Haloalkanes and Haloarenes Structure, Properties, and Reactions

Unit 6 Objectives After studying this Unit, you will be able to · name haloalkanes and haloarenes according to the IUPAC system of nomenclature from their given structures; · describe the reactions involved in the preparation of haloalkanes and haloarenes and understand various reactions that they undergo; · correlate the structures of haloalkanes and haloarenes with various types of reactions; · use stereochemistry as a tool for understanding the reaction mechanism; · appreciate the applications of organo-metallic compounds; · highlight the environmental effects of polyhalogen compounds. Haloalkanes and Haloar enes Haloarenes Halogenated compounds persist in the environment due to their resistance to breakdown by soil bacteria. The replacement of hydrogen atom(s) in an aliphatic or aromatic hydrocarbon by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively. Haloalkanes contain halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group whereas haloarenes contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group. Many halogen containing organic compounds occur in nature and some of these are clinically useful. These classes of compounds find wide applications in industry as well as in dayto-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. Chlorine containing antibiotic, chloramphenicol, produced by microorganisms is very effective for the treatment of typhoid fever. Our body produces iodine containing hormone, thyroxine, the deficiency of which causes a disease called goiter. Synthetic halogen compounds, viz. chloroquine is used for the treatment of malaria; halothane is used as an anaesthetic during surgery. Certain fully fluorinated compounds are being considered as potential blood substitutes in surgery. In this Unit, you will study the important methods of preparation, physical and chemical properties and uses of organohalogen compounds. 6.1 Classification Haloalkanes and haloarenes may be classified as follows: 6.1.1 On the Basis of Number of Halogen Atoms These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.) compounds depending on whether they contain one, two or more halogen atoms in their structures. For example, Monohalocompounds may further be classified according to the hybridisation of the carbon atom to which the halogen is bonded, as discussed below. 6.1.2 Compounds Containing sp3 C—X Bond (X= F, Cl, Br, I) This class includes (a) Alkyl halides or haloalkanes (R—X) In alkyl halides, the 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. If halogen is attached to a primary carbon atom in an alkyl halide, the alkyl halide is called primary alkyl halide or 1° alkyl halide. Similarly, if halogen is attached to secondary or tertiary carbon atom, the alkyl halide is called secondary alkyl halide (2°) and tertiary (3°) alkyl halide, respectively. (b) Allylic halides These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom adjacent to carbon-carbon double bond (C=C) i.e. to an allylic carbon. Allylic carbon Allylic carbon (c) Benzylic halides These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom attached to an aromatic ring. 6.1.3 Compounds Containing 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). (b) Aryl halides These are the compounds in which the halogen atom is directly bonded to the sp2-hybridised carbon atom of an aromatic ring. 6.2 Nomenclature Having learnt the classification of halogenated compounds, let us now learn how these are named. The common names of alkyl halides are derived by naming the alkyl group followed by the name of halide. In the IUPAC system of nomenclature, alkyl halides are named as halosubstituted hydrocarbons. For mono halogen substituted derivatives of benzene, common and IUPAC names are the same. For dihalogen derivatives, the prefixes o-, m-, p- are used in common system but in IUPAC system, as you have learnt in Class XI, the numerals 1,2; 1,3 and 1,4 are used. The dihaloalkanes having the same type of halogen atoms are named as alkylidene or alkylene dihalides. The dihalo-compounds having both the halogen atoms are further classified as geminal halides or gem-dihalides when both the halogen atoms are present on the same carbon atom of the chain and vicinal halides or vic-dihalides when halogen atoms are present on adjacent carbon atoms. In common name system, gem-dihalides are named as alkylidene halides and vic-dihalides are named as alkylene dihalides. In IUPAC system, they are named as dihaloalkanes. Some common examples of halocompounds are mentioned in Table 6.1. Table 6.1: Common and IUPAC Names of some Halides Structure Common name IUPAC name CH3CH2CH(Cl)CH3 sec-Butyl chloride 2-Chlorobutane (CH3)3CCH2Br neo-Pentyl bromide 1-Bromo-2,2-dimethylpropane (CH3)3CBr tert-Butyl bromide 2-Bromo-2-methylpropane CH2 = CHCl Vinyl chloride Chloroethene CH2 = CHCH2Br Allyl bromide 3-Bromopropene o-Chlorotoluene 1-Chloro-2-methylbenzene or 2-Chlorotoluene Example 6.1 Benzyl chloride Chlorophenylmethane CH2Cl2 Methylene chloride Dichloromethane CHCl3 Chloroform Trichloromethane CHBr3 Bromoform Tribromomethane CCl 4 Carbon tetrachloride Tetrachloromethane CH3CH2CH2F n-Propyl fluoride 1-Fluoropropane Draw the structures of all the eight structural isomers that have the molecular formula C5H11Br. Name each isomer according to IUPAC system and classify them as primary, secondary or tertiary bromide. Solution CH3CH2CH2CH2CH2Br CH3CH2CH2CH(Br)CH3 CH3CH2CH(Br)CH2CH3 (CH3)2CHCH2CH2Br o 1-Bromopentane (1 ) o 2-Bromopentane(2 ) o 3-Bromopentane (2 ) o 1-Bromo-3-methylbutane (1 )