Question

Anti-Markovnikov's rule is only applicable for addition of:
a.) $HCl$
b.) $HBr$
c.) $HI$
d.) All of these

Answer 1

Hint: Try to recall that Markonikov’s rule states that the addition of unsymmetrical reagents such as $HX,{H_2}O$ etc. to unsymmetrical alkenes occurs in such a way that the negative part of the addendum goes to that carbon atom of double bond which carries lesser number of hydrogen atoms.

Complete step by step solution: It is known to you that the markonikov’s states that negative part of addendum(adding molecule) gets attached to that carbon which possess lesser number of hydrogen atoms.
But according to anti-markovnikov's rule, in presence of peroxides such as benzoyl peroxide, the addition of $HBr$ (but not of $HCl$ or $HI$) to unsymmetrical alkenes takes place contrary to Markovnikov’s rule i.e. the negative part of addendum goes to that carbon atom which possess more number of hydrogen atoms.
Anti-Markovnikov's rule is also known as Kharasch effect or Peroxide effect.
The peroxide effect is only observed with $HBr$ and not with $HCl$, $HI$.

Therefore, from above we can say that option B is the correct option to the given question.
Additional information:
This happens because Peroxide is that compound which is having a nonpolar bond between two oxygen atoms. So, obviously the bond gets haemolysed (not hydrolysed) which results in serial chain propagation of homolytic bond cleavage resulting in formation of hydrogen free radical and bromine free radical. So, this time bromine free radical, since less stable, attacks the pi bond system. This is opposite of Markonikov’s rule where hydrogen cation first attacks the pi bond system.

Note: It should be remembered to you that Markonikov’s rule is based on ionic intermediates whereas anti- markovnikov's rule is based on free radical intermediates.
Also, you should remember that Markonikov’s rule is observed in all $HCl$ , $HI$, $HBr$ whereas anti-markovnikov's rule is observed only in $HBr$.