Which doesn’t follow Markovnikov’s rule
A.
B.
C.
D.

Hint: Markovnikov’s rule is used to determine the regioselectivity of electrophilic addition reactions of unsaturated compounds like alkenes and alkynes. Markovnikov’s rule is only applicable for an asymmetrical unsaturated compound. To approach this question we will have to determine the nature of alkene whether it is symmetric or asymmetric.

Complete Step by Step Answer:
According to Markovnikov’s rule when protic acid reacts with asymmetric alkenes, the positive part of the reagent is added to the carbon atom linked with a higher number of hydrogen atoms while the negative part of the addenda adds to the carbon atom attached with the least number of hydrogen atoms. This electrophilic addition reaction is regioselective in nature.
This rule is only applied to asymmetric alkenes. Now we have to draw the structures to predict their nature.

From (A) we have a propene which is symmetric in nature. Hence Markovnikov’s rule is applicable here.

But the alkene which is given in option (B) is not an asymmetric alkene but a symmetric alkene. Thus, Markovnikov’s rule is not applicable here.

Both the alkenes from (C) and (D) are asymmetric alkenes, thereby undergoing electrophilic addition reaction followed by Markovnikov’s rule.



Thus, option (B) is correct.

Note: The overall regioselectivity will be changed when peroxide is added to the reaction mixtures with protic acid. The major product is formed according to Anti-Markovnikov’s rule. Then this is called the peroxide effect or kharasch effect. The anti-rule is also applicable for asymmetrical alkenes. Among the hydrogen halides hydrogen chloride and hydrogen iodide do not show a peroxide effect.