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Halogeno-compounds

Halogeno-compounds. Chapter 33. R. R. H. R. C. R. R. C. H. R. C. H. X. X. X. Structures. Halogenoalkanes: X bond to sp 3 carbon. 1 o Primary 2 o Secondary 3 o Tertiary. X. Structures. Halobenzene: X bond to benzene, sp 2 carbon.

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Halogeno-compounds

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  1. Halogeno-compounds Chapter 33

  2. R R H R C R R C H R C H X X X Structures Halogenoalkanes: X bond to sp3 carbon 1o Primary 2o Secondary 3o Tertiary

  3. X Structures Halobenzene: X bond to benzene, sp2 carbon

  4. Reactions of Halogenoalkanes • Two major types: • Nucleophilic Substitution (SN) • Elimination (E)

  5. H R C+ H X- H R C H + X- Nu Nucleophilic Substitution (SN) :Nu- • Polar C-X bond • C+ is attacked by :Nu- • C-X bond is broken to • give out X-

  6. H H - HO- C+ Br- HO C Br H H CH3 CH3 H HO C + Br- H CH3 Bimolecular Nucleophilic Substitution (SN2) If C is chiral, completed stereochemical inversion.

  7. H - HO C Br H CH3 OH- + CH3CH2Br CH3CH2OH + Br- Bimolecular Nucleophilic Substitution (SN2) Rate law: Rate = k [OH-][CH3CH2Br] (Bimolecular, 2nd order)

  8. R R C+ Br- C+ + Br- R R R R (sp2 , trigonal planar) R R -H+ C+ + H2O: C OH R R R R Unimolecular Nucleophilic Substitution (SN1)  (rds)

  9. R3C---Br R3C---OH2+ R3C+ + Br- + H2O R3C-Br + H2O: R3C-OH + HBr Unimolecular Nucleophilic Substitution (SN1) Rate law: rate = k [R3CBr] (1st order, Unimolecular)

  10. Relative rates of SN1 and SN2

  11. Factors affecting relative rates Structure - Steric Factor The size of atoms or groups at/near the reactive site affects SN2. Bulky groups (-R) at the C-X site slow down SN2 reaction.

  12. Factors affecting relative rates Structure - Stability of carbocation R3C+ > R2CH+ > RCH2+ > CH3+ (R group is e- donating) Stable carbocation favours SN1 mechanism.

  13. Factors affecting relative rates

  14. Factors affecting relative rates Effect of nucleophile SN2 Strength and concentration have effect RO:-> :OH- > ROH > H2O: SN1 No effect

  15. Factors affecting relative rates • Effect of leaving groups • Relative rate of substitution • C-I > C-Br > C-Cl • Explanation : Bond energy • C-I 238 C-Br 276 C-Cl 338 • (*exp.1 p.235)

  16. Factors affecting relative rates • Effect of solvent: • Polar solvent stabilize the carbocation • and hence favour SN1 reaction • Increase in polarity: • CH3COCH3 << R-OH < H2O

  17. Synthetic applications Nitrile Formation ethanol, reflux R-Br + KCN  R-CN + KBr H+ 1.LiAlH4 R-CN  RCOOH  RCH2OH 2.H2O (Increase carbon chain length by one carbon)

  18. Synthetic applications Formation of C-O bond R-Br + NaOH  ROH R-Br + RO-Na+  ROR Formation of amine RI + NH3 R-NH2

  19. H H H H C C C H C H H H H X + H2O + X- Elimination HO:-

  20. H H C C H H H X Competition between SN and E E SN Nu:- Good Nu:- are also good B:- (SN always competes with E)

  21. Conditions favour E • Highly substitutedhaloalkanes is more • likely to undergo elimination (Steric Effect) • Favor SN • 3oRX 2oRX 1oRX • Favor E

  22. Conditions favour E 2. Use less polar solvent e.g. 75% ethanol + 25% water is better than 25% ethanol + 75% water Polar solvent favors the formation of highly concentrated charged particles. T.S. of SN2 reaction is Nu-….R….X- is more concentrated than Nu-…H – C - C….X-

  23. 45oC (47%) (53%) NaOH CH3CHBrCH3 CH3CH=CH2 + (CH3)2CH-OC2H5 (or OH) 100oC C2H5OH, H2O (36%) (64%) Conditions favour E 3. Higher temperature and prolonged refluxing Breaking of C-H bond and C-X bonds require greater Activation Energy.

  24. C2H5OH CH3 (19%) 25oC C Br CH3 (CH3)2C=CH2 CH3 (93%) C2H5O-/C2H5OH Conditions favour E 4. Stronger base: RO- > ROH

  25. Applications of Elimination Preparation of Alkenes e.g. C2H5O-Na+/C2H5OH C2H5Br  C2H5OC2H5 + CH2=CH2 heat 99% 1% C2H5O-Na+/C2H5OH (CH3 )2CHBr  C2H5OCH(CH3)2 + CH2=CHCH3 heat 21% 79%

  26. Applications of Elimination Preparation of Alkenes e.g. C2H5O-Na+/C2H5OH (CH3)3CBr  (CH3)2C=CH2 heat 100%

  27. Applications of Elimination Preparation of Alkynes e.g. Br2 CH3CH=CHCH3  CH3CHBrCHBrCH3 C2H5O-Na+/C2H5OH  CH3CCCH3 heat

  28. Uses of Halogeno-compounds Please refer to Section 33.6 on p.253

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