Chapter 6 Polar addition and Elimination Reaction

1. Chapter 6 Polar addition and Elimination Reaction

2. Introduction Advanced Organic Chemistry (Chapter 6) sh. javnshir

3. Bimolecular Electrophilic Addition (AdE2) Termolecular Electrophilic Addition (AdE3) Advanced Organic Chemistry (Chapter 6) sh. javnshir

4. 6.1 Addition of Hydrogen Halides to Alkenes Regioselective Reaction: Unsymmetrical alkene gives a predominance of one of the two possible addition products.

5. Regiospecific Reaction: One products is formed exclusively. Addition of Hydrogen Halides to Alkenes Markownikoff’s Rule Advanced Organic Chemistry (Chapter 6) sh. javnshir

6. Addition of HCl or HBr to Alkenes:

7. The stereochemistry of addition of hydrogen halides to unconjugated alkenes is predominantly ANTI. Advanced Organic Chemistry (Chapter 6) sh. javnshir

8. However, reaction can be modified by temperature and solvent. e.g.: Advanced Organic Chemistry (Chapter 6) sh. javnshir

9. Anti stereochemistry can be explained by a mechanism in which the alkene interacts simultaneously with the proton-donating HX and with the halide source. Advanced Organic Chemistry (Chapter 6) sh. javnshir

10. A significant variation in the stereochemistry is observed when the double bond conjugated with a group that can stabilize a carbocation intermediate. Because of greater stability of the carbocation (specially in aromatic conjugation) concerted attack by halide ion (AdE3 mechanism) is not required and formed ion pair collapses to product faster than rotation takes place, the result will be syn addition. The reaction is first order in HCl. Advanced Organic Chemistry (Chapter 6) sh. javnshir

11. Competing Reaction with Solvent in Nucleophilic Solvents: Addition of halide salt (e.g., Me4N+X-) increases the capture of a carbocation intermediate by halide ion.

12. Skeletal Rearrangement Advanced Organic Chemistry (Chapter 6) sh. javnshir

13. Addition to Conjugated Dienes Advanced Organic Chemistry (Chapter 6) sh. javnshir

14. Addition of HBr and HCl to Norbornene The excess of 1 over 2 indicates that some syn addition occurs by ion pair by ion pair collapse before the bridge ion achieves with respect to the chloride ion. Advanced Organic Chemistry (Chapter 6) sh. javnshir

15. Advanced Organic Chemistry (Chapter 6) sh. javnshir

17. 6.2 Acid Catalyzed Hydration and Related Addition Reactions 1) General acid catalysis 2) Solvent isotope effect (kH2O/kD2O=2-4) Rate Limiting Protonation

18. Addition of Nucleophilic Solvents: Effect of Acidity Alkene-acid complex with weaker acids Addition of DBr to cis- or trans-2-butene (AdE3):

19. Addition of CF3SO3D to cis- or trans-2-butene (AdE2):

20. 6.3 Addition of Halogens 1) Is there a discrete positively charged intermediate, or is the addition concerted? 2) If there is a positively charged intermediate, is it a carbocation or a cyclic halonium Ion? For Brominations: Anti addition is preferred for alkenes that can not stabilize a carbocation intermediate.

21. Syn addition is becomes much larger for alkenes conjugated with aryl groups. Aliphatic Systems: Bridged ion mechanism (Anti Addition). Aromatic Systems (Electron-Releasing): Stabilization of the carbocation and Ion pair mechanism (Syn Addition). Aromatic Systems (Electron-Attracting): Destabilization of the carbocation and bridged ion mechanism (Anti Addition). Styrene is Borderline.

22. Chlorination: Chlorine is less polarizable, more electronegative and then it is: Poor bridging group than bromine. Syn addition is slightly preferred for chlorination. For non-conjugated alkenes, stereo specific anti addition is usually observed for both halogens. Evidences for cyclic bronium ion: NMR Spectroscopy X-Ray crystallography Mechanism study

24. Kinetic of Bromination In methanol and high concentration of Br- kinetic is pseudo-second-order.

25. In non-polar solvents kinetic is third-order. Second-order term: Third-order term:

26. Kinetic of Chlorination: Overally 2ND-Order. First-order in both alkene and Cl2. Increasing the reaction rate with alkyl substitution. Table 6.3: Relative Reactivity of Alkenes toward Halogenation.

