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 $B{r}_2$ water
(iv) Addition of $HBr$ to alkenes in presence of peroxide proceeds via Markovnikov's rule

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

Answer 1

The correct option is C (i) and (iv)

While alkynes undergo electrophilic addition reaction, it forms vinylic carbocation ($\stackrel{+}{C}H=C{H}_2$). This carbocation is unstable as it is $s{p}^2$ hybridised. But while alkenes undergo electrophilic addition reaction, it forms $s{p}^3$ hybridised carbocation ($\stackrel{+}{C}{H}_2-C{H}_3$) 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).