Halogeno-compounds

1. Chapter 32 Halogeno-compounds 32.1Introduction 32.2Nomenclature of Halogeno-compounds 32.3Physical Properties of Halogeno-compounds 32.4Preparation of Halogeno-compounds 32.5Reactions of Halogeno-compounds 32.6Nucleophilic Substitution Reactions 32.7Elimination Reactions 32.8Uses of Halogeno-compounds

2. 32.1 Introduction (SB p.169) • Haloalkanes are organic compounds having one or more halogen atoms replacing hydrogen atoms in alkanes • Haloalkanes are classified into primary, secondary and tertiary, based on the number of alkyl groups attached to the carbon atom which is bonded to the halogen atom

3. 32.1 Introduction (SB p.169) Halobenzenes are organic compounds in which the halogen atom is directly attached to a benzene ring e.g.  not a halobenzene, because the chlorine atom is not directly attached to the benzene ring

4. 32.2 Nomenclature of Halogeno-compounds (SB p.170) • Naming haloalkanes are similar to those for naming alkanes • The halogens are written as prefixes: fluoro- (F), chloro- (Cl), bromo- (Br) and iodo- (I) • e.g.

5. 32.2 Nomenclature of Halogeno-compounds (SB p.170) When the parent chain has both a halogen and an alkyl substituent, the chain is numbered from the end nearer the first substituent regardless of what substituents are e.g.

6. 32.2 Nomenclature of Halogeno-compounds (SB p.171) In case of halobenzenes, the benzene ring is numbered so as to give the lowest possible numbers to the substituents e.g.

7. 32.2 Nomenclature of Halogeno-compounds (SB p.171) Solution: (a) Example 32-1 Draw the structural formulae and give the IUPAC names of all isomers with the following molecular formula. (a) C4H9Br Answer

8. 32.2 Nomenclature of Halogeno-compounds (SB p.171) Solution: (b) Example 32-1 Draw the structural formulae and give the IUPAC names of all isomers with the following molecular formula. (b) C4H8Br2 Answer

9. 32.2 Nomenclature of Halogeno-compounds (SB p.172) Check Point 32-1 Draw the structural formulae and give the IUPAC names for all the structural isomers of C5H11Br. Answer

10. 32.2 Nomenclature of Halogeno-compounds (SB p.172)

11. 32.3 Physical Properties of Halogeno-compounds (SB p.173)

12. 32.3 Physical Properties of Halogeno-compounds (SB p.173)

13. 32.3 Physical Properties of Halogeno-compounds (SB p.173)

14. 32.3 Physical Properties of Halogeno-compounds (SB p.174) Boiling Point and Melting Point

15. 32.3 Physical Properties of Halogeno-compounds (SB p.174) Haloalkanes have higher b.p. and m.p. than alkanes∵ dipole-dipole interactions are present between haloalkane molecules m.p. and b.p. increase in the order: RCH2F < RCH2Cl < RCH2Br < RCH2I ∵ larger, more polarizable halogen atoms increase the dipole-dipole interactions between the molecules No. of carbon   m.p. and b.p. 

16. 32.3 Physical Properties of Halogeno-compounds (SB p.174) Density • Relative molecular mass  •  density  • ∵ closer packing of the smaller molecules in the liquid phase • Bromo and iodoalkanes are all denser than water at 20°C

17. 32.3 Physical Properties of Halogeno-compounds (SB p.174) Solubility Although C — X bond is polar, it is not polar enough to have a significant effect on the solubility of haloalkanes and halobenzenes  Immiscible with water  Soluble in organic solvents

18. 32.4 Preparation of Halogeno-compounds (SB p.175) Preparation of Haloalkanes Substitution of Alcohols • Prepared by substituting –OH group of alcohols with halogen atoms • Common reagents used: HCl, HBr, HI, PCl3 or PBr3 • The ease of substitution of alcohols:3° alcohol > 2° alcohol > 1° alcohol > CH3OH • This is related to the stability of the reaction intermediate (i.e. stability of carbocations)

19. 32.4 Preparation of Halogeno-compounds (SB p.175) Reaction with Hydrogen Halides • Dry HCl is bubbled through alcohols in the presence of ZnCl2 catalyst • For the preparation of bromo- and iodoalkanes, no catalyst is required

20. 32.4 Preparation of Halogeno-compounds (SB p.176) • The reactivity of hydrogen halides: HI > HBr > HCl • e.g.

