Chapter 10: Haloalkanes and Haloarenes ; Haloalkanes and Haloarenes 10 10.0 Introduction **10.7 Stereochemistry 10.1 Classification 10.8 Nucleophilic substitution mechanism 10.9 Haloarenes 10.2 Monohalogen derivatives of alkanes 10.10 Nature of C X bond in haloarenes 10.3 Nomenclature of haloalkanes *10.11 Preparation of haloarenes 10.4 Nature of C X bond in haloalkanes 10.12 Physical and chemical properties of 10.5 Preparation of haloalkanes haloarenes 10.6 Physical and chemical properties of 10.13 Uses and environmental effects of some haloalkanes haloalkanes and haloarenes * marked section is only for JEE (Main) ** marked section is for NEET UG 10.0 Introduction Halogen derivatives of alkanes or of arenes: When one or more hydrogen atoms of alkanes or arenes are replaced by corresponding number of halogen atoms, the resulting compounds are called halogen derivatives of alkanes (haloalkanes) or halogen derivatives of arenes (haloarenes) respectively. Haloalkanes: The halogen derivatives of saturated aliphatic hydrocarbons are called as haloalkanes or alkyl halides. OR Haloalkanes are obtained by replacing one or more hydrogen atom(s) of an alkane with the corresponding number of halogen atom(s). eg. H3C Cl (Chloromethane) In haloalkanes, halogen atom(s) is/are bonded to sp3 hybridised carbon atom(s) of an alkyl group. Haloarenes: The halogen derivatives of aromatic hydrocarbons are called as haloarenes or aryl halides. OR Haloarenes are obtained by replacing one or more hydrogen atom(s) of an arene with corresponding number of halogen atom(s). In haloarenes, halogen atom(s) is/are bonded to sp2 hybridised carbon atom(s) of an aryl group. Note: Several organic compounds containing halogen exist in nature and some of them are clinically useful. Substance Contains halogen atom Description i. Chloramphenicol (Antibiotic) Chlorine a. Produced by soil micro-organisms. b. Used in treatment of typhoid fever. ii. Thyroxine (Hormone) Iodine a. Produced inside our body. b. Deficiency causes goiter. iii. Chloroquine (Synthetic halogen Chlorine Used in treatment of malaria. compound) iv. Halothane Used as an anaesthetic during surgery. v. Certain fully fluorinated Fluorine Being considered as potential blood compounds substitutes in surgery. 1 Chemistry Vol ‐ 2.2 (Med. and Engg.) 10.1 Classification Haloalkanes and haloarenes are classified as monohalogen derivatives or polyhalogen (di-, tri-, etc.) derivatives of alkanes and arenes respectively, based on the number of halogen atoms in their structure. Classification of haloalkanes on the basis of the number of halogen atoms: Haloalkanes Monohaloalkanes Polyhaloalkanes (Monohalogen derivatives of alkanes) (Polyhalogen derivatives of alkanes) One hydrogen atom of an alkane is More than one hydrogen atom of alkanes are substituted by one halogen atom. substituted by corresponding number of General formula: halogen atoms. C H X [n is an integer] n 2n+1 R X [X = F, Cl, Br, I and R = alkyl group] eg. CH CH Br 3 2 Ethyl bromide (Bromoethane) Dihalogen derivatives Trihalogen derivatives Tetrahalogen derivatives Two hydrogen atoms of an Three hydrogen atoms of Four hydrogen atoms of an alkane are substituted by an alkane are substituted alkane are substituted by two halogen atoms. by three halogen atoms. four halogen atoms. General formula: General formula: General formula: CnH2nX2 CnH2n1X3 CnH2n2X4 [X = F, Cl, Br, I and ‘n’ is [X = F, Cl, Br, I and n is an [X = F, Cl, Br, I and n is an an integer] integer] integer] eg. CHI3 eg. CCl 4 Iodoform Carbon tetrachloride Geminal dihalides Vicinal dihalides Both the halogen atoms are Both the halogen atoms are attached to same C-atom. attached to adjacent eg. H (vicinal) C-atom. eg. H2C CH2 H3C C Br Br Br Br Ethylene dibromide Ethylidene bromide (1,2-Dibromoethane) (1,1-Dibromoethane) 2 Chapter 10: Haloalkanes and Haloarenes Classification of monohalocompounds on the basis of nature of CX bond: Monohalocompounds Compounds containing Compounds containing 2 sp3CX bond sp CX bond Alkyl halides Allylic halides Benzylic halides Vinylic halides Aryl halides (Haloalkanes) Halogen atom is Halogen atom is Halogen atom is (Haloarenes) 3 3 2 Halogen atom is bonded to sp - bonded to sp - bonded to sp - Halogen atom is bonded 2 bonded to an hybridised carbon hybridised carbon hybridised carbon to sp -hybridised carbon alkyl group. atom next to C=C atom next to an atom of C=C. atom of an aro-matic General formula: i.e., to an allylic aromatic ring. eg. ring. X CnH2n+1X. carbon. eg. CH2X eg. X eg. CH2X 10.2 Monohalogen derivatives of alkanes Monohalogen derivatives of alkanes (alkyl halides) are obtained by substituting one hydrogen atom of an alkane by one halogen atom and are further classified as follows: Alkyl halides Primary alkyl halide (1) Secondary alkyl halide (2) Tertiary alkyl halide (3) Halide group is attached to Halide group is attached to secondary Halide group is attached to tertiary