What are Alkyl Halides?
Alkyl halides are organic compounds with the general formula RX, where R denotes the alkyl group and X denotes the halogen.
Alkyl halides and aryl halides are the two different types of substituted hydrocarbons (compounds composed of hydrogen and carbon). In these hydrocarbons, one or more of the hydrogen atom(s) is replaced by a halogen (group 17 elements). Alkyl halides are also known as haloalkanes and aryl halides are also known as haloarenes.
The key difference between alkyl halides and aryl halides is that haloalkanes are derived from alkanes (open chain hydrocarbons) while haloarenes are derived from aromatic hydrocarbons (hydrocarbons with sigma bonds and delocalized pi electrons between carbon atoms forming rings).
Haloalkanes and Haloarenes can be prepared from other organic compounds by numerous methods. Some of the methods of preparation of alkyl halides and aryl halides are underlined below:
Preparation of Alkyl Halides
1. Preparation of Alkyl Halides from Alkenes
The addition of hydrogen halides to alkenes follows either Markovnikov’s rule or exhibit Kharash effect. All the electrophilic addition reactions of alkenes following Markovnikov rule are known as Markovnikov addition reactions. A general example of such reaction is given below:
2. Preparation of Alkyl Halides from Alcohols
Alkyl halides can easily be prepared from alcohols upon the addition of halides. In this reaction hydroxyl group of alcohol is replaced with the halogen atom attached to the other compound involved. This reaction requires a catalyst for primary and secondary alcohols whereas it doesn’t require any catalyst for tertiary alcohols.
3. Preparation of Alkyl Halides by Free Radical Halogenation
A complex mixture of isomeric mono- and polyhaloalkanes upon free radical chlorination or bromination of alkanes.
Preparation of Aryl Halides
1. Preparation of Aryl Halides via Electrophilic Substitution Reactions
Aryl halides can be prepared by electrophilic aromatic substitution of arenes with halogens in the presence of a Lewis acid.
2. Preparation of Aryl Halides through Sandmeyer’s Reaction
Aryl halides can also be prepared by mixing the solution of freshly prepared diazonium salt from the primary aromatic amine with cuprous chloride or cuprous bromide.
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