UNIT-10 HALOALKANES AND HALOARENES

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Rolf Morrison
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1 Important terms and concepts: UNIT-10 HALOALKANES AND HALOARENES Alkyl halide or Haloalkane is the compound obtained by replacing one or more hydrogen atoms of an alkane by same number of halogen atoms. Eg: CH3CI,CH3Br R-H+X 2 R-X+H-X For C4H9Br, four isomeric compounds are possible out of which one(sec-butyl bromide) can show enantiomerism. For C5H11Br, eight isomeric compounds are possible out of which three can exhibit enantiomerism. Alcohols can be converted into chlorides by reaction with (i)hcl/zncl2 (ii)pcl5 (iii)pcl3 (iv) SOCl2/Pyridine.The reaction with SOCl2/Pyridine is preferred because in this case side products are gaseous and can be expelled during distillation. Halogenation of alkenes in the presence of peroxides takes place through free radicals as intermediates and results in anti-markownikoff s product. For same alkyl group boiling points of alkyl halide are in the order:ri RBr RCl RF Among isomeric halides boiling point decreases with increase in branching in alkyl group. Alkyl halides are insoluble in water due to their inability to form hydrogen bonds with water. Allylic halide : CH2=CH-CH2X Benzylic halide : C6H5CH2Cl Vinylic halide : CH2=CH-Cl Order of reactivity of Alkyl halide in SN1 reaction: 3 0 > 2 0 > 1 0 Order of reactivity of Alkyl halide in SN2 reaction: 1 0 > Finkelstein reaction: R X + NaI R I + NaX Swart reaction : R X + AgF R F +Ag

2 Saytzeff srule: It states that during dehydro halogenations, the preferred product is that alkene which has greater number of alkyl groups attached to the doubly bonded carbon atoms. Alc.KOH CH 3 -CH 2 CH- CH 3 CH 3 CH= CH- CH 3 Br 81% More highly substituted Alkenes will be more stable Some important name reaction: 1. Wurtz reaction R X+2Na + X R Dry ether R-R+2NaX 2. Wurtz- fittig reaction R X+2Na + R Ar Dryether R-Ar+2NaX 3.Sandmeyer s reaction: ArNH2 + HNO2+HCl 273 K ArN2Cl Cu2X2+HX ArX +N2 CCl4 is used as fire extinguisher under the name Pyrene. CONCEPTUAL QUESTIONS Q1. Why are halo alkanes more reactive than haloarenes?. Ans. In haloarenes, there is double bond character between carbon and halogen due to resonance effect which makes it less reactive. (ii)in benzene, carbon atom is sp2 hybridised which is shorter than sp3 present in halo alkanes. Hence C-Cl bond in aryl halides is shorter and stronger. Q2. Why do halo alkanes undergo nucleophillic substitution where as haloarenes undergo electophillic substitution. Ans. Due to more electro negative nature of halogen atom in halo alkanes carbon atom becomes slightly positive and is easily attacked by nucleophillic reagents. While in halo arenes due to resonance, carbon atom becomes slightly negative and attacked by electrophillic reagents.

3 Q3. When an alkyl halide is treated with ethanolic solution of KCN, the major product is alkylcyanide where as if alkyl halide is treated with AgCN, the major product is alkyl isocyanide. Ans. KCN is ionic they can attach through C or N but C-C bond is stronger than C-N bond. So RCN is major product. AgCN is covalent and so more electro negative N can attach to C and forms iso cyanides. Q4. Aryl halides cannot be prepared by the action of sodium halide on phenol in the presence H 2 SO 4. Why? Ans. Due to resonance the carbon-oxygen bond in phenol has partial double bond character and it is stronger than carbon oxygen single bond. Q5. Grignard reagent should be prepared under anhydrous conditions. Why? Ans. Grignard reagent reacts with H 2 O (moisture) to form alkanes, therefore they are prepared under anhydrous condition. Q6.Why is Sulphuric acid not used during the reaction of alcohols with KI? Ans. It is because HI formed will get oxidized to I 2 by concentrated Sulphuric acid which is an oxidizing agent. Q7. p-dichloro benzene has higher m.p.than those of ortho and m- isomers.? Ans. p-dichloro benzene is symmetrical, fits into crystal lattice more readily and has higher melting point. Q8. Although chlorine is an electron-with drawing group, it is ortho and para directing in electrophillic aromatic substitution reactions. Why? Ans. Chlorobenzene is resonance hybrid, there is ve charge at ortho and parapositions, electro phallic substitution reaction will takeplace at ortho and para position due to +R effect which dominating over the I effect

