Halogen Addition

Halogen addition is a simple organic reaction in which a halogen molecule is added to functional alkenes. Normally it gives rise to vicinal halides as a product. This is an electrophilic addition reaction, which should not be confused with halogenations. Robert Kimball prepared this concept of halogen addition (1937). The basic idea behind this is, double bonds in a functional alkene react with halogen to form sigma bonds.

Introduction to halogen addition

Halogen addition is the addition of halogen to a functional double-bonded carbon compound (alkenes). The halogens normally participating in halogen addition are chlorine and bromine. There are certain reasons for not including fluorine and iodine to react. As we know, fluorine is highly reactive. It replaces all the hydrogen with carbon. Since it is highly reactive, the reaction can be highly uncontrollable.  The iodine molecules can react with alkenes at low temperatures. But since most of this iodine incorporated compounds are unstable. They reverse back to parent alkenes and iodine as soon as possible.

The product of halogen addition is commonly vicinal dihalides. It means two halogens with adjacent carbon atoms. The halogen addition reactions are a type of halogenations with electrophilic addition. They are generally exothermic.

Let’s study the concept of halogen addition with the example of H- Br to But-2-ene.

CH3-CH=CH-CH3 + HBr → CH3-CH(Br)-CH2-CH3    

The pi bonds are situated above and below sigma bonds with an unpaired electron pair. These double bonds act as a nucleophile now. When a hydrogen atom (electrophile) comes close to the double bond, it gets attracted to the electrons in the nucleophile. This enhances the chances of bond formation. When hydrogen is attached to one of the carbon, it leaves the other carbon atom as a carbocation. Carbocations are carbon with one positive charge. The remaining bromine from H-Br is now acting as a nucleophile. It reacts with carbocation and forms a bond. Thus the halogen addition was completed.

The same reaction mechanism can be explained with Br-Br addition to alkenes.

First bromine is added to the nucleophile (double bond). The second bromine is added to carbocation and forms a sigma bond. This results in a vicinal dihalide called 1,2 dibromoethane. 

Orientation of Addition

In a halogen addition, there is a probability of forming two compounds. But only one of them is practically possible or correct. Let’s take the addition of H-Br to 2-methyl but-2-ene

CH3-C(CH3)=CH-CH3 + HBr → CH3-C(CH3)(Br)-CH2-CH3      +  CH3-C(CH3)-CH(Br)-CH3

(Major) (Minor)

Here first is the observed product with bromine added to the secondary carbon atom. The latter product is not observed. Because the secondary carbon is more stable to act as a carbocation. If there is a tertiary carbon, it will be more stable to act as a carbocation.

30 >20 > 10 – This is the order of stability to be a carbocation.

The reaction of H-Br to alkenes is known as regiospecific- the presence of only one possible orientation.

Importance of halogen addition

• Product of vinyl chloride is used to make products include PVC, furniture and automobile parts
• 1,2 dibromo ethane was used as a pesticide in the past.
• 1,2 Diiodoethane used to synthesise samarium (II) iodide- water complex
• 1,2 difluoro ethane is used as a refrigerant, aerosol propellant and foam blowing agent.

Are halogenation and halogen addition the same?

No. Halogenations are an addition of halogen to alkanes mostly. It follows a free radical pathway of chain reactions.

Halogen addition is an organic reaction where a halogen molecule or halogen-containing compounds are added to alkenes mostly. It is an electrophilic addition reaction.

Conclusion

Halogenations are an addition of halogen to alkanes mostly. It follows a free radical pathway of chain reactions. Halogen addition is an organic reaction where a halogen molecule or halogen-containing compound is added to alkenes mostly. It is an electrophilic addition reaction. Consider the addition of H-Br to But-2-ene. The pi bonds are situated above and below sigma bonds with an unpaired electron pair. These double bonds act as a nucleophile now. When a hydrogen atom (electrophile) comes close to the double bond, it gets attracted to the electrons in the nucleophile. This enhances the chances of bond formation. When hydrogen is attached to one of the carbon. It leaves the other carbon atom as a carbocation. Carbocations are carbon with one positive charge. The remaining bromine from H-Br is now acting as a nucleophile. It reacts with carbocation and forms a bond. Thus the halogen addition was completed.

The halogens normally participating in halogen addition are chlorine and bromine. There are certain reasons for not including fluorine and iodine to react. As we know, fluorine is highly reactive. It replaces all the hydrogen with carbon. Since it is highly reactive, the reaction can be highly uncontrollable.  The iodine molecules can react with alkenes at low temperatures. But since most of these iodine incorporated compounds are unstable. They reverse back to parent alkenes and iodine as soon as possible