medicinal chemistry?

• Direct synthesis of aromatic amines
• Formation of cyclic β-keto esters as precursors to substituted cyclopentanones or cyclohexanones used in drug scaffolds
• Preparation of glycosides from sugars
• Conversion of amides to nitriles

Correct Answer: Formation of cyclic β-keto esters as precursors to substituted cyclopentanones or cyclohexanones used in drug scaffolds

Q15. Who is credited with discovering the Dieckmann condensation?

• Adolf von Baeyer
• Wilhelm Dieckmann
• Robert Robinson
• Victor Grignard

Correct Answer: Wilhelm Dieckmann

Q16. After base-promoted cyclization but before acidic workup, the cyclic intermediate is best described as:

• A neutral β-keto ester
• An alkoxide-containing tetrahedral intermediate or its enolate form
• A carbocation stabilized by resonance
• A free radical centered on oxygen

Correct Answer: An alkoxide-containing tetrahedral intermediate or its enolate form

Q17. Which structural feature in the diester substrate will most likely hinder successful Dieckmann cyclization?

• A flexible chain enabling close approach of reactive centers
• Bulky substituents near the reacting α-carbon that block deprotonation or approach
• Having both esters as ethyl esters
• A single unsubstituted methylene between ester groups

Correct Answer: Bulky substituents near the reacting α-carbon that block deprotonation or approach

Q18. Which of the following analytical observations indicates that cyclization in a Dieckmann reaction has occurred?

• Disappearance of ester carbonyl stretch in IR and appearance of a new β-keto carbonyl signature
• Immediate gas evolution of CO2 at room temperature
• Formation of a bright yellow precipitate due to aromaticity
• Complete insolubility in common organic solvents

Correct Answer: Disappearance of ester carbonyl stretch in IR and appearance of a new β-keto carbonyl signature

Q19. What happens to the alkoxide leaving group generated during Dieckmann condensation under typical reaction conditions?

• It is permanently consumed and cannot act further
• It acts as a base and may deprotonate additional substrate molecules (propagating reaction)
• It is immediately converted to carbon dioxide
• It coordinates strongly to the product making it inert

Correct Answer: It acts as a base and may deprotonate additional substrate molecules (propagating reaction)

Q20. Which statement about tautomerism of the cyclic β-keto ester formed by Dieckmann condensation is most accurate under neutral conditions?

• The enol form always predominates regardless of solvent
• The keto form is usually predominant under neutral conditions, though enolization can occur and is favored under certain conditions
• The molecule exists only as an enolate permanently
• Tautomerism is impossible for β-keto esters

Correct Answer: The keto form is usually predominant under neutral conditions, though enolization can occur and is favored under certain conditions

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