Chapter - 10

Haloalkanes and Haloarenes

Syllabus:

  ΓΌ  Introduction to Haloalkanes and Haloarenes

  ΓΌ  Classification

·       On the basis of number of halogen atoms

·       Compounds containing sp3 C-X bond

·       Compounds having sp2 C-X bond

  ΓΌ  Nomenclature

  ΓΌ  Nature of C-X Bond C-X Bond

  ΓΌ  Methods of preparation

·       From Alcohols

·       From Hydrocarbons

  ΓΌ  Halogen Exchange

  ΓΌ  Physical Properties

  ΓΌ  Chemical Reactions

·       Nucleophilic substitution reactions

·       Elimination reactions

·       Reaction with metals

  ΓΌ  Reactions of Haloarenes

·       Nucleophilic substitution

·       Electrophilic substitution reactions

·       Reaction with metals

  ΓΌ  Polyhalogen Compounds

·       Dichloromethane (Methylene chloride)

·       Trichloromethane (Chloroform)

·       Triiodomethane (Iodoform)

·       Tetrachloromethane (Carbon tetrachloride)

·       Freons

·       p,p’-Dichlorodiphenyltrichloroethane(DDT)

 

    ΓΌ  Introduction:

The replacement of hydrogen atom(s) in a hydrocarbon, aliphatic or aromatic, by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively.

These are compounds containing halogen atoms attached to an alkyl or aryl group. The general representation of haloalkanes is R-X and that of haloarenes is Ar-X [where X = F, Cl, Br, I].

These classes of compounds find wide applications in industry as well as in day-to-day life. They are used as solvents for relatively non-polar compounds and as starting materials for the synthesis of wide range of organic compounds.

ΓΌ  Classification:

Haloalkanes and haloarenes may be classified as follows:

·       On the Basis of Number of Halogen Atoms: Alkyl halides and aryl halides both are classified as mono-, di-, tri- or polyhalogen derivatives depending on the number of halogen atoms present in their structure.

Mono-halogen compounds contain only one halogen atom, dihalo-compounds contain 2 halogen atoms and poly-halogen compounds contain more than 2 halogen atoms.

C2H5X     ------    Mono-halogen Compound         

XCH2 – CH2X       -------     Di-halogen Compound

XCH2 – CHX - CH2X      -------    Poly-halogen Compound


·       Compounds Containing sp3 C-X Bond:

a). Alkyl Halides or Haloalkanes: Here the halogen atom is directly bonded to sp3 hybridized C atom of an alkyl group. They are further classified as primary, secondary or tertiary according to the nature of carbon to which halogen atom is attached. Their general formula may be:

Primary haloalkane: R-CH2-X,           

Secondary haloalkane: R2CH-X

Tertiary haloalkane:    R3C-X

 

b). Allylic Halides: These are the compounds in which the halogen atom is bonded to sp3 hybridized carbon atom next to a C = C bond.  E.g.: CH2=CH-CH2X.

c). Benzylic Halides: These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom next to an aromatic ring. For example. C6H5-CH2-X.

 

·   Compounds having sp2 C-X bond: This class includes:

(a) Vinylic Halides: These are the compounds in which the halogen atom is bonded to a sp2-hybridised carbon atom of a carbon-carbon double bond (C = C).

E.g.: CH2=CH-X.

(b) Aryl halides: Here the halogen atom is directly bonded to sp2 hybridized carbon atom of an aromatic ring.  E.g.: C6H5-X.