Question

The negative part of the addenda adds on to the carbon atom linked with the least number of hydrogen atoms. This statement is called
A. Thiele’s principle
B. Bayer’s strain theory
C. Markovnikov’s rule
D. Peroxide effect

Answer 1

Hint: In a hydrogenation process, alkenes react with hydrogen molecules in presence of a metal catalyst, thus hydrogen molecules are added to the double bond of alkene in such a way that each carbon atom is bonded with one hydrogen atom. This is an example of electrophilic addition reactions.

Complete Step by Step Answer:
When a hydrogen halide reacts with asymmetric alkenes the product formed by this electrophilic addition reaction is mainly governed by Markovnikov’s rule.

According to Markovnikov’s rule when hydrogen halide or protic acid reacts with asymmetric alkenes, the positive part of the reagent is added to the carbon atom attached with a higher number of hydrogen atoms while the negative part of addenda adds to the carbon atom linked with least number of hydrogen atoms. This electrophilic addition reaction is regioselective in nature which means one product is formed with a higher percentage as a major product while another one with a lower percentage is the minor product.

For example, propene reacts with hydrogen bromide to form $2-bromopropane$ the major product and $1-bromopropane$ the minor product. Here negative bromide ion $B{{r}^{-}}$gets attached to the carbon with the least number of hydrogen atoms and the positive ${{H}^{+}}$ion is linked with carbon with a higher number of hydrogen atoms and hence gives the $2-bromopropane$ major product according to Markovnikoff’s rule.

Thus,option (C) is correct.

Note: The overall reaction mechanism will be changed when a peroxide is present in the reaction medium. Alkene reacts with protic acid in presence of peroxide according to anti-Markovnikov’s rule. Here the overall regioselectivity will be changed as compared to Markovnikov’s rule.