4 Q9.The treatment of alkyl chlorides with aqueous KOH lead to the formation of alcohols but in presence of alcoholic KOH alkenes are major products. Explain? Ans. In aqueous KOH, (OH-) is nucleo phile which replaces another nucleophile. R-X+KOH Where as in alcoholic KOH C 2 H 5 OH+KOH CH 3 CH 2 -Cl+alco.KOH R-OH+KX C 2 H 5 O- +K+ CH 2 =CH 2 +C 2 H 5 OH Q10. Explain why vinylchloride is unreactive in nucleophillic substitution reaction? Ans. Vinylchloride is unreactive in nucleophillic substitution reaction because of double bond character between C=C-Clbond which is difficult to break. Q11. Arrange the following compounds according to reactivity towards nucleophilic substitution reaction with CH 3 ONa- 4-nitro chlorobenzene, 2, 4 di nitro chlorobenzene, 2, 4, 6, tri nitro chlorobenzene Ans- 2, 4, 6, trinitro chlorobenzene>2, 4 di nitro chlorobenzene>4-nitro chlorobenzene Q12. Which compound will react faster in SN 2 reaction with OH-? Ans-(a)CH 3 Br and CH 3 I CH 3 I will react faster than CH 3 Br since I is a easily leaving group (b)(ch 3 ) 3 C-Cl or CH 3 Cl CH 3 Cl will react faster than 3 0 halide since (CH 3 ) 3 C has more steric

5 hindrance Q13. Arrange in order of boiling points. (a)bromo benzene, Bromoform, chloro methane, Dibromo-methane (b)1-chloro propane, Iso propyle chloride,1-chlorobutane. Ans. (a)chloro methane<bromo benzene<dibromo-methane<, Bromoform (b), Iso propyl chloride<1-chloropropane<1-chlorobutane (As branching increases, boiling point decreases) Q14. Predict the reactivity in SN 1 (a) C 6 H 5 CH 2 Br, C 6 H 5 CH(C 6 H 5 )Br, C 6 H 5 CH(CH 3 )Br, C 6 H 5 C(CH 3 )(C 6 H 5 )Br Ans. 3 0 >2 0 >1 0 (SN 1 ) C 6 H 5 C(CH 3 )(C 6 H 5 )Br >C 6 H 5 (C 6 H 5 )Br > C 6 H 5 CH(CH 3 )Br >C 6 H 5 CH 2 Br (3 0 ) (2 0 ) (2 0 ) (1 0 ) VERY SHORT ANSWER TYPE QUESTION[1MARKS] 1. Write the IUPAC name of : CH2=CHCH2Br - ( All India 2011) 2. An alkyl halide having molecular formula C 4 H 9 Cl is optically active. What is its structure? 3. Why is vinyl chloride less reactive than ethyl chloride? 4.Write the structural isomers of C 3 H 6 Cl 2 which can exihibit enantiomerism? 5. Write down the structure of the following compounds; (a)1-chloro-4-ethylcyclohexane (b)1,4-dibronebut-2-ene (c)4-terbutyl-3-iodoheptane (d)1-bromo-4-secbutyl-2-methylbenzene 6. A hydro carbon C 5 H 12s does not react with chlorine in dark but it gives a single mono chloro compound in bright sunlight. Identify the compound. 7. Why is sulphuric acid not used during the reaction of alcohols with KI? 8. Out of C 6 H 5 CH 2 Cl &C 6 H 5 CH(Cl)C 6 H 5 which is more easily hydrolysed with aq.koh & why? 9. What is meant by the following terms: (1)chirality (2)enantiomerism (3)Ambident nucleophiles.

