What is Markovnikov’s rule?
“When an unsymmetrical reagent adds to an unsymmetrical alkene or alkyne, the positive part of the reagent becomes attached to the double or triple bonded carbon which bears the greatest number of hydrogen atoms and vice versa.”
OR
“When an asymmetric molecule is added to an asymmetric alkene or alkyne then the negative part of the molecule to be added goes to that multiple bonded carbon which has less number of hydrogen atoms and vice versa.”
In above mentioned later case (propene) addition took place according to Markovnikov’s rule.
In case-I:
primary carbonium ion will be formed while in a later case secondary carbonium ion will be formed. The stability of carbonium ions is:
Ter-carbonium ion > Sec-carbonium ion > Pri-carbonium ion
As in case-II:
stable carbonium ion is formed so addition takes place according to Markownikoff’s rule.
Mechanism:
When the halogen acid molecule approaches alkene its +ve end pulls π-electrons and as a result, the halogen acid molecule breaks up and +ve hydrogen ion (acts as electrophile) forms a temporary bond with two multiple bonded carbons.
The electrophile (Hydrogen) makes a permanent bond with one of the carbon and leaves a +ve charge on the other carbon.
The –ve ion (nucleophile i.e bromide) attacks the carbonium ion and offers its electrons to positively charged carbon and makes a bond with it.
Recommended video:
Markovnikov rule statement and example ( in Urdu / Hindi)
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