Pre-Requisite Reading: Identifying most and least substituted alkene, Difference Between Primary, Secondary, Tertiary Carbon and Nitrogen, Alkene Addition Reaction, Electrophiles, Nucleophiles.
Alkenes are double bonds containing molecules. These double bonds are called pie bonds and are made up of two electrons. Due to the presence of the pie electron cloud, alkenes are electron-rich species and interact with electron-deficient species, the electrophiles. The reaction between alkenes and the electrophiles is an addition reaction.
The most common electrophilic addition reaction of an alkene is the addition of the reagent HX across the double bond to form an alkyl halide. The reagent HX is made of two components- H+ and X-.
Let’s assume a reaction between HX and 2-methylbut-2-ene, an unsymmetrical trisubstituted alkene. Since alkene is electron-rich, it will add to the H+ in the first step of the reaction. There are two possibilities for the H+ addition, at point A and point B.
The addition of H+ at point A generates a secondary (20) carbocation, whereas at point B, a tertiary (30) carbocation.
The carbocation is electron-deficient species; therefore, electron-donating groups that push their electron richness (by the +I effect denoted with the sign V) to the deficient carbocation help to increase its stability. The 30 carbocation has three alkyl groups increasing the electron density over the 20 carbocation, which has only two alkyl groups. Therefore, tertiary (30) carbocation is more stable than secondary carbocation (20).
The stability of the 30 carbocation makes the electrophile (H+) prefer to attack the alkene at point B.
In the next step of the reaction, the electron-rich X- attaches to the 30 carbocation to generate an alkyl halide molecule.
Therefore, the carbocation stability makes the most substituted carbon of the alkene a preferred site for the negative part of a reagent to attack in electrophilic addition reactions.
In 1870 Vladimir Markovnikov studied the carbocation stability for electrophilic addition reactions. He proposed a rule known as Markovnikov’s rule that suggests that the negative part of the reagent always attacks the most substituted carbon.
Next: Stability Order of Substituted versus Unsubstituted alkenes