Title SP-3_Ch-16_Haloalkanes and Haloarenes.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456  Introduction  Classification  IUPAC Nomenclature  Methods of Preparation of Haloalkanes  Physical Properties  Chemical Properties  Stereochemical Aspects of Nucleophilic Substitution Reactions  Reactions of Haloarenes  Polyhalogen Compounds Introduction Alkyl halides or haloalkanes and aryl halides or haloarenes are organic compounds obtained by replacement of one or more H-atoms of aliphatic and aromatic hydrocarbons respectively by halogen atom(s). Haloalkanes have each of the halogen atom attached to sp3 hybridised C-atom of the alkyl group. Haloarenes have each of halogen atom attached to sp2 hybridized C-atom of aryl group. CLASSIFICATION These compounds may be classified as : Based on the Number of Halogen Atoms (a) Monohaloalkanes and monohaloarenes are compounds containing one halogen atom only, e.g., chloroethane, chlorobenzene etc. CH Cl 3 CH2 (Chloroethane) C Cl 6H5 (Chlorobenzene) Monohaloalkanes may be further classified as primary (1°), secondary (2°) or tertiary (3°) depending on whether halogen atom is bonded to 1°, 2° or 3° C-atom, e.g., CH Cl 3 CH CH 2 2 1-Chloropropane (1°) CH 3 CHCl CH3 2-Chloropropane (2°) or Isopropylchloride CH C Cl 3 CH3 CH3 2-Chloro-2-methylpropane (3°) or Tert. butyl chloride (b) Dihaloalkanes and dihaloarenes are compounds containing two halogen atoms. Dihaloalkanes may be further classified as (i) Gem-dihalides containing two halogen atoms attached to the same C-atom and Chapter 16 Haloalkanes and Haloarenes
140 Haloalkanes and Haloarenes NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (ii) Vicinal dihalides containing two halogen atoms attached to the adjacent C-atoms. Gem-dihalides are also known as alkylidene halides and vicinal dihalides are also known as alkylene halides. e.g., CH Cl 2 CH2Cl 1, 2-Dichloroethane or Ethylene chloride (vic-dihalide) 1, 1-Dichloroethane or Ethylidene chloride (gem-dihalide) CH CHCl 3 2 1, 2-Dichlorobenzene or -Dichlorobenzene o Cl Cl (c) Polyhaloalkanes and polyhaloarenes are compounds containing three or more halogen atoms. They may be attached to the same or different C-atoms, e.g., CHBr3 CHI CHCl 3 3 Chloroform Bromoform Iodoform CCl4 Carbon tetrachloride Cl Cl Cl 1, 2, 3-Trichlorobenzene Based on the type of sp3 Hybridised C-atom containing Halogen Atom(s) (a) Allylic halides are compounds containing halogen atom bonded to sp3 hybridised C-atom next to carbon, carbon double bond, e.g., 1-Chlorobut-2-ene (1°) CH 3 CH CH CH Cl CH 2 CH CH Cl 2 2 3-Chloropropene (1°) (b) Benzylic halides are compounds containing halogen atom bonded to sp3 hybridised C-atom next to an aromatic ring. For example CH2Cl Benzyl chloride (1°) 1-Chloro-1-phenylethane (2°) CH CH Cl 3 Based on sp2 hybridised C-atom containing Halogen Atom(s) (a) Vinylic halides are compounds containing halogen atom bonded to sp2 hybridised C-atom of an aliphatic compound, e.g., Chloroethene CH 2 CH Cl 1-Chlorocyclopentene Cl (b) Aryl halides are compounds containing halogen atom bonded to sp2 hybridised C-atom of an aromatic ring. For example 1-Chloronaphthalene Cl (m-Chlorotoluene) 1-Chloro-3-methylbenzene Cl CH3
NEET Haloalkanes and Haloarenes 141 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 IUPAC NOMENCLATURE Haloalkanes and haloarenes are named as halogen-substituted hydrocarbons in the IUPAC system of nomenclature. The rules of naming such compounds are same as used for hydrocarbons considering halogens as substituents. The common names of these compounds are derived by writing the appropriate name of alkyl or aryl group followed by halide, e.g., Common name : IUPAC : CH CH Cl 3 CH2 CH3 Isobutyl chloride 1-Chloro-2-methylpropane Neopentyl bromide 1-Bromo-2-2-dimethylpropane CH C Br 3 CH2 CH3 CH3 IUPAC : 2-Chloro-3-methylbutane CH CH 3 CH CH3 Cl CH3 2, 2-Difluorobutane (gem) CH CF 32 2 3 CH CH IUPAC : 2, 3-Difluorobutane (vic) CH CHF CHF CH 3 3 1-Chloro-3-methylbut-2-ene (CH ) C 3 2 2 CH CH Cl IUPAC : 1-Chloromethyl-1, 3-dimethyl cyclopentane CH3 CH Cl H C3 2 H 3-Chlorocyclohex-1-ene Cl Example 1 : Draw all possible geometrical isomers of molecular formula C2FClBrI. Solution : There are six possible geometrical isomers of C2FClBrI as shown below. C C ; ; C C C C Br I Cl I Br I FF F Cl Cl Br C C ; C C ; C C I Cl Br Cl Br Cl FF F Br I I Nature of C-X Bond Alkyl halides are polar molecules having permanent dipole moment due to higher electronegativity of halogen than carbon. The carbon-halogen bond length increases and hence bond energy decreases with the size of halogen atom. In vapour phase dipole moment of Me–X varies as Me–Cl > Me–F > Me–Br > Me–I because dipole moment depends on electronegativity of halogen atom as well as on C–X bond length as shown in the following table. C–X bond Bond length (pm) Bond energy (kJ mole ) –1 Dipole moment (D) Me–F Me–Cl Me–Br Me–I 139 178 193 214 452 351 293 234 1.847 1.860 1.830 1.636