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Markownikoff’s rule dictates the preferred mode of addition of unsymmetrical addendum (like HCl) to unsymmetrical alkenes. This states that during the addition of an unsymmetrical addendum to unsymmetrical alkenes, the negative part of the addendum gets added to that carbon atom of the C=C double bond which is attached to the least number of hydrogen atoms. For example HCl adds on to chloroethene to give 1,1-Dichloroethane and 1,2-Dichloroethane. Out of them, the Markownikoff’s choice would be 1,1-dichloroethane. The choice is explicable by considering the relative stability of the carbocation formed by initial attack of H+ on the parent alkene. Considering the addition reaction between chloroethene and HCl, attack of H+ on C1 (carbon attached to the chlorine atom) produces a primary carbocation, whereas, attack of H+ on C2 produces another primary carbocation, but this time the lone pair over chlorine stabilizes it through its +M effect. The latter pathway thus proceeds preferably towards producing 1,1-dichloroethane as the major product.
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