As an organic chemistry student, you will be faced with a number of electrophilic alkene addition mechanisms. One in particular is the halogenation mechanism, which is an important intermediate when working through the synthesis of simple to complex product. In this article I will help you understand the details in the hopes that this helps you better understand the nature of alkene mechanisms in general
The Halogenation mechanism, is the addition of two halogen atoms across a pi bond, to a carbon to carbon double bond. The halogenation reaction mechanism is an example of an electrophilic alkene addition, since the alkene reactant in question will break its carbon to carbon double bond in order to accept a halogen atom to replace each atom within the former pi bond
Since this is an electrophilic alkene addition, it is important to understand the nature and reactivity of each of the reactants. The alkene, with its negative pi electrons is considered the nucleophile in this reaction. The dihalide, while non-polar to begin with, will obtain and induced polarity when in proximity to the alkene. The partially positive halide, while temporary, will be the electrophile in this reaction
When the nucleophilic pi electrons reach out for the electrophilic halogen the double bond between the carbon atoms will break. The former pi electrons will now form a bond between one of the carbon atoms and one of the halogen atoms. The second halogen will collect the former halogen bond electrons and disappear into solution with a full octet and negative charge. The carbon atom that does not get the halogen bond will form a carbocation which is simply a carbon atom that has a positive charge
The Halogen that is attached to one of the carbon atoms is still quite electronegative. It will therefor use one of its lone electron pairs to reach out to the carbocation and form a second bond. Since the halogen is now bound to both carbons it forms a halogen bridge. This in turn places a positive charge on the halogen.
Halogen atoms are quite electronegative and therefor very unstable when given a positive charge. The halogen bridged atom will pass some of its positivity to both of the connected carbon atoms. This in turn will attract the lone halide from solution to attack. The negative halide attacks one of the carbon atoms, breaking its bond with the bridged halogen. This in turn yields the desired halogenation product with two halogens bound to neighboring carbon atoms where the pi bond used to be
The Halogenation mechanism, is the addition of two halogen atoms across a pi bond, to a carbon to carbon double bond. The halogenation reaction mechanism is an example of an electrophilic alkene addition, since the alkene reactant in question will break its carbon to carbon double bond in order to accept a halogen atom to replace each atom within the former pi bond
Since this is an electrophilic alkene addition, it is important to understand the nature and reactivity of each of the reactants. The alkene, with its negative pi electrons is considered the nucleophile in this reaction. The dihalide, while non-polar to begin with, will obtain and induced polarity when in proximity to the alkene. The partially positive halide, while temporary, will be the electrophile in this reaction
When the nucleophilic pi electrons reach out for the electrophilic halogen the double bond between the carbon atoms will break. The former pi electrons will now form a bond between one of the carbon atoms and one of the halogen atoms. The second halogen will collect the former halogen bond electrons and disappear into solution with a full octet and negative charge. The carbon atom that does not get the halogen bond will form a carbocation which is simply a carbon atom that has a positive charge
The Halogen that is attached to one of the carbon atoms is still quite electronegative. It will therefor use one of its lone electron pairs to reach out to the carbocation and form a second bond. Since the halogen is now bound to both carbons it forms a halogen bridge. This in turn places a positive charge on the halogen.
Halogen atoms are quite electronegative and therefor very unstable when given a positive charge. The halogen bridged atom will pass some of its positivity to both of the connected carbon atoms. This in turn will attract the lone halide from solution to attack. The negative halide attacks one of the carbon atoms, breaking its bond with the bridged halogen. This in turn yields the desired halogenation product with two halogens bound to neighboring carbon atoms where the pi bond used to be
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To read about this mechanism in greater detail, visit my page about the Halogenation Of Alkenes
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