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Among the following compounds, which one is most reactive toward addition of HBr? (i) CH3-C≡C-CH3 (but-2-yne) (ii) CH2=CH-CH=CH2 (1,3-butadiene) (iii) Benzene (iv) None of these

1. CH3—C≡C—CH3
2. CH2=CH—CH=CH2
3. benzene ring
4. None of these

Correct answer: CH2=CH—CH=CH2

Solution

1,3-Butadiene (conjugated diene) is the most reactive toward HBr addition because the conjugated pi system stabilizes the intermediate carbocation and allows both 1,2- and 1,4-addition. Benzene is aromatic and strongly resists addition reactions.

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