Introduction
The halogen derivatives of aliphatic and aromatic hydrocarbons are called haloalkanes and haloarenes. These are obtained when the hydrogen atom of an alkane and arene is replaced by a halogen atom (F, Cl, Br and I).
Nature of C-X Bond
The general formula of Haloalkane (CnH2n+1 - X) or Haloarene is 'RX' where R - is an alkyl or aryl group and X is a halogen atom (F, Cl, Br or I). The halogen derived hydrocarbons are a highly reactive class of compounds. The reactivity is due to the presence of polar carbon-halogen bond in these molecules. For a given alkyl or aryl group, the order of reactivity is: Chlorides > Bromides > Iodides.
Nomenclature of Haloalkanes and Haloarenes
The monohalogen derivatives of alkanes are called alkyl halides. Their names are derived by naming the attached alkyl group to halogen and adding the name of the halide. For example,
Alkyl Halides or Haloalkanes
Chain isomerism
Haloalkanes containing four or more carbon atoms exhibit chain isomerism in which the isomers differ in the chain of carbon atoms. For example, C4H9Br has three chain isomers, such as:
Physical Properties of Haloalkanes
Physical state and smell
Haloalkanes are colorless, sweet-smelling liquids. The lower members like methyl chloride, methyl bromide and ethyl chloride are colorless gases while members having very high molecular masses are solids.
Chemical Properties - Nucleophilic Substitution Reactions
Nucleophilic substitution reactions
In the C-X bond there is a partial positive charge on the carbon atom and negative on the halogen atom. Thus nucleophilies attack the electron deficient carbon resulting in the displacement of the weaker nucleophile, the halide ion. Reactions of alkyl halides are generally nucleophilic substitution reactions.
Elimination Reactions (Dehydrohalogenation)
Haloalkanes undergo elimination of hydrogen halide (HX) when boiled with alcoholic solution of potassium hydroxide, resulting in the formation of alkenes. As the hydrogen atom present at b-position of haloalkane (i.e., on the carbon atom next to the one carrying the halogen) is removed, these are called b-elimination reactions or 'dehydro halogenation reactions' (removal of hydrogen halide).
Aryl Halides or Haloarenes
Direct chlorination or bromination of benzene or other aromatic hydrocarbon gives chloroarenes and bromoarenes. These reactions are carried out in the presence of Lewis acids such as ferric or aluminium halides (FeCl3, FeBr3, AlCl3) in the dark, at ordinary temperatures (310-320 K).
Chemical Properties of Haloarenes
Replacement by hydroxy group (formation of phenol)
When aryl halides are heated at 623 K and under pressure (300 atm), with aqueous solution of sodium hydroxide, the halogen atom is replaced by hydroxyl group forming phenol. Firstly sodium phenoxide is formed, which on acidification gives phenol.
This reaction forms the basis for the manufacture of phenol by Dow's process.
Reactivity of C-X Bond
The halogen atom in haloalkane is very reactive and can be easily replaced by nucleophiles whereas the halogen atom in haloarenes is strongly held to the nucleus and cannot be replaced easily with nucleophiles.
Derivatives of Haloalkanes and Haloarenes - Dichloroethanes
Polyhalogen derivatives of alkanes are the halogen derivatives of alkanes containing more than one halogen atom per molecule. For example, ethylene dichloride, ethylidene chloride, chloroform, iodoform, carbon tetrachloride, etc.
Chloroform
Laboratory method of chloroform preparation
Ethanol (ethyl alcohol) or propanone (acetone) when treated with bleaching powder gives chloroform. This reaction is known as haloform reaction.
Iodoform
Laboratory preparation
It is prepared in the laboratory by the action of iodine on ethyl alcohol or acetone, in the presence of alkali. This is called haloform reaction.
Polyhalogen Derivatives - Carbon tetrachloride
Many polyhalogenated hydrocarbons find important use as industrial solvents, pesticides, refrigerants, medicines, inert lining of cooking utensils, etc.
D.D.T. 2,2-bis (p-chlorophenyL) -1,1,1 trichloroethane
Properties and uses of D.D.T.
D.D.T. is almost insoluble in water but it is moderately soluble in polar solvents.
It is a powerful insecticide widely used for killing mosquitoes and other insects.
BHC (benzene hexachloride)
BHC is an important agricultural pesticide mainly used for exterminating white ants, leaf hopper termite, etc. It is also known by the common name gammexane or Undone or 666.
Dichlorodifluoromethane
It is prepared by the action of antimony trifluoride on carbon tetrachloride in the presence of antimony pentachloride.
Freon is non-toxic, non-inflammable, non-corrosive and highly stable even at high temperature and pressures. It has low boiling point and can easily be liquefied by applying pressure at room temperature. Therefore, it is widely used as a refrigerant (cooling agent) in refrigerators and air conditioners. It is also used as a propellant in aerosols and foams.
