JEE Challenger
More from Aldehydes, Ketones and Carboxylic Acids
ChemHardAdv 2014 • Paper 2

Match Peroxyester Thermal Decomposition Pathways to Molecular Structures

Various feasible thermal decomposition pathways for peroxyesters are illustrated below. Match each pathway presented in List I with the corresponding structure in List II and choose the correct code from the options given.

List-I P. Pathway P
Q. Pathway Q
R. Pathway R
S. Pathway S

List-II
(Refer to structures 1, 2, 3, and 4 in the diagram)

Question Diagram 1
Question Diagram 2

Options

A

P →\rightarrow 1, Q →\rightarrow 3, R →\rightarrow 4, S →\rightarrow 2

Correct
B

P →\rightarrow 2, Q →\rightarrow 4, R →\rightarrow 3, S →\rightarrow 1

C

P →\rightarrow 4, Q →\rightarrow 1, R →\rightarrow 2, S →\rightarrow 3

D

P →\rightarrow 3, Q →\rightarrow 2, R →\rightarrow 1, S →\rightarrow 4

Topics & Concepts

Aldehydes, Ketones and Carboxylic AcidsCarboxylic Acids and Derivatives

Step-by-Step Solution

To determine the correct thermal decomposition pathway for each peroxyester, we analyze the mechanism of homolytic cleavage and subsequent radical reactions:

  1. Nature of the First Cleavage Step (One-bond vs. Two-bond Cleavage):

    • When R=C6H5CH2\text{R} = \text{C}_6\text{H}_5\text{CH}_2 (benzyl), the resulting benzyl radical C6H5CH2∙\text{C}_6\text{H}_5\text{CH}_2^\bullet is highly resonance-stabilized. This stability promotes a concerted two-bond homolytic cleavage involving the simultaneous rupture of the O−O\text{O}-\text{O} bond and the R−C\text{R}-\text{C} bond, leading directly to the extrusion of CO2\text{CO}_2 in the initial step: R−C(=O)−O−O−R′→−CO2R∙+R′O∙\text{R}-\text{C}(=\text{O})-\text{O}-\text{O}-\text{R}' \xrightarrow{-\text{CO}_2} \text{R}^\bullet + \text{R}'\text{O}^\bullet This corresponds to Pathways P and Q.
    • When R=C6H5\text{R} = \text{C}_6\text{H}_5 (phenyl), the phenyl radical is significantly less stable, and concerted cleavage is energetically unfavorable. Therefore, it undergoes a stepwise single-bond cleavage of the weak O−O\text{O}-\text{O} bond first to form an aroyloxy radical and an alkoxy radical: R−C(=O)−O−O−R′⟶RCO2∙+R′O∙\text{R}-\text{C}(=\text{O})-\text{O}-\text{O}-\text{R}' \longrightarrow \text{RCO}_2^\bullet + \text{R}'\text{O}^\bullet Subsequent loss of CO2\text{CO}_2 occurs in a later step: RCO2∙→−CO2R∙\text{RCO}_2^\bullet \xrightarrow{-\text{CO}_2} \text{R}^\bullet This corresponds to Pathways R and S.

  2. Fate of the Alkoxy Radical (R′O∙\text{R}'\text{O}^\bullet):

    • When R′=−CH3\text{R}' = -\text{CH}_3 (Structures 1 and 2), the methoxy radical CH3O∙\text{CH}_3\text{O}^\bullet does not undergo β\beta-scission because the expulsion of a hydrogen atom (H∙\text{H}^\bullet) is thermodynamically unfavorable. Hence, no carbonyl compound is produced.
    • When R′\text{R}' is a tertiary alkyl group (Structures 3 and 4), the alkoxy radical undergoes facile β\beta-scission (fragmentation) to yield a stable radical X∙\text{X}^\bullet and a carbonyl compound:
      • For Structure 3 (R′=−C(CH3)2CH2C6H5\text{R}' = -\text{C}(\text{CH}_3)_2\text{CH}_2\text{C}_6\text{H}_5): C6H5CH2C(CH3)2O∙⟶CH3COCH3+C6H5CH2∙ (X∙)\text{C}_6\text{H}_5\text{CH}_2\text{C}(\text{CH}_3)_2\text{O}^\bullet \longrightarrow \text{CH}_3\text{COCH}_3 + \text{C}_6\text{H}_5\text{CH}_2^\bullet \, (\text{X}^\bullet)
      • For Structure 4 (R′=−C(CH3)2C6H5\text{R}' = -\text{C}(\text{CH}_3)_2\text{C}_6\text{H}_5): C6H5C(CH3)2O∙⟶C6H5COCH3+CH3∙ (X∙)\text{C}_6\text{H}_5\text{C}(\text{CH}_3)_2\text{O}^\bullet \longrightarrow \text{C}_6\text{H}_5\text{COCH}_3 + \text{CH}_3^\bullet \, (\text{X}^\bullet)

  3. Matching the Pathways:

    • Pathway P: Direct CO2\text{CO}_2 extrusion without β\beta-scission   ⟹  \implies Structure 1 (R=C6H5CH2, R′=CH3\text{R} = \text{C}_6\text{H}_5\text{CH}_2, \, \text{R}' = \text{CH}_3).
    • Pathway Q: Direct CO2\text{CO}_2 extrusion followed by β\beta-scission to form a carbonyl compound   ⟹  \implies Structure 3 (R=C6H5CH2, R′=−C(CH3)2CH2C6H5\text{R} = \text{C}_6\text{H}_5\text{CH}_2, \, \text{R}' = -\text{C}(\text{CH}_3)_2\text{CH}_2\text{C}_6\text{H}_5).
    • Pathway R: Stepwise CO2\text{CO}_2 loss accompanied by β\beta-scission to form a carbonyl compound   ⟹  \implies Structure 4 (R=C6H5, R′=−C(CH3)2C6H5\text{R} = \text{C}_6\text{H}_5, \, \text{R}' = -\text{C}(\text{CH}_3)_2\text{C}_6\text{H}_5).
    • Pathway S: Stepwise CO2\text{CO}_2 loss without β\beta-scission   ⟹  \implies Structure 2 (R=C6H5, R′=CH3\text{R} = \text{C}_6\text{H}_5, \, \text{R}' = \text{CH}_3).

Therefore, the correct matching is: P→1,Q→3,R→4,S→2\text{P} \rightarrow 1, \quad \text{Q} \rightarrow 3, \quad \text{R} \rightarrow 4, \quad \text{S} \rightarrow 2

This corresponds to option (A).

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