Synthesis of Alkyl Halides

Alkyl halides are organic compounds that are generally derived directly from alkanes and usually have one or more than one halogen atom present. 

They are one of the fundamental building blocks of organic chemistry as they are involved in the production of numerous complex compounds and polymeric chains. 

They are also used in various industries such as refrigerants, aerosols, plastics, and many more. For the synthesis of alkyl halides, the primary chemicals needed are also basic and are readily available. These chemicals only need to be put through a single process to successfully produce alkyl halides. 

Important compounds

Before we learn about the synthesis of alkyl halides, we need to learn about the various compounds that will be utilised in various ways in chemical reactions. 

Alkanes

Alkanes are the most basic kind of organic compound. They consist of simply carbon atoms directly single-bonded with other carbon atoms or hydrogen atoms. They consist only of carbon atoms and hydrogen atoms, and are fully saturated compounds. 

Alkenes

Alkenes are compounds very similar to alkanes other than one noteworthy difference. Alkenes have double bonds present between carbon atoms, apart from the regular single bonds. 

There is unsaturation in these chemicals due to the presence of the double bond, which paves the way for new molecules to come into the compound.

It is also worth noting that the carbon attached to another carbon, involved in a double bond, through a single bond, is said to be an allylic carbon atom.

Alcohols

Alcohols are simple organic compounds with a hydroxyl group (-OH) attached to a carbon atom in the place of a hydrogen atom. They are mostly basic.

Hydrogen halides

Hydrogen halides are diatomic molecules that consist of a single hydrogen atom bonded with a halogen atom. 

The general formula for a hydrogen halide is (HX), where X is a character used for representing the entire halogen group.

Hydrogen halides are generally acidic.

Methods of synthesis of Alkyl Halides

Now that we have a general idea of what alkyl halides are and what the need for them is, let us look at the various ways in which we can synthesise them.

Addition of hydrogen halides to alkenes

Addition of a hydrogen halide (HX) to an alkene results in a monohalogenated alkane, with the hydrogen atom and the halogen breaking the double bond homogeneously and occupying the space so created.

CH2=CH2 + HX → CH3-CH2X

Addition of an element to alkenes

The addition of a pure element compound such as Cl2 or Br2 to an alkene also results in the synthesis of an alkyl halide. The pure element added directly attacks the unsaturation and breaks the double bond. The newly formed vacancy is filled by the atoms of the halogen to create the needed product. The product obtained in this manner is a vicinal alkyl dihalide. 

CH2=CH2 + X2 → CH2X-CH2X

Addition of an element to alkanes

The addition of pure elements to an alkane also results in the synthesis of an alkyl halide. The product is obtained by one atom of the halogen replacing a hydrogen atom, which bonds with the other hydrogen atom to create a hydrogen halide.

The only downside to this method is that we have no control over the chemicals present and multiple substitutions can take place, with the final product being a mixture of multiple compounds.

CH4 + Cl2 → CH3Cl + HCl

CH3Cl + Cl2 → CH2Cl2 + HCl

CH2Cl2 + Cl2 → CHCl3 + HCl

CHCl3 + Cl2 →CCl4 + HCl

Substitution with NBS

N-bromosuccinimide (NBS) is a chemical compound that acts in a  specific way around certain chemicals. Because of such a specific action, this compound only creates alkyl halides when there are allylic hydrogens present in an alkene.

NBS attacks the hydrogen present at the allylic positions in an alkene and replaces them with a bromine atom, thus resulting in a monohalogenated alkyl halide.

CH3CH=CH2 + NBS → CH2BrCH=CH2 + Residual compounds

Treatment of alcohols

The final way for the synthesis of alkyl halides is to treat alcohols with a hydrogen halide such as HCl, HBr, or HI. The whole chemical reaction can be seen as a neutralisation reaction, with the alcohol acting as a base, and the hydrogen halide acting as an acid. The results obtained are the alkyl halide (salt) and water.

CH3CH2OH + HCl → CH3CH2Cl + H2O

Conclusion

Synthesis of Alkyl Halides is important for the formation of different compounds. 

In fact, alkyl halides allow the synthesis of alcohols, organic acids, and unsaturated carbon compounds. The synthesis of alkyl halides is to treat alcohols with a hydrogen halide such as HCl, HBr, or HI. The synthesis of alkyl halides, the primary chemicals needed are also basic and are readily available. These chemicals only need to be put through a single process to successfully produce alkyl halides.