Alkyl Halides, Preparation of Alkyl Halides, Question, Electrophilic Substitution Reactions (For CBSE, ICSE, IAS, NET, NRA 2022)

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Title: Alkyl Halides

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

Preparation of Alkyl Halides from Alkenes

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:

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.

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

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.

Electrophilic Substitution Reactions

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.

Preparation of Aryl Halides through Sandmeyer Reaction

Preparation of aryl halides through Sandmeyer՚s reaction


What is Reactivity of Alkyl Halides?


  • Alkyl Halide Reactions The functional group of alkyl halides is a carbon-halogen bond, the common halogens being fluorine, chlorine, bromine and iodine.
  • With the exception of iodine, these halogens have electronegativities significantly greater than carbon.

What Are the Common Names of Alkyl Halides?


  • The common names of alkyl halides consist of two parts: the name of the alkyl group plus the stem of the name of the halogen, with the ending -ide.
  • Give the common and IUPAC names for each compound. Give the IUPAC name for each compound. The alkyl group is a propyl group, and the halogen is bromine (Br) .

What is the Substitution for Alkyl Halides?


  • In alcohol: Substitution to form alkyl halides Alkyl halides are often synthesized from alcohols, in effect substituting a halogen atom for the hydroxyl group.
  • Hydrochloric hydrobromic , and hydroiodic acids are useful reagents for this substitution, giving their best yields with tertiary alcohols.

Why Are Alkyl Halides Good Electrophiles?


  • Alkyl halides are excellent electrophiles because halogens share a polar bond with carbon, are polarizable, and form relatively stable leaving groups as halide anions.
  • In the example below, 2-bromopropane is converted into propan-2-ol in a substitution reaction. Alkyl halides can also undergo elimination reactions in the presence of strong bases.

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