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JEE Main 2025 Jan 22 Shift 2, Chemistry Q20: Naming of Alkanes

JEE Main2025Jan 22, Shift 2Chemistry
Q.

The alkane from below having two secondary hydrogens is :

  1. A

    4-Ethyl-3,4-dimethyloctane

  2. B

    2,2,4,4-Tetramethylhexane

  3. C

    2,2,3,3-Tetramethylpentane

  4. D

    2,2,4,5-Tetramethylheptane

Solution

Count the number of hydrogens (hydrogens on carbons bonded to exactly two other carbons) in each structure.

(1) 4-Ethyl-3,4-dimethyloctane: the octane chain contains several internal groups, giving 10 secondary hydrogens.

(2) 2,2,4,4-Tetramethylhexane: . The single internal between the quaternary carbons gives 2H, and the terminal contributes another 2H, totalling 4 secondary hydrogens.

(3) 2,2,3,3-Tetramethylpentane: . Only the group between the quaternary carbon and the terminal methyl is secondary, giving exactly 2 secondary hydrogens.

(4) 2,2,4,5-Tetramethylheptane: contains two groups (2H each), giving 4 secondary hydrogens.

Only 2,2,3,3-tetramethylpentane has exactly 2 secondary hydrogens, option (3).

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