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Question

Identify the incorrect statement/statements :
(i) Alkynes are more reactive than alkenes towards electrophilic addition reaction
(ii) Alkynes are less reactive than alkenes towards electrophilic addition reaction
(iii) Alkynes decolourise Br2 water
(iv) Addition of HBr to alkenes in presence of peroxide proceeds via Markovnikov's rule

A
(i) and (ii)
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B
(ii) and (iii)
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C
(i) and (iv)
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D
(ii) and (iv)
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Solution

The correct option is C (i) and (iv)
While alkynes undergo electrophilic addition reaction, it forms vinylic carbocation (+CH=CH2). This carbocation is unstable as it is sp2 hybridised. But while alkenes undergo electrophilic addition reaction, it forms sp3 hybridised carbocation (+CH2CH3) which is more stable. So, alkynes are less reactive than alkenes towards electrophilic addition reaction. Hence, option (i) is wrong.

Reddish orange colour of the bromine solution is decolourised after reaction with an alkyne. This is used as a test for unsaturation. Hence, option (iii) is correct.

Addition of HBr to alkenes in presence of peroxide proceeds via anti-Markovnikov's rule. Hence, option (iv) is wrong.

Therfore, the incorrect statements are option (i) and (iv).

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