Chapter 10 Haloalkanes and Haloarenes

Chapter_10 Haloalkanes and Haloarenes 1. Haloalkanes and haloarenes are obtained by the replacement of a hydrogen atom of an alkane and arene respectively by a halogen atom (F, Cl, Br, I). 2. Haloalkanes are named as X-haloalkanes where, X represents the position of halogen atom. e.g. CH 3CH 2CH 2 I (1-iodopropane). 3. Methods of preparation These are as follows : ● Haloalkanes are obtained from alcohols by treating them with HX (+ anhy. ZnCl 2 ),/ PCl 5, / PCl 3, / SOCl 2 etc. HX (anhy. ZnCl 2 ) R  OH  → R  Cl PCl 5 / PCl 3 / SOCl 2 ● Haloalkanes are also prepared by the action of halogen acid (HX) on alkene. The reaction follows Markownikoff’s rule, which states that the negative part of the reagent goes to the carbon bearing less number of hydrogen atoms. Fluoroalkanes are prepared by treating alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg 2F2, CoF2 or SbF3. This reaction is known as Swarts reaction. + CH 3Br Bromo methane ● AgF Silver fluoride → CH 3F + AgBr Silver Fluoro methane bromide Generally, bromoalkanes are prepared by the reaction known as Borodine-Hunsdiecker reaction. −+ CCl , 350 K 4 CH 3COOAg + Br2   → CH 3Br + CO 2 + AgBr Reflux ● Sandmeyer’s reaction NH2 + – N2X NaNO2 + HX ● X Cu2X2 (i) SN1 mechanism involves formation of carbocation as intermediate. The reactant that gives more stable carbocation, is more reactive towards SN1 reactions. The order of reactivity of alkyl halides towards these reactions is 3°>2°>1°. These reactions give racemisation product, i.e. retention as well as inversion of configurations. (ii) SN2 mechanism involves formation of transition state. Less hindered alkyl halides readily undergo these reactions. These reactions result in the inversion of configuration. The order of reactivity of alkyl halide will be 1° > 2° > 3°. In the presence of polar solvent, KCN readily ionises to furnish CN − ions. As, C C bond is more stable than C N bond, so cyanide is predominantly formed. R  Br + CN − → RCN + Br − ● 273-278 K (X = Cl, Br) ● ● SOCl 2 is the best reagent as by products are gases (SO 2 and HCl). ● Density : Fluoro and chloroalkanes are lighter than water while bromo, iodo and polychloro derivatives are heavier than water. From F to I, the electronegativity of halogen decreases, therefore, polarity in the C  X bond and thus, dipole moment also decreases accordingly. However, fluorides have lower dipole moment than chlorides because of very small size of F. (Strong electron ... electron-repulsion). CH 3Cl > CH 3F > CH 3 Br > CH 3I Alkyl halides are slightly soluble in water, because they do not form H-bonds with water. Due to better symmetry of para isomers as compared to ortho and meta isomers, para isomers have high melting points as compared to their ortho and meta isomers. 5. Chemical properties : With the increase in size of halogen atom, carbon-halogen bond length increases, hence, reactivity increases. Thus, R  I is most reactive towards SN1 and SN 2 reactions. These are as follows: ● + N 2↑ However, AgCN does not ionise, so it attacks through N and results in the formation of isocyanide. R  Br + AgCN → RNC + AgBr Aryl halide ● Finkelstein reaction (For preparation of alkyl lodides) Silver cyanate Acetone CH 3CH 2Br + NaI → CH 3CH 2 I+ NaBr Ethyl bromide Sodium iodide ∆ Ethyl iodide Sodium bromide ● 4 Physical properties ● For same alkyl group, the boiling point of alkyl halides decreases in the order, RI > RBr > RCl > RF This is because with the increase in size and mass of the halogen atom, the magnitude of van der Waals’ forces of attraction increases. As the size of alkyl group decreases, boiling point decreases for the same halogen. CH 3CH 2CH 2 X > CH 3CH 2 X > CH 3 X For isomers of RX having same number of C-atoms, more be the branching, lesser would be the boiling point. Silver bromide Reaction of KNO 3 with RBr results in the formation of RONO 2 (nitrite) whereas, reaction of RBr with AgNO 3 results in the formation of RNO 3 (nitrate) predominantly. 6. Stereochemical Principles and Notation ● ● An object or molecule which is non-superimposable on its mirror image is called chiral and the property is called chirality. A chiral or asymmetric carbon is that, in which all the four valencies are satisfied by four different groups. Such a carbon bearing molecule is generally optically active. An equimolar mixture of two enantiomers (d and l-forms) is called racemic mixture. It is represented as dl or ± forms and will be optically inactive. The process of converting the d or l-form of an optically active compound into racemic mixture (dl ) is called racemisation. 7. Grignard reagents are highly reactive species and react with water (a good source of proton) to give hydrocarbons. (alkanes) ● RMgX + H2O → RH + Mg(OH) X Hence, traces of moisture must be avoided during formation of a Grignard reagent. 8. In electrophilic substitution reactions of benzene ring, Cl, Br, etc. are ortho/para directing groups and direct the incoming group at ortho and para positions of benzene ring. 9. Haloarenes are less reactive than haloalkanes towards nucleophilic substitution reactions, due to resonance effect (double bond character between C Cl bond), sp2-hydridisation of C bearing halogen atom and instability of phenyl cation. 10. Wurtz reaction : Alkyl halides react with sodium metal in the presence of dry ether to give hydrocarbon containing double the number of carbon atoms present in the alkyl halide. Dry ether 2 RX + 2Na → R  R + 2 NaX 11. When aryl halide is heated with alkyl halide in the presence of sodium and dry ether, halogen atom is replaced by alkyl group and alkylarene is formed. This is called Wurtz-Fittig reaction. Cl + 2Na + Cl—CH3 Chlorobenzene Dry ether Methyl chloride CH3 +2NaCl Toluene Sodium chloride 12. When haloarenes react with sodium in the presence of dry ether, two aryl groups are joined together and diphenyl is formed. This reaction is called Fittig reaction. Dry ether X + 2Na + X Iodobenzene + 2NaX Diphenyl 13. Carbon compounds having more than one halogen atom are usually referred to as polyhalogen compounds e.g. CH 2Cl 2, CHCl 3, CHI3, CCl 4, freons, DDT. Some polyhalogen compounds are as follows : Chloroform : In the presence of sunlight, chloroform is slowly oxidised by air to produce a highly poisonous gas, carbonyl chloride or phosgene, COCl 2. That’s why, it is stored in dark coloured bottles filled up to the brim. Freons Chlorofluorocarbon compounds of methane and ethane are collectively known as freons. ● ● ● ● In stratosphere, freon is able to initiate chain reactions that can result in depletion of ozone layer. Since, freon has been found to be one of the factors responsible for the depletion of ozone layer, they are being replaced by other harmless compounds in many countries. DDT : (p, p′ dichlorodiphenyltrichloro ethane) is an insecticide, highly toxic and chemically stable. It is not metabolised rapidly by animals. In many countries DDT is used as an insecticide. Cl Cl Cl Cl Cl H