21. 32.4 Preparation of Halogeno-compounds (SB p.176) Reaction with Phosphorus Halides Haloalkanes can be prepared from the vigorous reaction between cold alcohols and phosphorus(III) halides

22. 32.4 Preparation of Halogeno-compounds (SB p.177) Addition of Alkenes and Alkynes Addition of halogens or hydrogen halides to an alkene or alkyne can form a haloalkane e.g.

23. 32.4 Preparation of Halogeno-compounds (SB p.177) Preparation of Halobenzenes Halogenation of Benzene Benzene reacts readily with chlorine and bromine in the presence of catalysts (e.g. FeCl3, FeBr3, AlCl3)

24. 32.4 Preparation of Halogeno-compounds (SB p.177) From Benzenediazonium Salts

25. 32.4 Preparation of Halogeno-compounds (SB p.178) (a) CH3CHBrCH2CH3 (b) CH3CHBrCH3 (c) CH3CBr2CH3 (d) Check Point 32-2 State the major products of the following reactions: (a) CH3CHOHCH2CH3 + PBr3  (b) CH3CH = CH2 + HBr  (c) CH3C  CH + 2HBr  (d) Answer

26. 32.5 Reactions of Halogeno-compounds (SB p.178) • Carbon-halogen bond is polar • Carbon atom bears a partial positive charge • Halogen atom bears a partial negative charge

27. 32.5 Reactions of Halogeno-compounds (SB p.178) • Characteristic reaction: • Nucleophilic substitution reaction • Alcohols, ethers, esters, nitriles and amines can be formed by substituting – OH, – OR, RCOO –, – CN and – NH2 groups respectively

28. 32.5 Reactions of Halogeno-compounds (SB p.179) Haloalkane Base Alkene • Another characteristic reaction: • Elimination reaction • Bases and nucleophiles are the same kind of reagents • Nucleophilic substitution and elimination reactions always occur together and compete each other

29. 32.6 Nucleophilic Substitution Reactions (SB p.179) Reaction with Sodium Hydroxide The reactions proceed in 2 different reaction mechanisms:bimolecular nucleophilic substitution (SN2) unimolecular nucleophilic substitution (SN1)

30. 32.6 Nucleophilic Substitution Reactions (SB p.180) Experiment number Initial [CH3Cl] (mol dm–3) Initial [OH–] (mol dm–3) Initial rate (mol dm–3 s–1) 1 2 3 4 0.001 0.002 0.001 0.002 1.0 1.0 2.0 2.0 4.9  10–7 9.8  10–7 9.8  10–7 19.6  10–7 Results of kinetic study of reaction of CH3Cl with OH– Bimolecular Nucleophilic Substitution (SN2) Example: CH3– Cl + OH–  CH3OH + Cl– Rate = k[CH3Cl][OH–] Order of reaction = 2 both species are involved in rate determining step

31. 32.6 Nucleophilic Substitution Reactions (SB p.181) Reaction mechanism of the SN2 reaction: • The nucleophile attacks from the backside of the electropositive carbon centre • In the transition state, the bond between C and O is partially formed, while the bond between C and Cl is partially broken

32. 32.6 Nucleophilic Substitution Reactions (SB p.181) Energy profile of the reaction of CH3Cl and OH- by SN2 mechanism Transition state involve both the nucleophile and substrate second order kinetics of the reaction

33. 32.6 Nucleophilic Substitution Reactions (SB p.182) Stereochemistry of SN2 Reactions • The nucleophile attacks from the backside of the electropositive carbon centre • The configuration of the carbon atom under attack inverts

34. 32.6 Nucleophilic Substitution Reactions (SB p.182) Unimolecular Nucleophilic Substitution (SN1) Example: • Kinetic study shows that: • Rate = k[(CH3)3CCl] • The rate is independent of [OH–] • Order of reaction = 1  only 1 species is involved in the rate determining step

