Understanding the Alkene Hydrohalogenation Mechanism in Organic Chemistry responses
As an organic chemistry student you will be faced with dozens if not hundreds of reaction mechanisms. Instead of memorizing them all, I highly recommend that you attempt to understand what’s going on in every step. This will ensure that you have what it takes to apply concepts to tricky exam questions. In this article I will help you understand the electrophilic addition reaction: Hydrohalogenation
Hydrohalogenation, as the name implies, involves the addition of a hydro or hydrogen atom, and Halogen to an alkene
The reaction can be written out as follows:
Alkene + H-X —> Haloalkane (also known as alkyl halide)
To understand the mechanism we must first understand the character of the responding molecules.
The H-X Electrophile
H-X is a molecule composed of hydrogen bound to a halogen. The halogen can include Fluorine, Chlorine, Bromine, and Iodine
Since the halogen is highly electronegative it will pull on the electrons that bind it to the carbon atom. This will consequence in an unequal dispensing of charge between the atoms giving the halogen a uncompletely negative charge, and the hydrogen a uncompletely positive charge
The uncompletely 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 lies between the 2 bound atoms, a pi bond sits very high and very low in the p orbitals. This allows it to get easily ‘distracted’ by nearby electrophiles
Electrophilic Alkene Addition
When an H-X comes close to the alkene, the pi electrons will reach out for the positive hydrogen atom. In doing so the pi bond breaks as the 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 away as a negative halide in solution.
The carbon which did not get the hydrogen atom is now a deficient in electrons resulting in a positive formal charge. This is called a carbocation
The carbocation will form on the more substituted carbon as it is more stable and therefor more capable of holding the positive charge. This follows Markovnikov’s rule
Finally the negative halide in solution will reach out to attack the carbocation with a lone pair of electrons. In doing so it forms a pi bond between itself and carbon.