When studying alkene reactions in organic chemistry, you will often come across a mechanism that requires you to place two halogen atoms on an alkene. Understanding the mechanism will help you visualize the products and allow you to perform this reaction flawlessly on your upcoming quiz or exam. In this article I will help you understand that what and how of the halogenation mechanism
This mechanism, as the name implies is related to halogens or halides. These atoms are of particular importance to organic chemistry due to their high electronegativity and reactivity. You will particularly see the halogenation mechanism carried out using a dibromine or dichlorine added across a carbon to carbon double bond. This results in a halogenated product contained two neighboring halogens
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
The mechanism begins when the nucleophilic pi electrons reach out for and grab the partially positive halogen. This causes both the dihalide bond, and the carbon to carbon double bond to break. Only one of the carbon atoms will be attached to the halogen via a sigma bond formed from the former pi electrons. The second carbon remains deficient, and as a result has a formal charge of plus one. There will also be one lone halide ion in solution with a negative charge
Despite being attached to a carbon atom with a full octet, the halogen bound to carbon is attracted to the nearby carbon atom's positive charge. It will therefor reach out and attack this carbocation with one of it's lone electron pairs forming a bridge between the two carbon atoms and itself. Since the halogen now has two bonds and two lone pairs, it gets a formal charge of positive one.
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
This mechanism, as the name implies is related to halogens or halides. These atoms are of particular importance to organic chemistry due to their high electronegativity and reactivity. You will particularly see the halogenation mechanism carried out using a dibromine or dichlorine added across a carbon to carbon double bond. This results in a halogenated product contained two neighboring halogens
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
The mechanism begins when the nucleophilic pi electrons reach out for and grab the partially positive halogen. This causes both the dihalide bond, and the carbon to carbon double bond to break. Only one of the carbon atoms will be attached to the halogen via a sigma bond formed from the former pi electrons. The second carbon remains deficient, and as a result has a formal charge of plus one. There will also be one lone halide ion in solution with a negative charge
Despite being attached to a carbon atom with a full octet, the halogen bound to carbon is attracted to the nearby carbon atom's positive charge. It will therefor reach out and attack this carbocation with one of it's lone electron pairs forming a bridge between the two carbon atoms and itself. Since the halogen now has two bonds and two lone pairs, it gets a formal charge of positive one.
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 this page about Alkene Halogenation Mechanism
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