Q.7 The major product(s) produced by gas phase UV irradiation of 2-pentanone is (are)
(A) acetone and ethene
(B) acetic acid and propionic acid
(C) 2-pentanol
(D) cyclopentane

The major product(s) from gas phase UV irradiation of 2-pentanone are acetone and ethene, produced via the dominant Norrish Type II reaction pathway.​

Mechanism Overview

2-Pentanone (CH₃COCH₂CH₂CH₃) absorbs UV light (n→π* transition), exciting the carbonyl group, primarily to the triplet state in gas phase. The Norrish Type II process involves intramolecular γ-hydrogen abstraction by the excited oxygen, forming a 1,4-biradical that cleaves the Cβ-Cγ bond, yielding acetone (CH₃COCH₃) and ethene (CH₂=CH₂) as major fragments in ~54-62% and ~61% yields, respectively. Norrish Type I (α-cleavage) competes but is minor (~12-38% branching), producing CH₃CO- and – CH₂CH₂CH₃ radicals that disproportionate or recombine without matching options directly.​

Option Analysis

• (A) Acetone and ethene: Correct, as these directly form from Norrish Type II cleavage (88% combined yield in some reports), confirmed in tropospheric simulations at 311 nm UV.​

• (B) Acetic acid and propionic acid: Incorrect; these are photooxidation products (with OH/O₂) under atmospheric conditions, not primary photolysis products in pure gas phase UV irradiation.​

• (C) 2-Pentanol: Incorrect; this requires H-atom addition or reduction, absent in gas phase photolysis without H₂ or sensitizers.​

• (D) Cyclopentane: Incorrect; no ring closure to cyclopentane occurs—minor cyclic products like 1-methylcyclobutanol (~12%) arise from biradical cyclization, but not this hydrocarbon.​

Introduction to Gas Phase UV Irradiation of 2-Pentanone

Gas phase UV irradiation of 2-pentanone triggers key photochemical reactions like Norrish Type I and II, producing acetone and ethene as major products. This process is vital for understanding atmospheric chemistry and ketone photolysis in exams like CSIR NET Life Sciences.​

Photochemical Mechanism Breakdown

Upon UV absorption (240-360 nm), 2-pentanone (CH₃COCH₂CH₂CH₃) undergoes triplet excitation. Norrish Type II dominates in gas phase: γ-H abstraction forms a biradical, cleaving to CH₃COCH₃ + CH₂=CH₂ (yields ~54-88%).​

• Type I α-cleavage yields radicals (minor).

• No oxidation without O₂/OH.​

Why Acetone and Ethene Are Major Products

Experimental yields confirm: acetone (54±2.7%), ethene (61.7±3.4%) from photodissociation. This matches option (A) for MCQs on gas phase UV irradiation of 2-pentanone.​

Exam-Relevant Option Elimination

Atmospheric and Exam Implications

In troposphere, these products contribute to SOA/acidity; for CSIR NET, focus on Norrish II for acyclic ketones with γ-H.​