Halogenoalkane compounds and Nucleophilic Substitution

1. Halogenoalkane compounds and Nucleophilic Substitution 30.1 Introduction 30.2 Nomenclature of Halogeno-compounds 30.3 Physical Properties of Halogeno-compounds 30.4 Preparation of Halogeno-compounds 30.5 Nucleophilic Subsititution Reaction 30.6 Elimination Reactions

2. 30.1 Introduction

3. 30.1 Introduction (SB p.208) Haloalkanes • Organic compounds having one or more halogen atoms replacing hydrogen atoms in alkanes • If there is one halogen atom replacing a hydrogen atom •  General formula of haloalkanes: CnH2n+1X

4. 30.1 Introduction (SB p.208) Haloalkanes • According to the number of alkyl groups attached to the carbon atom which is bonded to the halogen atom •  haloalkanes are classified into primary, secondary or tertiary

5. 30.2 Nomenclature of Halogeno-compounds

6. 30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds • The IUPAC rules for naming halogeno-compounds are similar to those for naming alkanes

7. 30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds • The halogens are written as prefixes: fluoro- (F), chloro- (Cl), bromo- (Br) and iodo- (I) • Numbers are assigned to the halogen substituent in the same way as the alkyl substituent

8. 30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds e.g.

9. 30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds • When the parent chain has both a halogen and an alkyl substituent •  the chain is numbered from the end closest to the first substituent

10. 30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds • When the parent chain has both a halogen and an alkyl substituent •  the chain is numbered from the end closest to the first substituent •  all the prefixes are listed in alphabetical order

11. 30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds e.g.

12. 30.3 Physical Properties of Halogeno-compounds

13. 30.3 Physical Properties of Halogeno-compounds (SB p.211) Physical properties of some halogeno-compounds

14. 30.3 Physical Properties of Halogeno-compounds (SB p.211) Physical properties of some halogeno-compounds

15. 30.3 Physical Properties of Halogeno-compounds (SB p.211) Physical properties of some halogeno-compounds

16. 30.3 Physical Properties of Halogeno-compounds (SB p.212) Boiling Point and Melting Point • CX bond is polar in nature •  Difference in electronegativity between carbon and halogens • Molecules of haloalkanes are held together by dipole-dipole interactions • Haloalkanes have higher b.p. and m.p. than alkanes of comparable relative molecular masses

17. 30.3 Physical Properties of Halogeno-compounds (SB p.212) Boiling Point and Melting Point • B.p. and m.p. of fluoro-, chloro-, bromo- and iodo- compounds increase in the order: • RCH2F < RCH2Cl < RCH2Br < RCH2I • Larger, more polarizable halogen atoms lead to •  increase in dipole-dipole interactions between molecules

18. 30.3 Physical Properties of Halogeno-compounds (SB p.212) Boiling Point and Melting Point • When the number of carbon atoms in the alkyl groups increases •  the molecular size also increases •  stronger dipole-dipole interactions between molecules •  higher b.p. and m.p.

19. 30.3 Physical Properties of Halogeno-compounds (SB p.212) Variation of boiling points with the number of carbon atoms of straight-chain haloalkanes

20. 30.3 Physical Properties of Halogeno-compounds (SB p.212) Density • When the relative molecular mass of haloalkanes increases •  the densities of haloalkanes decrease • Closer packing of the smaller molecules in the liquid phase • Bromoalkanes and iodoalkanes are denser than water at 20 oC

21. 30.3 Physical Properties of Halogeno-compounds (SB p.213) Solubility • Haloalkanes are immiscible with water • But dissolve in organic solvents

22. 30.5 Reactions of Halogeno-compounds

23. 30.5 Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds • CX bond is polar •  halogens are more electronegative than carbon • The carbon atom bears a partial negative charge • The halogen atom bears a partial positive charge

24. 30.5 Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds • One of the characteristic reactions of haloalkanes is nucleophilic substitution reactions:

25. 30.5 Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds • In the reaction, •  the nucleophileattacks the electropositive carbon centre •  displaces a halide ion from the haloalkane •  a kind of substitution reactions

26. 30.5 Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds •  The reaction is initiated by a nucleophile •  called nucleophilic substitution reaction

