GATE 2026 CY – Question 58

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. · 2 marks · Numerical answer

On heating, the compound Z undergoes a series of 1,5-H/D shifts to give a mixture of its isomers. The total number of isomers present in the mixture (including Z) is ______ (in integer).

Dideuterated cyclopentadiene with one deuterium at its saturated carbon.

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Correct answer: 9 to 9

Explanation

With neither D at the saturated carbon, reflection symmetry leaves four positional classes. With one D there, the other can be adjacent or one position farther, each giving an enantiomeric pair: four more. Both D at the saturated carbon gives one achiral form. Total = 9.

Official GATE 2026 answer key: https://gate2026.iitg.ac.in/doc/download/2026/Keys/CY_Keys.pdf