Edexcel A Level Chemistry:复习笔记3.3.4 Electrophilic Addition Mechanism
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 result 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 a 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
Exam Tip
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.
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