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Dr. Manal Fawzy Abou Taleb

Organic Chemistry , 7 th Edition L. G. Wade, Jr. Dr. Manal Fawzy Abou Taleb. Alkyl Halides: Nucleophilic Substitution and Elimination. Alkyl Halides. Introduction Nomenclature of Alkyl Halides Physical Properties of Alkyl Halides Preparation of Alkyl Halides

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Dr. Manal Fawzy Abou Taleb

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  1. Organic Chemistry, 7th EditionL. G. Wade, Jr. Dr. Manal Fawzy Abou Taleb Alkyl Halides: Nucleophilic Substitution and Elimination

  2. Alkyl Halides Introduction Nomenclature of Alkyl Halides Physical Properties of Alkyl Halides Preparation of Alkyl Halides Reactions of Alkyl Halides NucleophilicSubstitution Reactions Elimination Reactions Uses of Alkyl Halides

  3. Introduction What Is an Alkyl Halide • Alkyl Halides are organic compounds having one or more halogen atoms bonded to a carbon atom.

  4. Identify the longest continuous carbon chain It must contain any double or triple bond if present Number from end nearest any substituent (alkyl or halogen) If any multiple bonds are present, number from end closest to these. The halogens are written as prefixes: fluoro- (F), chloro- (Cl), bromo- (Br) and iodo- (I) IUPAC Nomenclature • Name as haloalkane.

  5. If more than one of the same kind of halogen is present, use prefix di, tri, tetra If there are several different halogens, number them and list them in alphabetical order Naming with Multiple Halides

  6. Begin at the end nearer the substituent whose name comes first in the alphabet Naming if Two Halides or Alkyl Are Equally Distant from Ends of Chain In case of halobenzenes, the benzene ring is numbered so as to give the lowest possible numbers to the substituents e.g.

  7. Systematic Common Names • Common names are often used for simple alkyl halides. To assign a common name: • Name all the carbon atoms of the molecule as a single alkyl group. • Name the halogen bonded to the alkyl group. • Combine the names of the alkyl group and halide, separating the words with a space. iso-butyl bromide sec-butyl bromide

  8. Chloroform Carbon tetrachloride Methylene chloride Methyl iodide Trichloroethylene Many Alkyl Halides That Are Widely Used Have Common Names

  9. A halogen attached to a carbon next to a doubly bonded carbon is an Allylic Halide • A halogen is attached directly to a doubly bonded carbon is called: Vinylic halides

  10. A halogen one carbon away from an aromatic ring is Benzylic Halide • A halogen attached directly to a benzene ring is an Aryl halide

  11. 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

  12. Classes of alkyl halides • Haloalkanes are classified into primary, secondaryand tertiary, based on the number of alkyl groups attached to the carbon atom which is bonded to the halogen atom

  13. Physical Properties of Organic Halides 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.  The boiling points increases with increasing in molecular weights.

  14. 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 • Explain Why • Alkyl halides have higher melting point than the corresponding alkanes, alkenes, and alkynes • because: • 1. Polarity 2. Molecular weight

  15. Preparation of Halogeno-compounds 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)

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

  17. Preparation of Halogeno-compounds • The reactivity of hydrogen halides: HI > HBr > HCl • e.g.

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

  19. Reaction with Phosphorus Halides Addition of Thionyl Chloride to Alcohols

  20. Alkane + Cl2 or Br2, heat or light replaces C-H with C-X but gives mixtures Hard to control Via free radical mechanism It is usually not a good idea to plan a synthesis that uses this method—multiple products Preparing Alkyl Halides from Alkanes: Radical Halogenation

  21. Preparation of Halogeno-compounds Addition of Alkenes and Alkynes • Alkyl dihalides are prepared from anti addition of bromine (Br2) or chlorine (Cl2) (addition of halogen)

  22. Addition of Alkenes and Alkynes The most effective means of preparing an alkyl halide is from addition of HCl, HBr, HI to alkenes or alkyne to give Markovnikov product Anti-Markinikoff’s rule.

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

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

  25. Reactions of Halogeno-compounds • Carbon-halogen bond is polar • Carbon atom bears a partial positive charge • Halogen atom bears a partial negative charge

  26. Reactions of Halogeno-compounds • Characteristic reaction: • Nucleophilic substitution reaction • Alcohols, ethers, esters, nitriles and amines can be formed by substituting – OH, – OR, RCOO –, – CN and – NH2 groups respectively Nu= OH, OR, OCOR, NH2, RNH, SH, SR, RC=C, CN, X’

  27. Haloalkane Base Alkene Reactions of Halogeno-compounds • 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

  28. Reaction of Grignared reagent a- Formation of Grignard reagent b- Reaction of Grignard reagent

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

  30. 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– Nucleophilic Substitution Reactions 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. Nucleophilic Substitution Reactions 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. Energy profile of the reaction of CH3Cl and OH- by SN2 mechanism Nucleophilic Substitution Reactions Transition state involve both the nucleophile and substrate second order kinetics of the reaction

  33. Nucleophilic Substitution Reactions 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. Nucleophilic Substitution Reactions UnimolecularNucleophilic 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. Nucleophilic Substitution Reactions Reaction mechanism of SN1 reaction involves 2 stepsand 1 intermediate formed • Step 1: • Slowest step (i.e. rate determining step) • Formation of carbocation and halide ion

  36. Nucleophilic Substitution Reactions • 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. Nucleophilic Substitution Reactions For some cations, different products may be formed by either mode of attack e.g. The reaction is called racemization

  40. Nucleophilic Substitution Reactions The above SN1 reaction leads to racemization ∵ formation of trigonal planar carbocationintermediate

  41. Nucleophilic Substitution Reactions 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. Nucleophilic Substitution Reactions Unreactivity of Halobenzene • Halobenzenes are comparatively unreactive to nucleophilic substitution reactions • ∵ the p orbital on the carbon atom of the benzene ring and that on the halogen atom overlap side-by-side to form a delocalized  bonding system

  43. Nucleophilic Substitution Reactions • Delocalized electrons repel any approachingnucleophiles unreactive towards SN2 reactions • Benzene cations are highly unstable because of loss of aromaticity unreactive towards SN1 reactions

  44. Nucleophilic Substitution Reactions Reaction with Potassium Cyanide A nitrile is formed when a haloalkane is heated under reflux with an aqueous alcoholic solution of potassium cyanide e.g.

  45. 32.6 Nucleophilic Substitution Reactions (SB p.194) • Cyanide ion (CN–) acts as a nucleophile • Halobenzenes do not react with potassium cyanide • The reaction is very useful because the nitrile can be hydrolyzed to carboxylic acidswhich can be reduced to alcohols • A useful way of introducing a carbon atom into an organic molecule, so that the length of the carbon chain can be increased

  46. 32.6 Nucleophilic Substitution Reactions (SB p.195) Reaction with Ammonia When a haloalkane is heated with an aqueous alcoholic solution of ammonia under a high pressure, an amine is formed e.g. • Ammonia is a nucleophile because the presence of a lone pair of electrons on the nitrogen atom

  47. The END

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