As an organic chemistry student, you will be faced with dozens, if not hundreds, of reaction mechanisms. Rather than memorize them all, I recommend that you try to understand what happens at each step. This will ensure that you have what it takes to apply concepts to difficult test questions. In this article I will help you understand the electrophilic addition reaction: Hydrohalogenation
Hydrohalogenation, as its name implies, involves the addition of a water or hydrogen atom, and halogen to an alkene
The reaction can be written as follows:
Alkene + HX —> Haloalkane (aka alkyl halide)
To understand the mechanism, we must first understand the nature of the reacting molecules.
The HX Electrophile
HX is a molecule made up of hydrogen bonded to a halogen. Halogen can include fluorine, chlorine, bromine and iodine.
Since halogen is highly electronegative, it will attract the electrons that bind it to the carbon atom. This will result in an uneven distribution of charge between the atoms, giving the halogen a partial negative charge and the hydrogen a partial positive charge.
Partial positive hydrogen is considered an electrophile.
The alkene nucleophile
An alkene is a molecule that contains a double bond or pi bond between 2 carbon atoms. Unlike a sigma bond that sits between the 2 bonded atoms, a pi bond sits very high and very low in the p orbitals. This allows you to easily “get distracted” by nearby electrophiles.
Addition of electrophilic alkenes
When an HX approaches the alkene, the pi electrons will reach the positive hydrogen atom. In doing so, the pi bond is broken when electrons are used to form a new bond between hydrogen and carbon.
Since hydrogen can only have one bond, it is forced to drop the electrons that bind it to the halogen. This allows the halogen to float as a negative halide in solution.
The carbon that did not receive the hydrogen atom is now electron deficient, resulting in a positive formal charge. This is called a carbocation.
The carbocation will form on the more substituted carbon, since it is more stable and therefore better able to retain the positive charge. This follows the Markovnikov rule
Eventually, the negative halide in solution will extend to attack the carbocation with a lone pair of electrons. In doing so, it forms a pi bond between itself and carbon.