6 10. Chloroform is stored in dark coloured & sealed bottle. Why? Short answer type questions 1. Give the IUPAC names of the following compounds? A) ClCH 2 C= CCH 2 Br b) (CCl 3 )3CCl C)CH 3 CH(Cl)CH(Br)CH 3 2. How would you carry out the following conversions? ( All India 2008) (i) Ethyl magnesium chloride to propanol. (ii) Benzyl chloride to benzyl alcohol. 3. What is significance of 12 in Freon 12? 4. Predict the product of the following reaction CH2=CH-CH3 + HBr +Peroxide 5. Write the reaction involved in: (A) Sandmeyers reaction (B) Swarts reaction (C) Finkelstein reaction (D) Fittigs reaction 6. Rearrange the following in order of increasing ease of dehydrohalogenations CH 3 CH 2 CH 2 Cl, CH 3 CHClCH 3, CH 3 C-Cl(CH 3 )2 1. Account for the following: ( C.B.S.E. 2013) (i)the C Cl bond length in chlorobenzene is shorter than that in CH3 Cl? (ii)chloroform is stored in closed dark brown bottles. 8. Give the uses of (a)ccl 4 (b)iodoform 9. Propose the mechanism of the following reaction: CH 3 -CH 2 -Br+CH 3 OH CH 3 -CH 2 -OCH 3 +HBr 10.Suggest a possible mechanism for the following reaction: (C.B.S.E. DELHI 2009) n-bubr + KCN n- BuCN 11. How will you bring the following conversion? (a)propene to Propyne (b)toluene to Benzyl Alcohol(c) Aniline to Phenyl iso cyanide 12. What happen when;

7 (a)n-butyl chloride is treated with alc.koh. (b) ethyl chloride is treated with aq.koh. (c)methyl chloride is treated with KCN. 13. How can we produce nitro benzene from phenol? Ans.(I)First convert phenol to benzene by heating with Zn dust. (II) Nitration of benzene with conc.nitric acid in presence of conc.sulphuric acid. 14. Alcohols react with halogen acids to form haloalkanes but phenol does not form halo benzene. Explain. Ans. The C-O bond in phenol acquires partial double bond character due to resonance. Hence it is not Cleaved by X- ions to form halobenzenes. But in alcohols a pure single bond C O bond is maintained and can be cleaved by X ions. LONG ANSWER TYPE QUESTIONS Q1) How do you convert the following? (i)propenetopropan-1-ol (ii)1-bromopropaneto2-bromopropane (iii)toluenetobenzylalcohol (iv)benzeneto4-bromonitrobenzene (v)benzylalcoholto2-phenylethanoicacid 1. CH3-CH=CH2+HBr Peroxide CH3CH2CH2Br aq.koh CH3CH2CH2OH 2.

8 Q2) How are the following conversions carried out? (i)ethanol to propane nitrile (ii)aniline to chloro benzene (iii)2-chloro butane to 3, 4-dimethyl hexane (iv)2-methyl-1-propene to 2-chloro-2-methylpropane (v)ethyl chloride to propanoic acid 1. 2.

9 Q3) How are the following conversions carried out? (i) But-1-ene to n-butyliodide (ii) 2-Chloropropane to1-propanol (iii) Iso propyl alcohol to iodoform (iv) Chloro benzene to p-nitrophenol (v) 2-Bromopropane to 1-bromopropane

11 5.. Q4.How will you carry out the followingconversions? (i)chloro ethane to butane (ii)benzene to diphenyl 1. 2.

HALOALKANES AND HALOARENES

Unit - 10 HALOALKANES AND HALOARENES 1. Write the IUPAC names of the following compounds. Br CH = CH C CH (vi) (vii) (ix) (CCl 3 ) 3 CCl 103 XII Chemistry 2. Write the structure of following halogen compounds