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In the presence of platinum catalyst, hydrocarbon A adds hydrogen to form n-hexane. When hydrogen bromide is added to A instead of hydrogen, only a single bromo compound is formed. Which of the following is A?

1. CH3—CH2—CH=CH—CH=CH3
2. CH3—CH=CH2—CH=CH—CH3
3. CH3—CH=CH—CH2—CH=CH3
4. CH2=CH—CH2—CH2—CH2—CH3

Correct answer: CH3—CH=CH—CH2—CH=CH3

Solution

Compound A must have a structure that allows for the addition of hydrogen bromide to form a single bromo compound. Option C, CH3—CH=CH—CH2—CH=CH3, has conjugated double bonds, which ensure regioselectivity in the addition of HBr, leading to a single product.

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