Electrophilic Addition Mechanism (Edexcel A Level Chemistry): Revision Note
Exam code: 9CHO
Electrophilic Addition Mechanism
Electrophilic addition of hydrogen halides
Hydrogen halides such as hydrogen bromide (HBr) are polar as the hydrogen and halogen atoms have different electronegativities
The bromine atom has a stronger pull on the electrons in the H-Br bond
As a result of this, the Br atom has a partial negative and the H atom a partial positive charge
Due to differences in electronegativities of the hydrogen and bromine atom, HBr is a polar molecule
In an addition reaction, the H atom acts as an electrophile and accepts a pair of electrons from the C=C bond in the alkene
The H-Br bond breaks heterolytically, forming a Br- ion
This results in the formation of a highly reactive carbocation intermediate which reacts with the Br- (nucleophile)
Example of an electrophilic addition reaction of HBr and propene to form 1-bromopropane and 2-bromopropane
Electrophilic addition of Halogens
Halogens such as bromine (Br2) are non-polar molecules as both atoms have similar electronegativities and therefore equally share the electrons in the covalent bond
However, when a bromine molecule gets closer to the double bond of an alkene, the high electron density in the double bond repels the electron pair in Br-Br away from the closest Br atom
As a result of this, the closest Br atom to the double bond is slightly positive and the further Br atom is slightly negatively charged
Br2 is a non-polar molecule however when placed close to an area of high electron density it can get polarised
In an addition reaction, the closest Br atom acts as an electrophile and accepts a pair of electrons from the C=C bond in the alkene
The Br-Br bond breaks heterolytically, forming a Br- ion
This results in the formation of a highly reactive carbocation intermediate which reacts with the :Br- (nucleophile)
Example of an electrophilic addition reaction of Br2 and ethene to form dibromoethane
Examiner Tips and Tricks
The stability of the carbocation intermediate is as follows:
tertiary > secondary > primary
When more than one carbocations can be formed, the major product of the reaction will be the one that results from the nucleophilic attack of the most stable carbocation.
Electrophilic addition of water
Water is a weak electrophile, so does not undergo addition reactions with alkenes unless in the presence of a strong acid which can act as a catalyst
H3O+ acts as the electrophile
The reaction occurs in two steps:
Step 1
The π electrons in the C=C are attracted to H3O+
Heterolytic fission occurs and a carbocation is formed
Step 2
Water acts as a nucleophile and donates a pair of electrons to the positive carbon atom forming the C-O bond
An equilibrium is established between the positive product and the deprotonated product (the alcohol)
The H3O+ is regenerated as the catalyst
This mechanism is catalysed by concentrated acid
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