Haloalkanes and Haloarenes

Haloalkanes Haloalkanes are the halogen derivatives of alkanes. On the basis of number of halogen atoms, haloalkanes are classified as mono, di, tri or polyhalogen compounds. On the basis of nature of carbon atom, haloalkanes are classified as primary, secondary and tertiary. In common system, monohalogen derivatives of alkanes are named as alkyl halides. In the IUPAC system, the monohalogen derivatives of the alkanes are named as haloalkanes. Haloalkanes are usually prepared in the laboratory from alcohols, hydrocarbons and by halogen exchange (Finkelstein reaction, Swarts reaction). In the preparation of alkyl halide from alkene, the major product is predicted by Markovnikov’s rule or Anti Markovnikov rule (Peroxide effect or the Kharasch effect). The boiling points of haloalkanes are comparatively higher than the corresponding hydrocarbons. In spite of the polarity within the molecules, haloalkanes, are insoluble in water. The reactions of the haloalkanes are divided into three categories: nucleophilic substitution reactions, elimination reactions and reactions with metals. Nucleophilic substitution reactions are classified in two categories: Substitution Nucleophilic Unimolecular (SN1) and Substitution Nucleophilic Bimolecular (SN2). SN1 is a two-step reaction, which proceeds via the carbocation intermediate. SN2 is a single step reaction and it proceeds via the transition state (unstable state). SN2 reactions proceed via the inversion of the configuration of the product relative to the configuration of the reactants called ‘Walden inversion’. High concentration of strong nucleophile and presence of polar aprotic solvent favours SN2 mechanism. Weak nucleophile and presence of polar solvent favours SN1 mechanism. In case of optically active reactant, a SN2 reaction proceeds with complete stereochemical inversion. In case of optically active reactant, a SN1 reaction proceeds with racemisation. When a beam of plane polarized light is passed through a solution of organic compound, the plane of polarisation of the light that emerges is rotated relative to the original plane. This phenomenon is known as optical activity. The carbon attached with four different group or atoms is called asymmetric carbon which is responsible for the optical activity. The objects which are non-superimposable on their mirror image are said to be chiral. The stereo-isomers which are related to each other as non-superimposable mirror images are called enantiomers. If one of the enantiomers is dextrorotatory, the other will be laevorotatory. A mixture containing two enantiomers, if in equal proportions, will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer. Such a mixture is known as racemic mixture. A racemic mixture is represented by prefixing dl or (±) before the name. The process of conversion of enantiomer into a racemic mixture is known as racemisation. Retention of configuration is the preservation of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation. The formation of a product during the dehydration and dehydrohalogenation reaction is governed by Saytzeff rule. Alkyl halides react with metallic sodium in the presence of dry ether to form symmetrical alkanes. This is called Wurtz reaction. Haloarenes Haloarenes are the halogen derivatives of the aromatic hydrocarbons. In haloarene (aryl halide), the carbon atom attached to the C–Cl bond is sp2 hybridised. On the basis of the number of the halogen atoms present, the haloarenes may be classified as mono, di or polyhalogen (tri-, tetra-, etc.) compounds. In case of monohalogen compounds, prefix halo is added before the name of the aromatic hydrocarbon in common as well as in IUPAC system. For the dihalogen derivative, the prefixes o-, m-, and p- are used before the name of the haloarenes in the common system. In the IUPAC system, the position of the substituents is defined by numbering the carbon atoms of the ring such that the substituents are located at the lowest possible numbers. Haloarenes can be prepared from diazonium salt or by the electrophilic substitution reactions of benzene. Halobenzene can be prepared from the diazonium salt by Sandmeyer reaction, Gattermann reaction, Replacement of diazonium group by iodine and Replacement of diazonium group by fluorine. Chlorobenzene as well as bromobenzene can be prepared by the electrophilic substitution reaction of benzene. This reaction is carried out at low temperature, in the absence of sunlight and in the presence of Lewis catalyst such as FeCl3 and anhydrous AlCl3. Fluoro compounds are not prepared by this method because of the high reactivity of fluorine.Electrophilic substitution reaction of benzene with I2 is reversible in nature. Hence, the presence of an oxidising agent is required to oxidise the HI formed during iodination. For the same aryl group, the melting and boiling point increases as the size of the halogen atom increases. The melting point of the p-isomer of dihalobenzene is always higher than that of o- and m- isomer. Although the C–X bond is polar, the haloarenes are insoluble in water because of the greater effect of the non-polar nature of the aromatic ring. These are however soluble in non-polar solvents. Reactions of haloarenes can be broadly classified into three categories and these are Nucleophilic Substitution Reactions, Electrophilic Substitution Reactions and Reactions with Metals. Haloarenes are comparatively unreactive towards nucleophillic reagents under ordinary laboratory conditions. The presence of an electron withdrawing group at ortho and para positions (but not at m-positions) increases the reactivity of the haloarenes towards the nucleophilic substitution reaction as electron withdrawing group stabilises the intermediate carbanion formed during the reaction. Haloarenes undergo the usual electrophilic substitution reactions of the benzene ring such as halogenation, nitration, sulphonation, Friedel-Craft alkylation and Friedel-Craft acylation. In the electrophilic substitution reaction of haloarenes, halogen atom is slightly deactivating and o, p-directing. Besides the nucleophilic and electrophiic substitution reactions, haloarenes also undergo Wurtz–Fittig reaction and Fittig reaction. Polyhalogen Compounds Organic compounds, containing more than one halogen atom, are referred as polyhalogen compounds. Some commercially important polyhalogen compounds are dichloromethane (methylene chloride), trichloromethane (chloroform), tetrachloromethane (carbon tetrachloride), triiodomethane (Iodoform), freons and p,p’-dichlorodiphenyltrichloroethane (DDT). The chemical formula of dichloromethane is CH2Cl2. Dichloromethane is widely used as a solvent, paint remover and propellant in aerosols. In humans, direct skin contact with methylene chloride causes intense burning and mild redness in the skin. The chemical formula of trichloromethane is CHCl3.The major use of chloroform today is in the production of the freon refrigerant R-22. It was once used as a general anesthetic in surgery but has been replaced by less toxic, safer anesthetics, such as ether. Chloroform is slowly oxidised by air in the presence of light to an extremely poisonous gas, phosgene. It is therefore stored in completely filled, closed dark colored bottles so that the air is kept out. The chemical formula of triiodomethane is CHI3. Triiodomethane was used as an antiseptic due to the liberation of free iodine for dressing wounds. The chemical formula of tetrachloromethane is CCl4. It is used as a solvent in laboratory. When carbon tetrachloride is released into the air, it depletes the ozone layer, leading to increased skin cancer, eye diseases and disorders and possible disruption of the immune system. The chlorofluorocarbon compounds of methane and ethane in which all the H-atoms are replaced by the halogen atoms are collectively known as freons. Freons are extremely stable, unreactive, non-toxic, non-corrosive and easily liquefiable gases. Freon 12 (CCl2F2) is one of the most common freons in industrial use. p,p’-Dichlorodiphenyltrichloroethane (DDT) is the first chlorinated organic insecticide. The use of DDT increased enormously after World War II, primarily because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus. The chemical stability of DDT and its fat solubility compounded the problem. Animals do not metabolise DDT rapidly and it is deposited and stored in the fatty tissues.

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