primary carbon atom of an carbon atom of an alkyl group. carbon atom of an alkyl group. alkyl group. eg. eg. CH3 CH3 eg. CH3CH2CH2Br n- Propyl bromide H3C C Br H3C C Br (1-Bromopropane) H CH3 Isopropyl bromide tert-Butyl bromide (2-Bromopropane) (2-Bromo-2-methylpropane) 10.3 Nomenclature of haloalkanes Common and IUPAC names of some monohalogen derivatives: No. Formula Common name IUPAC name i. CH3Br Methyl bromide Bromomethane ii. CH3CH2Cl Ethyl chloride Chloroethane iii. CH3CH2CH2Br n-Propyl bromide 1-Bromopropane iv. Br Isopropyl bromide 2-Bromopropane | (sec-Propyl bromide) CH3 CH CH3 v. CH3CH2CH2CH2Cl n-Butyl chloride 1-Chlorobutane vi. CH3CH CH2CH3 sec-Butyl bromide 2-Bromobutane | Br vii. CH3 Isobutyl chloride 1-Chloro-2-methylpropane CHCH2Cl CH3 3 Chemistry Vol ‐ 2.2 (Med. and Engg.) viii. Br tert-Butyl bromide 2-Bromo-2-methylpropane | CH3 C CH3 | CH3 ix. Br tert-Pentyl bromide 2-Bromo-2-methylbutane | CH3 C CH2 CH3 | CH3 x. CH3 Isobutyl bromide 1-Bromo-2-methylpropane CHCH2Br CH3 xi. CH3 Neopentyl iodide 1-Iodo-2,2-dimethylpropane | H3C C CH2I | CH3 xii. CH2 = CHCl Vinyl chloride Chloroethene xiii. CH2 = CH CH2 Br Allyl bromide 3-Bromopropene xiv. CH2Cl2 Methylene chloride Dichloromethane xv. CHCl3 Chloroform Trichloromethane xvi. CCl4 Carbon tetrachloride Tetrachloromethane xvii. CH2Cl Benzyl chloride Chlorophenylmethane 10.4 Nature of C X bond in haloalkanes i. In an alkyl halide, highly electronegative halogen atom is bonded to less electronegative carbon atom. Therefore, C X bond in alkyl halide is polar in nature. ii. The carbon atom carries partial positive charge (+) as it is less electronegative than halogen and halogen atom carries a partial negative charge (). + C X iii. In the formation of CX bond, sp3 hybrid orbital of carbon atom overlaps with half filled p-orbital of halogen atom. iv. C X bond strength decreases down the group 17 of the periodic table because orbital overlap is most efficient between orbital of same principle quantum number (i.e., in the same row of periodic table) and efficiency decreases as difference in principle quantum number increases. Halogen atom Its overlapping orbital in C–X bond F 2pz Cl 3pz Br 4pz I 5pz v. The size of the halogen atom increases from F to I, as a result of which, the bond length also increases and the bond formed is weaker. Hence, C X bond strength in CH3 X decreases in the order: CH3F > CH3Cl > 3 CH3Br > CH3I as the 2sp orbital of carbon cannot penetrate into the larger p-orbitals sufficiently to form strong bonds. 4 Chapter 10: Haloalkanes and Haloarenes Bond enthalpy, bond length and dipole moment of CX bond in CH X: 3 Bond Bond Enthalpy (kJ/mol) Bond Length (Å) Dipole moment (Debye) CH3 F 452 1.42 1.847 CH3 Cl 351 1.77 1.860 CH3 Br 293 1.91 1.830 CH I 234 2.12 1.636 3 10.5 Preparation of haloalkanes Monohalogen derivatives of alkanes (haloalkanes) can be prepared by the following methods: i. From halogenation of alkanes: a. Direct halogenation of alkanes in the presence of UV light, heat or suitable catalyst gives the corresponding alkyl halides. b. The displacement of H-atom from hydrocarbon during halogenation follows the order: Benzylic allylic > 3 H-atom > 2 H-atom > 1 H-atom > H-atom of methane > vinylic arylic c. The reactivity of halogens decreases in the order: F2 > Cl2 > Br2 > I2 d. Fluorination of alkanes is highly exothermic and violent, resulting in the cleavage of CC bonds. Chlorination is fast and exothermic while bromination is slow, as bromination of alkanes is less exothermic than chlorination. Direct iodination is not possible as reaction is reversible and highly endothermic. 1. Chlorination: Alkanes react with chlorine in the presence of UV light or diffused sunlight or at high temperature to yield the corresponding alkyl chlorides. h ,UV light R H + Cl2 orΔ R Cl + HCl Alkane Alkyl chloride h,UVlight eg. CH3 H + Cl2 or CH3 Cl + HCl Methane Methyl chloride 2. Bromination: Alkanes are heated with bromine in the presence of anhydrous aluminium tribromide to give the corresponding alkyl bromides. Anhydrous AlBr3 R H + Br2 R Br + HBr Alkane Alkyl bromide Anhydrous AlBr3 eg. CH3 CH2 H + Br2 CH3 CH2 Br + HBr Ethane Ethyl bromide Note: i. Direct halogenation of an alkane is a chain reaction and follows free radical mechanism. ii. This method of preparation gives the mixture of mono, di, tri and tetra halogen derivatives of an alkane and it is difficult to separate each component in pure form. eg. Preparation of methyl chloride by direct halogenation method results in the formation of mono, di, tri and tetra chloromethane derivatives. h Cl2 Cl2 Cl2 CH4 + Cl2 HCl CH3 Cl HCl CH2Cl2 HCl CHCl3 HCl CCl4 Methane Methyl Dichloromethane Trichloromethane Tetrachloromethane chloride ` Therefore, halogenation (chlorination and bromination) of an alkane is not useful for laboratory preparation of alkyl halide, because it gives mixture of different alkyl halides which are difficult to separate.