Electron pair acceptors (electrophiles) are attracted to the pi-bonded electrons in a carbon double bond and react through electrophilic addition reactions with an alkene.
In electrophilic addition, an electrophile causes the double carbon bond to break and a new bond is formed between the electrophile and one of the carbons.
Alkene + Bromine --> Dibromo-alkene
Alkene + Hydrogen Bromide --> Bromo-alkene
A carbocation (contains positively charged carbon atom) intermediate is formed that a negatively charged species forms a bond with.
Primary carbocations are less stable than secondary and tertiary carbocations, as they experience less of an inductive effect, meaning they are less likely to form during electrophilic addition.
Major and minor products of electrophilic addition reactions are determined by the stability of the intermediate carbocation that forms.
Electrophilic Addition Reactions of Alkenes
The high electron density within a carbon-carbon double bond makes it very vulnerable to ‘attack’ from electron deficient species. Electron deficient species are called electrophiles; they are attracted to the electrons within the carbon-carbon double bond.
The bonding pair of electrons from the pi-bond in the alkene can leave the double bond and form a new covalent bond between the electrophile and one of the carbon atoms. The single (sigma) bond between the carbon atoms is left.
Remember, a carbon atom can form four covalent bonds. The carbon that the electrophile is bonded to now has four bonds, but the other carbon atom from the double bond only has three. As one its electrons was taken from the pi-bond to form a new bond between the electrophile and other carbon atom, it carries a positive charge.
A positively charged carbon atom in a molecule is called a carbocation and is highly reactive.
Because we are ‘opening’ a double bond and adding an electrophile to the original alkene, addition is taking place. The reaction type is electrophilic addition.
Addition of bromine
When bromine comes into contact with a carbon-carbon double bond, electrophilic addition takes place. This is the basis of the ‘bromine water test’ for an alkene. If an alkene is present, bromine (Br ) is removed from the bromine water and thus decolourises the bromine water.
Remember, to start an addition reaction with a carbon-carbon double bond, an electrophile is required. Bromine (Br ) is a diatomic molecule and is non-polar, so it is not an electrophile. The bromine can only act as an electrophile when it is arranged in a certain way with the double bond.
The high electron density of the carbon-carbon double bond induces a dipole in the bromine molecule. The electrons in the bromine molecule are repelled by the electrons in the double bond, resulting in the bromine atom nearest the double bond exhibiting a slightly positive charge. It is this slight positive charge that enables the bromine molecule to act as an electrophile.
Electrons from the carbon-carbon double bond move to the slightly positive bromine atom and a bond forms. Electrons from the Br-Br bond move to the slightly negative bromine atom, leaving a bromide ion with a lone pair of electrons.
This leaves the ‘other’ carbon atom with a positive charge (carbocation). The lone pair of electrons from bromide ion we formed above then forms a bond with this carbon atom.
Addition of hydrogen bromide
When addition occurs to some alkenes, there are two possible products that can be formed.
When hydrogen bromide (HBr) and but-1-ene undergo an addition reaction, 1-bromo-butane and 2-bromo-butane can be formed. The two products are not formed in equal amounts. More 2-bromo-butane is formed than 1-bromo-butane, so 2-bromo-butane is called a major product and 1-bromo-butane is called a minor product.
We can see the reason for this formation of a major and a minor product from the reaction mechanism. In particular, the stabilities of the carbocations formed.
The H-Br bond is polar, so the slightly positive hydrogen atom acts as an electrophile – with a bonding pair of electrons from the carbon bond moving to form a new bond with the hydrogen atom. The hydrogen atom can bond to either one of the carbon atoms in the carbon-carbon double bond.
This now leaves the other carbon with a positive charge (carbocation). Remember, a carbocation is highly reactive and unstable. There are two possible carbocations that can be formed and they both have different stabilities.
Alkyl groups have a slightly electron donating effect, in this case that means they can ‘feed’ electron density towards the carbocation, effectively diluting its positive charge and making it more stable. This is called an inductive effect. Carbocations can fall into three categories – primary, secondary and tertiary.
In the first carbocation, there is only one alkyl group that can ‘feed’ electron density towards the carbocation. So, it is unable to stabilise the positive charge as much as the second carbocation, which has two alkyl groups that can ‘feed’ electron density. In nature, stability wins!
If two intermediates (something formed in the middle of a reaction that is not a product) are possible in a reaction, the one which is most stable (lowest energy) will be formed more readily.
Since the bromide ion, with a lone pair, bonds to the carbocations in each reaction pathway, the position of the carbocation dictates the final position of the bromine in the molecule.
Because the secondary carbocation is more stable than the primary carbocation, it will form more readily. This means more 2-bromo-butane is formed than 1-bromo-butane, so 2-bromo-butane is the major product.