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MHT - CET : Chemistry - Halogen Derivatives of Alkanes Page 1

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Halogen Derivatives of Alkanes

The compounds formed by the replacement of one or more hydrogen atoms of alkanes by halogen atoms are called halogen derivatives of alkanes.
Examples: CH3Cl (methyl chloride), C2H5 Br (ethyl bromide)

 

 

Classification

 

Halogen derivatives of alkanes

Monohalogen
derivatives contain
one halogen atom
per molecule,
e.g. CH3Cl

Dihalogen
derivatives contain
two halogen atoms
per molecule,
e.g. CH2Cl2

Trihalogen
derivatives contain
three halogen atoms
per molecule,
e.g. CHCl3

Tetrahalogen
derivatives contain
four halogen atoms
per molecule,
e.g. CCl4

 

 

Monohalogen Derivatives


They are also called alkyl halides. Their general formula is CnH2n+1 X where X = Cl, Br or I. They are also represented as R
- X, where R stands for an alkyl group.
Nomenclature according to:
Common system
- alkyl halide
IUPAC system
- haloalkane

 

 

Classification of Monohalogen Derivatives of Alkanes or Alkyl Halides


Alkyl halides are classified into the following three classes depending on the number of alkyl groups attached to the carbon atom bearing the halogen atom:

  1. Primary Alkyl Halide: The alkyl halide in which the carbon atom bearing the halogen atom is attached to not more than one other carbon atom or alkyl group is called a primary alkyl halide.
    General Structure: R
    - CH2 - X
    Examples: CH3
    - CH2 - Cl, CH3 - CH2 - Br
  2. Secondary Alkyl Halide: The alkyl halide in which the carbon atom bearing the halogen atom is attached to two other carbon atoms or alkyl groups is called a secondary alkyl halide. In such halides, the halogen bearing carbon atom is attached to only one hydrogen atom.
    General Structure:
    Example: (CH3)2CH
    - Cl (isopropyl chloride)
  3. Tertiary Alkyl Halide: The alkyl halide in which the carbon atom bearing the halogen atom is attached to three other carbon atoms or alkyl groups is called a tertiary alkyl halide. In such halides, the halogen bearing carbon atom is devoid of any hydrogen atom.
    General Structure:
    Example: (CH3)3 C
    - Cl (tertiary butyl chloride)

 

 

Preparation

  1. By Halogenation of Alkanes

Halogenation is a substitution reaction. The order of reactivity of halogens is
F2 > Cl2 > Br2 > I2

    1. R - H + Cl2 R - Cl + HCl

      It is a free radical substitution reaction.
    2. R - H + Br2 R - Br + HBr
      It is an electrophilic substitution reaction.
    3. R-H + I2 R - I + HI
      or
      5R
      - H + 2I2 + HIO3 5R - I + 3H2O
    4. Direct iodination of alkanes using iodine is a highly reversible reaction. However Iodic acid (HIO3) used in the reaction acts as an oxidising agent and removes HI. Thus, the reverse reaction is prevented.
      It is an electrophilic substitution reaction.

 

  1. By Addition of Halogen Acid to Alkenes:

It is an addition reaction.

    1. R - CH = CH2 + HBr

      The order of reactivity of halogen acids is HI > HBr > HCl > HF.
      It is an electrophilic addition reaction.
    2. R - CH = CH2 + HBr R - CH2 - CH2Br
      It is a free radical addition reaction.
      Peroxide effect is applicable only for HBr and not for
      HCl, HI, H2SO4, etc.

 

  1. By the Action of Phosphorus Halides and Thionyl Chloride to Alcohols:
    It is a nucleophilic substitution reaction.
    1. 3R - OH + PCl3 3R - Cl + H3PO3

      R
      - OH + PCl5 R - Cl + HCl + POCl3
      R
      - OH + SOCl2 R - Cl + HCl + SO2
    2. 2P + 3Br2 2PBr3

      3R
      - OH + PBr3 3R - Br + H3PO3
    3. 2P + 3I2 2PI3

      3R
      - OH + PI3 3R - I + H3PO3

 

 

 

Reactions of Alkyl Halides


As the electronegativity difference between the carbon and halogen atoms is high, alkyl halides are highly polar in nature.
The order of reactivity of alkyl halides is: Tertiary > Secondary > Primary

  1. Nucleophilic Substitution Reactions
    1. With Alkali (Alkaline Hydrolysis):
    2. With Aqueous Ag2O:
      2R
      - X + Ag2O + H2O 2AgX + 2R - OH
      (Ag2O + H2O 2AgOH)
    3. With Potassium Cyanide (KCN):
    4. With Ammonia [Ammonolysis]:

    5. With Sodium Alkoxide (Na - OR) [Williamson's Synthesis]:
    6. With Silver Salt of Carboxylic Acids (R - COOAg):
      R
      - COOAg + R - X R - COOR + AgX
    7. Wurtz Reaction:
  2. Formation of Grignard Reagent

    R
    - X + Mg

    Grignard reagent is an organo metallic compound. Formation of Grignard reagent is a nucleophilic addition reaction.
  3. b -Elimination Reaction [Dehydrohalogenation]:



    Homolytic Fission (Homolysis)
    The symmetrical breaking of a covalent bond in such a way that each partner atom acquires one electron from the bonding pair is called homolytic fission.

    A
    - B

    The fission takes place in the presence of uv. light, peroxide or due to high temperature.
    The carbon atom in alkyl free radical shows sp2 hybridisation and has a planar structure.
    The order of stability of carbon free radical is:
    Tertiary > Secondary > Primary > Methyl group

    Heterolytic Fission (Heterolysis):

    The asymmetrical breaking of a covalent bond in such a way that one of the partner atoms retain the complete bonding pair of electrons is called a heterolytic fission.

    A
    - B A+ + B:- or A:- + B+

    The fission takes place in presence of a polar solvent.

Carbonium ion (Carbocation)

Carbanion

1.

It is an organic species in which the carbon atom carries a positive charge.

It is an organic species in which the carbon atom carries a negative charge.

2.

It is an electron deficient species.

It is an electron rich species.

3.

It is a planar ion.

It is a tetrahedral ion.

4.

The carbon atom involved is sp2- hybridised.
Example: (CH3)3C+

The carbon atom involved is sp3 - hybridised.
Example:
- : CH3

5.

Order of stability is: 3 > 2 > 1 > CH3-

Order of stability is: CH3-> 1 > 2 > 3

6.

Carbon atom has 6 electrons.

Carbon atom has 8 electrons.

  1.  

Electrophile

Nucleophile

1.

An electrophile is an electron-deficient species.

A nucleophile is an electron rich species.

2.

It attacks an electron rich centre of the substrate. Examples: H+, CH3+, R+, Lewis acids.

It attacks an electron deficient centre of the substrate. Examples: OH-, Cl-, RCOO-, Lewis bases.

  1.  

Inductive effect

Electromeric effect

1.

The polarisation of a single (s) covalent bond due to the electronegativity difference between the atoms joined by it is called inductive effect.

The shift of the p-bond electrons in a double bond to one of the atoms joined by it in the presence of a suitable reagent is called electromeric effect.

2.

It is present in a s-bond.

It is possible only in a p-bond.

3.

It is a permanent effect.

It is a temporary effect.

 

 

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