35. 32.6 Nucleophilic Substitution Reactions (SB p.183) Reaction mechanism of SN1 reaction involves 2 steps and 1 intermediate formed • Step 1: • Slowest step (i.e. rate determining step) • Formation of carbocation and halide ion

36. 32.6 Nucleophilic Substitution Reactions (SB p.183) • Step 2: • Fast step • Attacked by a nucleophile to form the product

37. 32.6 Nucleophilic Substitution Reactions (SB p.183) Energy profile of the reaction of (CH3)3CCl and OH- by SN1 mechanism • Rate determining step involves the breaking of the C – Cl bond to form carbocation • Only 1 molecule is involved in the rate determining step  first order kinetics of the reaction

38. 32.6 Nucleophilic Substitution Reactions (SB p.184) Stereochemistry of SN1 Reactions • The carbocation formed has a trigonal planar structure • The nucleophile may either attack from the frontsideor the backside

39. 32.6 Nucleophilic Substitution Reactions (SB p.184) For some cations, different products may be formed by either mode of attack e.g. The reaction is called racemization

40. 32.6 Nucleophilic Substitution Reactions (SB p.184) The above SN1 reaction leads to racemization ∵ formation of trigonal planar carbocationintermediate

41. 32.6 Nucleophilic Substitution Reactions (SB p.185) The attack of the nucleophile from either side of the planar carbocation occurs at equal rates and results in the formation of the enantiomers of butan-2-ol in equal amounts

42. 32.6 Nucleophilic Substitution Reactions (SB p.185) Factors Affecting the Rates of SN1 and SN2 Reactions Most important factors affecting the relative rates of SN1 and SN2 reactions: 1.Thestructure of the substrate 2. The concentration and strength of the nucleophile (for SN2 reactions only) 3. The nature of the leaving group

43. 32.6 Nucleophilic Substitution Reactions (SB p.186) The Structure of the Substrate • 1. SN2 reactions • The reactivity of haloalkanes in SN2 reactions: CH3X > 1° haloalkane > 2° haloalkane > 3° haloalkane • Steric hindrance affects the reactivity∵ bulky alkyl groups will inhibit the approach of nucleophile to the electropositive carbon centre  energy of transition state  activation energy 

44. 32.6 Nucleophilic Substitution Reactions (SB p.186) Steric effects in the SN2 reaction

45. 32.6 Nucleophilic Substitution Reactions (SB p.187) 2. SN1 reactions • Critical factor: the relative stability of the carbocation formed • Tertiary carbocations are the most stable∵ 3 electron-releasing alkyl groups stabilize the carbocation by releasing electrons • Methyl, 1°, 2° carbocation have much higher energy activation energies for SN1 reactions are very large and rate of reaction become very small

46. 32.6 Nucleophilic Substitution Reactions (SB p.187) The Concentration and Strength of the Nucleophile • Only affect SN2 reactions • Concentration of nucleophile   rate 

47. 32.6 Nucleophilic Substitution Reactions (SB p.187) • Relative strength of nucleophiles can be correlated with two structural features: • (I) A negatively charged nucleophile (e.g. OH–) is always a stronger nucleophile than a neutral nucleophile (e.g. H2O) • (II) In a group of nucleophiles in which the nucleophilic atom is the same, the order of nucleophilicity roughly follows the order of basicity: • e.g. RO– > OH– >> ROH > H2O • Strength   rate 

48. 32.6 Nucleophilic Substitution Reactions (SB p.188) The Nature of Leaving Group • Halide ion departs as a leaving group • For the halide ion, the ease of leaving: I– > Br– > Cl– > F– • This is in agreement with the order of bond enthalpies of carbon-halogen bonds C – I bond is weakest  I– is the best leaving group

49. 32.6 Nucleophilic Substitution Reactions (SB p.188) • Uncharged or neutral compounds are better leaving groupse.g. The ease of leaving of oxygen compounds: • H2O >> OH– > RO– • Strongly basic ions rarely act as leaving groupe.g.

50. 32.6 Nucleophilic Substitution Reactions (SB p.188) When an alcohol is dissolved in a strong acid, it can react with a halide ion ∵ the acid protonates the –OH group, and the leaving group becomes a neutral water moleculee.g.