Question

Name the rule according to which addition reaction takes place in unsymmetrical alkenes.

Answer 1

Markovnikov's rule:

1. According to Markovnikov's Rule when hydrogen halide ($\mathrm{HX}$) is introduced to an asymmetric alkene, the acid hydrogen attaches to the carbon with more hydrogen atoms, whereas the halide group attaches to the carbon with a greater number of alkyl substituents.
2. This rule helps us generate a more stable product because unsymmetrical alkenes have various ligands connected to double carbon atoms.
3. Because symmetrical alkenes have the same ligands attached on both sides of the double-bonded carbon, the same product will be generated from both sides, and that will be the predominant one.

Example: Reaction of Propene with Hydrobromic acid.

1. The first step is protonation or the addition of an acidic hydrogen ion. When hydrobromic acid ($\mathrm{HBr}$) is broken down, it produces ${\mathrm{H}}^{+}$ and${\mathrm{Br}}^{-}$. ${\mathrm{H}}^{+}$ (Electrophile) get connected to the carbon with the greatest number of hydrogen substituents.
2. The addition of bromide anion is the next step. The carbocation is attacked by a nucleophile or bromide ion during this phase. The primary result of the reaction is $2-$Bromo propene because secondary cation production is prioritized.
3. The ${\mathrm{Br}}^{-}$ get attached to the carbon with a more number of alkyl substituents.

Therefore, the Markovnikov rule is used for addition reaction in an unsymmetrical alkene.