Question

Which of the following gives 2-methylbutane on hydrogenation?
(i) 2-methylbut-1-ene (ii) 3-methylbut-1-yne (iii) 2-methylbut-2-ene
(iv) 3-methylbut-1-ene (v) pent-2-ene

• A. (i) and (iii)
• B. (ii), (iii), (iv)
• C. (ii), (iii), (v)
• D. (i), (ii), (v)
• E. (i), (iii), (iv)

Hint:

Hydrogenation: - Does NOT change carbon skeleton - Only removes unsaturation

Answer 1

Answer: (E)

Concept: Hydrogenation adds $\mathrm{H_2}$ across double or triple bonds without changing the carbon skeleton.

Step 1: Check each compound.

• (i) 2-methylbut-1-ene $\rightarrow$ hydrogenation $\rightarrow$ 2-methylbutane

• (ii) 3-methylbut-1-yne $\rightarrow$ hydrogenation $\rightarrow$ 2-methylbutane (rearranged chain? No, gives straight chain equivalent)

• (iii) 2-methylbut-2-ene $\rightarrow$ hydrogenation $\rightarrow$ 2-methylbutane

• (iv) 3-methylbut-1-ene $\rightarrow$ hydrogenation $\rightarrow$ 2-methylbutane

• (v) pent-2-ene $\rightarrow$ hydrogenation $\rightarrow$ pentane (not required)

Step 2: Select correct ones.
\[ (i), (iii), (iv) \]