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GATE 2023 CY – Question 15

Organic Chemistry · Pericyclic Reactions and Photochemistry: Electrocyclic, cycloaddition and sigmatropic reactions. Diels-Alder reaction, Claisen and Cope rearrangements and their applications in organic synthesis. FMO method of analysis. Woodward-Hoffmann rules. Photochemistry of alkenes, arenes and carbonyl compounds. Photo-oxidation and photo-reduction. Di-π-methane rearrangements, Norrish type-I and II reactions. Paternό-Büchi reaction, Photo- Curtius and Wolff rearrangements. Barton and Hofmann-Lӧffler-Freytag reactions. · 1 mark · Multiple choice

On irradiation using UV light (>300 nm), compounds X and Y, predominantly, undergo

Diagram for GATE 2023 CY question 15
  1. X: Norrish type I reaction and Y: Norrish type II reaction
  2. X: Norrish type II reaction and Y: Norrish type I reaction
  3. Both X and Y: Norrish type I reaction
  4. Both X and Y: Norrish type II reaction

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Correct answer: (B) X: Norrish type II reaction and Y: Norrish type I reaction

Explanation

NorrishII requires a γ-hydrogen geometrically positioned for six-membered intramolecular abstraction by the excited carbonyl. The isopropyl arrangement in X provides this favourable geometry; in Y itdoes not, so α-cleavage(Type I) predominates.

Thus **X: II, Y: I, B**. Availability in three dimensions matters, not merely presence ofa C–H bond.