Chemistry of Alkyl Halides

Alkyl halides, also known as halo alkanes, are organic compounds in which one or more hydrogen atoms in an alkane are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). Commonly, the halogen is indicated by the -X symbol. So, the general formula of an alkyl halide is R-X.

Alkyl halides are widely used as solvents, refrigerants, pharmaceuticals, and in organic synthesis.

Due to the halogen, haloalkanes are polar compounds. There are dipolar–dipolar interactions as a secondary interaction force in addition to van der Waals forces. Therefore, alkyl halides have higher boiling points and melting points when compared to alkenes with approximately the same molecular masses.

Although alkyl halides are polar compounds, they hardly dissolve in water due to the hydrophobic alkyl group. But they can be easily dissolved in organic solvents.

Classification of alkyl halides

Alkyl halides are categorized into three groups: primary, secondary, and tertiary haloalkanes. According to the number of alkyl groups that are attached to the halogen-bonded carbon, alkyl halides are categorized. If the halogen-bonded carbon is attached to only one alkyl group, such molecules are called primary alkyl halides.

If there are two alkyl groups attached to the halogen-bonded carbon, such molecules are known as secondary alkyl halides.

When the halogen-bonded carbon is attached to three alkyl groups, such molecules are categorized into tertiary alkyl halides.

IUPAC nomenclature of alkyl halides

The halogen in an alkyl halide is not considered a functional group. Therefore, alkyl halides are named similarly to the alkenes. The halogen is considered a substitute for the alkane. If the alkane is attached to halogens of Fluorine, Chlorine, Bromine, or Iodine, they are named fluoro, chloro, bromo, and iodo, respectively.

1. Identify the longest hydrocarbon chain with the maximum number of substituent groups.
2. Number the hydrocarbon chain where the carbon atoms with substituent groups get the lowest possible number,
3. Name the substituent groups according to alphabetical order. When naming a substituent group, it writes the number of the carbon atom and then puts a dash, and then writes the name of the alkyl group.
4. Finally, it writes the stem name of the hydrocarbon chain and adds the suffix “ane” at the end.

• All the letters should be simple letters, and the name must be written as a single word.
• If there are similar substituent groups in different carbon atoms, they are named as follows.

• There must be dashes between numbers and letters, and there must be commas between two numbers.

Preparation of alkyl halides

In the presence of light, alkanes react with Cl₂ to form alkyl chlorides. The reaction occurs through a free radical chain reaction.  From this reaction, a mixture of alkyl halides results. As an example, if methane is used as the initial alkane, it can prepare a mixture of methyl chloride (CH₃Cl), dichloromethane (CH₂Cl₂), chloroform (CHCl₃), and carbon tetrachloride (CCl₄)

When alkenes and alkynes are treated with hydrogen halides of HCl, HBr, and HI, they undergo an Electrophilic addition reaction and form alkyl chlorides, alkyl bromides, and alkyl iodides, respectively. When alkynes are used it is added two halogens are added to each triple bond.

Alkenes and alkynes are treated with Br₂ in the CCl₄ medium. It forms alkyl bromides. One mole of double bonds is added to one mole of Br_2, and one mole of triple bonds is added to two moles of Br₂.

Alkyl chlorides are prepared by treating alcohols with PCl₃ or PCl₅. The -OH group of the alcohol is substituted with Cl and forms an alkyl chloride when PCl₃ is used. Phosphoric(III) acid, H₃PO₃, is formed as a byproduct. If PCl₅ is used, it forms POCl and HCl as byproducts.

Alcohols are heated with PBr₃ or HBr to obtain alkyl bromides. Here, the -OH group of the alcohol is substituted by Br and results in an alkyl bromide.

Alcohols are heated with PI₃ or HI to obtain alkyl iodides. Here, the -OH group of the alcohol is substituted by Iodine (I) and results in an alkyl iodide.