Haloalkanes and Haloarenes
MCQsHaloalkanes are derivatives of hydrocarbon in which hydrogen atom is replaced by halogen atom (F, Cl, Br, I). We can also say that the compounds containing a halogen atom bonded to a saturated (sp3) carbon are haloalkanes. Haloalkanes are also called alkyl halide as halide is directly attached to alkyl group.
Haloarenes are derivatives of benzene in which hydrogen atom is replaced by halogen atom (F, Cl, Br, I). In haloarenes, the halogen atom is bonded to unsaturated (sp2) carbon. Haloalkanes are also called aryl halide as halide is directly attached to aryl group. Haloalkanes and haloarenes does not occur in nature and they are synthesized in the laboratory.
Classification of Haloalkanes and Haloarenes
On the Basis of Number of Halogen Atoms
These may be classified as mono, di, or poly halogen (tri-, tetra-, etc.) compounds depending on whether they contain one, two or more halogen atoms in their structures. Mono halo compounds may further be classified according to the hybridisation of the carbon atom to which the halogen is bonded, as discussed below-
Compounds Containing sp3 C—X Bond (X = F, Cl, Br, I)
Halogen atom is bonded to an alkyl group (R). They form a homologous series represented by CnH2n+1X. They are further classified as primary, secondary or tertiary according to the nature of carbon to which halogen is attached.
Allylic halides
Halogen atom is bonded to an sp3-hybridised carbon atom next to carbon-carbon double bond (C=C) i.e. to an allylic carbon.
Benzylic halides
Halogen atom is bonded to an sp3-hybridised carbon atom next to an aromatic ring.
Compounds Containing sp2 C—X Bond
Vinylic halides
Halogen atom is bonded to an sp2-hybridised carbon atom of a carbon-carbon double bond (C = C).
Aryl halides
Halogen atom is bonded to the sp2 -hybridised carbon atom of an aromatic ring.
Nomenclature
Nomenclature of Haloalkanes
The following rules are used for naming haloalkanes according to the IUPAC system.
1. The longest chain of the carbon atoms bearing the halogen atom is selected.
2. Numbering of the carbon atoms in the chain is done in such a way that the carbon atom bearing the halogen atom gets the lowest number.
3. The word chloro, is prefixed to the parent hydrocarbon name.
4. In case of alkyl substituted haloalkanes, the longest chain containing halogen atom is selected for numbering.
5. When two or more halogen atoms are present in a compound, the longest chain selected must contain the maximum number of halogen atoms. The multiplicative prefixes (di, tri, tetra, etc.) are added before the name of halogen atom to indicate the number of halogen atoms.
Nomenclature of Haloarenes
In naming a haloarene, the prefix chloro, bromo-or iodo- etc. is are added to name of arene according to halogen(s) present. The relative positions of halogen atoms are indicated by appropriate numbers. The prefixes ortho (o-), meta(m-) and para (p-) are also commonly used respectively to indicate the relative positions i.e. 1,2- ;1,3 − and 1,4- of substituents in a benzene ring.
Nature of C-X Bonds
Halogens are more electronegative than carbon. Thus, the electron density along the C–X bond is displaced in the direction of the halogen. Thus, the C–X bond in polar in nature. The carbon atom bears a partial positive charge (δ+) and the halogen atoms bears a partial negative charge (δ–). Depending upon the type of halogen attached to the haloalkanes, the partial charge on C–X bond and the polarity may differ.
Haloarenes have the halogens bonded to sp2 hybridized carbon of the benzene ring. The C–X bond is polar, too, as in haloalkanes. However, the difference between the C–X bond in haloalkanes and haloarenes is that the C–X bond acquires a partial double bond character because the partial charge on the halogens is involved in resonance with the ring. Hence, the polarity of the C–X bond in haloarenes is lesser than the haloalkanes, and the bond length of the C–X is also shorter than the haloalkanes.
Some Useful Poly Halogen Compounds
Large number of polyhalogen aliphatic and aromatic halogen compounds are known. These are extensively used as solvents, pesticides, anaesthetics etc. Some important compounds are chloroform (CHCl3), iodoform (CHI3), carbon tetrachloride (CCl4), benzene hexachloride (BHC), DDT, etc.
Chloroform (CHCl3)
Chlorofom is a derivative of the simplest hydrocarbon, methane. Its IUPAC name is trichloromethane. Chlorofom is prepared in the laboratory by treating ethanol or propanone with chlorine gas in the presence of an alkali. Chlorofom is a colourless sweet smelling liquid (b.p. 334K). CHCl3 is slowly oxidized by air in the presence of light to a poisonous gas, phosgene. Therefore, chloroform is stored in dark coloured bottles to protect it from light. Chloroform is used in isocyanide test for the detection of primary amines.
Iodoform (CHI3)
Iodoform is a pale yellow solid with a distinct smell. Its IUPAC name is triiodomethane. Iodoform is prepared by heating ethanol or acetone with iodine in the presence of alkali. Yellow crystals of iodoform can easily be recognized by the characteristic smell. Formation of iodoform is used to test compounds containing methyl ketone and secondary alcohol.
Dichlorodiphenyltrichloroethane (DDT)
DDT is available in powder, aerosols, granules etc. forms. DDT is used mainly to control mosquito-borne malaria. It is also used as an argicultural insecticide. The use of DDT has been banned in many countries because being non-biodegradable, it accumulates in environment. It is toxic to other living organisms such as: mammals, birds, fishes, etc.
