Question

Name the unsaturated hydrocarbon for which Anti-Markovnikov’s rule for addition of $HBr$ is not applicable.

Answer 1

Hint : Markovnikov Rule: According to this rule, if hydrohalogenation of an unsymmetrical alkene takes place, then the halide ion will attack the more substituted unsaturated carbon atom. Anti-Markovnikov rule is opposite to that of the Markovnikov rule.

Complete Step By Step Answer:
Anti-Markovnikov rule: It is a special case of addition of alkenes. In presence of hydrogen peroxide, $HBr$ reacts with an unsymmetrical alkene to give the product in which bromide ion i.e., $B{r^ - }$ ion attacks the unsaturated carbon which is comparatively less substituted. It is also termed as peroxide effect.
Some important points regarding Anti-Markovnikov rule:
The Peroxide effect or Anti-Markovnikov rule is only applied in the case of $HBr$ . Hydrohalogenation addition reactions, except hydrogen bromide, follow only Markovnikov rule.
The reaction must be carried out in the presence of peroxide to achieve the Anti-Markovnikov product.
Alkene must be unsymmetrical i.e., the two unsaturated carbon atoms must be differently substituted.
Anti-Markovnikov rule is also known as Kharasch effect.
Example:
$C{H_3}CH = C{H_2} + HBr + {H_2}{O_2} \to C{H_3}C{H_2}C{H_2}Br$
Here $1 -$ propene reacts with hydrogen bromide in presence of hydrogen peroxide, then bromide ion attacks an unsaturated carbon atom which is less substituted to give the product $1 -$ bromopropane.
Hence, the unsaturated hydrocarbon for which Anti-Markovnikov rule for addition of $HBr$ is not applicable is a symmetrical unsaturated hydrocarbon or alkene.

Note :
The addition reaction of hydrogen halides to unsaturated hydrocarbons follows a free radical mechanism which involves three steps that are initiation, propagation and termination. It is important to note that in absence of peroxide, a simple electrophilic addition reaction takes place and the product is formed according to Markovnikov rule.