Q.24 The major product in the following reaction is

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Introduction

The question shows diethyl 3‑oxoglutarate treated successively with NaOEt/EtOH, Br(CH₂)₂Br, followed by acidic hydrolysis and heating, and asks for the major product from four cyclic β‑keto acid structures. This transformation is a classic example of Dieckmann condensation followed by decarboxylation, leading to a five‑membered β‑keto acid ring. Understanding each step helps to eliminate incorrect options systematically.

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Stepwise reaction mechanism

1. Base‑catalyzed enolate formation

   • NaOEt in EtOH removes an acidic α‑proton from diethyl 3‑oxoglutarate to give its stabilized enolate anion. This enolate is nucleophilic at the α‑carbon.

2. Alkylation with Br(CH₂)₂Br

   • The enolate undergoes SN2SN2 alkylation with one end of 1,2‑dibromoethane (BrCH₂–CH₂Br), installing a –CH₂–CH₂–Br side chain α to the carbonyl.

   • Because only one equivalent of base is typically used, mono‑alkylation dominates and introduces a pendant bromoethyl group on the β‑keto diester.

3. Intramolecular Dieckmann condensation

   • The remaining ester group within the same molecule is attacked intramolecularly by the regenerated enolate.

   • This intramolecular Claisen (Dieckmann) reaction closes a five‑membered ring, because the tether now has four atoms between the two reacting centers (favorable 5‑membered ring closure).

4. Acidic hydrolysis (H₃O⁺)

   • Both ester groups in the cyclic β‑keto diester are hydrolyzed to carboxylic acids, giving a cyclic β‑dicarboxylic acid (actually a β‑keto diacid).

5. Heating (decarboxylation)

   • β‑Keto acids are unstable on heating and lose CO₂ readily.

   • One carboxyl group (the one β to the carbonyl) is lost as CO₂, giving a cyclic β‑keto acid with a five‑membered ring fused to a carbonyl and a carboxylic acid group.

The final product is therefore a five‑membered ring β‑keto acid: a cyclopentane bearing an internal ketone and a carboxylic acid at the β‑position with respect to that ketone.

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Analysis of each option

Option (A)

• Shows a five‑membered ring (cyclopentane) with a ketone and a carboxylic acid separated by one carbon (β‑keto acid).

• This matches the expected outcome of intramolecular Dieckmann condensation followed by hydrolysis and decarboxylation, where a 5‑membered ring is most favorable and only one CO₂ is lost.

• Therefore, Option (A) is the correct major product.

Option (B)

• Depicts a six‑membered ring (cyclohexane) with a ketone and a carboxylic acid (β‑keto acid on a six‑membered ring).

• Formation of a six‑membered ring would require a different tether length (five atoms between reacting centers) than provided by 1,2‑dibromoethane; with –CH₂–CH₂–, the system preferentially forms a five‑membered ring.

• Thus, Option (B) does not correspond to the correct ring size and is incorrect.

Option (C)

• Shows a five‑membered ring with a single ketone and no carboxylic acid group.

• After Dieckmann condensation and decarboxylation of a β‑keto diacid, at least one carboxylic acid group should remain because only the β‑carboxyl group decarboxylates easily.

• The absence of –CO₂H makes this structure inconsistent with the reaction sequence, so Option (C) is incorrect.

Option (D)

• Illustrates a seven‑membered ring (cycloheptane) containing a β‑keto acid.

• Neither the length of the bromoalkyl chain nor the geometry of intramolecular attack favors formation of a seven‑membered ring; such rings are less favorable and would not be the major product in this Dieckmann system.

• Consequently, Option (D) is also incorrect.

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Key points about Dieckmann condensation here

• Starting ester: Diethyl 3‑oxoglutarate provides two ester groups and an internal carbonyl, making its α‑position highly acidic and enolate‑forming.

• Alkylating agent: 1,2‑Dibromoethane introduces a two‑carbon tether essential for 5‑membered ring closure.

• Ring size control: The number of atoms in the tether determines whether a 5‑, 6‑, or 7‑membered β‑keto ester forms; with –CH₂–CH₂– between centers, a five‑membered ring product is most favored.

• Final functional group: Heating a cyclic β‑keto diacid removes only one carboxyl group, leaving a stable cyclic β‑keto acid, as shown in Option (A).

So, the major product of the given NaOEt/EtOH – Br(CH₂)₂Br – H₃O⁺, heat sequence is the five‑membered ring β‑keto acid in Option (A).