27. 30.5 Reactions of Halogeno-compounds (SB p.219) Reactions of Halogeno-compounds • Another characteristic reaction of haloalkanes is elimination reactions:

28. 30.5 Reactions of Halogeno-compounds (SB p.219) Reactions of Halogeno-compounds • A baseremoves a hydrogen atom from the carbon atom adjacent to the CX bond • C=C bond is formed

29. 30.6 Nucleophilic Substitution Reactions

30. 30.6 Nucleophilic Substitution Reactions (SB p.219) Reaction with Sodium Hydroxide Consider the following reactions:

31. 30.6 Nucleophilic Substitution Reactions (SB p.219) Reaction with Sodium Hydroxide • In both reactions, •  the nucleophile (OH-) attacks the haloalkane •  replacing the halogen atom with a hydroxyl group

32. 30.6 Nucleophilic Substitution Reactions (SB p.221) 2. Stereochemistry of SN2 Reactions • The nucleophile attacks the electropositive carbon centre from the backside •  the configuration of the carbon atom under attack inverts

33. 30.6 Nucleophilic Substitution Reactions (SB p.226) The Concentration and Strength of the Nucleophile A negatively charged nucleophile (e.g. OH-) is always a stronger nucleophile than a neutral nucleophile (e.g. H2O)

34. 30.6 Nucleophilic Substitution Reactions (SB p.227) The Concentration and Strength of the Nucleophile 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. oxygen compounds show the following order of reactivity: RO- > OH- >>ROH > H2O

35. 30.6 Nucleophilic Substitution Reactions (SB p.227) The Nature of the Leaving Group Bond enthalpies of carbon-halogen bonds

36. 30.6 Nucleophilic Substitution Reactions (SB p.228) 6. Comparison of Rates of Hydrolysis of Haloalkanes Experiment 1: Comparison of the Rates of Hydrolysis of 1-Chlorobutane, 1-Bromobutane and 1-Iodobutane Three test tubes containing ethanol, silver nitrate solution and different haloalkanes

37. AgBr(s) AgI(s) AgCl(s) Test tube A Test tube B Test tube C 30.6 Nucleophilic Substitution Reactions (SB p.228) Results and Observation:

38. 30.6 Nucleophilic Substitution Reactions (SB p.229) Discussion: • The ease of leaving of halide ionsdecreases in the order: • I- > Br- > Cl- • The order of precipitates appeared follows the order of ease of leaving of the halide ions • Ag+(aq) + X-(aq)  AgX(s)

39. 30.6 Nucleophilic Substitution Reactions (SB p.233) Reaction with Potassium Cyanide • When a haloalkane is heated under reflux with an alcoholic solution of KCN or KCN in ethanol •  a nitrile is formed

40. 30.6 Nucleophilic Substitution Reactions (SB p.233) Reaction with Potassium Cyanide e.g.

41. 30.6 Nucleophilic Substitution Reactions (SB p.233) Reaction with Potassium Cyanide • CN- acts as a nucleophile • Halobenzenes do not react with KCN

42. 30.6 Nucleophilic Substitution Reactions (SB p.233) Reaction with Potassium Cyanide • Nitriles can be hydrolyzed to carboxylic acids which can be reduced to alcohols • Provides a useful way of introducing a carbon atom into an organic molecule •  length of carbon chain can be increased

43. 30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia • Ammonia is a nucleophile •  Presence of a lone pair of electrons on the nitrogen atom

44. 30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia • When a haloalkane is heated with an aqueous alcoholic solution of ammonia under high pressure •  an amine is formed

45. 30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia e.g.

46. 30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia •  Ethylamine will compete with ammonia as the nucleophile •  a series of further substitution reactions takes place •  a mixture of products is formed

47. 30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia • The reaction stops at the formation of a quaternary ammonium salt

48. Let's Think 2 30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia • The competing reactions can be minimized by using excess ammonia • Not a satisfactory method for preparing amines •  a mixture of amines is always formed

49. 30.7 Elimination Reactions

50. 30.7 Elimination Reactions (SB p.236) Formation of Alkenes • Heating a haloalkane with a strong base (e.g. sodium hydroxide in ethanol) •  causes the elimination of HX from adjacent carbon atoms of a haloalkane