27. Competitive Reactions of Cationic Intermediate in Chlorination: a) Loss of a Proton

28. b) Alkyl Migration

29. Fluorination F2 Reaction with alkenes is violent giving carbon chain degradation products. Electrophilic addition of F2 to alkenes: a) XeF2 or other electrophilic fluorine derivatives. b) Highly dilute F2 at low temperature (syn addition, ion pair collapsing). Stereo chemical results and theoretical calculations: Bridged Fluoronium Ion is involved. Iodination Reaction is reversible even in the presence of excess alkene. The addition is stereo specifically anti. Polar or radical mechanism is involved.

30. Halogenation of Conjugated Dienes 1,4-Addition and cationic intermediate with Br2 (Stereo specifically syn). 1,2-Addition (AdE3 Mechanism) with mild reagents (Stereo specifically anti).

31. 6.4 Electrophilic Additions Involving Metal Ions Oxymercuration: Usual nucleophile is the solvent, but other nucleophiles can compete in less nucleophilic solvents. Because of steric effects, reaction can not be accelerated by alkyl substituents on the alkene.

32. Bridged Mercurium Ion

33. Comparison with other bridged intermediates The proton is hard acid and has no unshared electrons. The carbocation or hydrogen bridged is electron deficient and reactive.

34. The positive bromine is softer and has unshared electron pairs which can permit a total of four electrons to participate in the bridged ion. Therefore: a) The ion is more strongly bridged and more stable than hydrogen bridged. b) The bridged ion can be represented as having two covalent bonds to bromine and is electrophilic but not electron deficient.

35. The Hg2+ is a soft acid and strongly polarizing. It polarizes the p electrons of an alkene to the extent that a three-center, two-electron bond is formed between mercury and the two carbons of the double bond. Therefore: a) A three-center, two-electron bond implies weaker bridging in the mercurinium ion than the three-center, four-electron bonding of the bromonium ion. b) Oxymercuration of simple alkenes is usually a stereo specific anti addition.

36. Reactivity of mercury salts: Solvent Counter ion (CF3COO)2Hg > (CH3COO)2Hg > HgCl2 Soft anions reduce the reactivity of the Hg2+ ion by coordination, which reduce the electrophilicity of the cation. The harder oxygen anions, like NO3- and ClO4-, leave the mercuric ion in more reactive state.

38. 6.5 Addition to Alkynes and Allenes How reactive are alkynes in comparison with alkenes? What is the stereochemistry and regiochemistry of addition? Considered basic mechanisms: Mechanisms A and C are of the AdE2 type while mechanism B would be classified as AdE3.

39. Syn Addition of HCl to Aryl Acetylenes in AcOH

40. Electrophilic Addition Alkyl Acetylenes: It can be AdE3( anti addition) or AdE2, depend on individual structure and reaction conditions. Solvent Isotope Effect: Protonation is rate determining Step

41. Addition of CF3COOH to Alkynes The reaction are proceed through a vinyl cation. Halogenation of Phenyl Acetylenes

42. Chlorination of Alkyl Acetylenes a) T.S. in di-alkyl acetylenes is stabilized by both alkyl substituents. b) T.S. has bridged ion character.

43. Bromination of Acetylenes a) Alkyl acetylenes Mechanism: Termolecular (AdE3) b) Aryl acetylenes Mechanism: Through vinyl cation intermediate

44. Effect of Br- ion Addition on Mechanism Alkyne-bromine complex: a) Collapsing to bridged ion b)dissociation to vinyl cation when the cation is stable. c) Reaction with nucleophile (Br-)

45. Chlorination of 1-Hexene in AcOH: Chlorination of 1-Hexene in CH2Cl2:

46. Reaction of Acetylenes with Hg(OAc)2 in AcOH Kinetic: First order in both alkyne and Hg(OAc)2 .

47. Summary Addition to Alkynes: Vinyl cation intermediate: non-stereospecific (depending upon the life time of the vinyl cation and concentration of the nuclephiles). Bridged ion intermediate or alkyne-electrophile complex: stereospecific anti addition

48. Addition to alkenes is faster than Alkynes. a) Vinyl Cation Intermediate: Vinyl cation has around 10-15 kcal/mol higher energy than cation with sp2 hybridization. This can be partially compensated by higher G.S. of alkynes. b) Bridged ion intermediates: Larger rate retardation for the alkyne addition than that for alkenes. Greater destabilization of bridged species by strain and electronic effects in the case of alkynes.

49. Electrophilic Addition to Allenes Protonation at terminal sp2 carbon: Protonation at central sp carbon:

50. Initial Protonation of the central carbon: The twisted cation is about 17 kcal/mol higher in energy than the vinyl cation. Vinyl cation formation is kinetically favored.