Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza

1. Organic A Chapter 8 Alkenes (I) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza

2. Alkenes and Alkynes Hydrocarbons(contain only carbon and hydrogen) • Saturated: (Contain only single bonds) Alkanes (CnH2N + 2 ) Cycloalkanes (CnH2N ) b)Unsaturated: contain Alkenes: double bonds (,,,CnH2N) Alkynes: triple bonds ((CnH2N - 2) Aromatic: benzene like compounds

3. Facts about double and triple bonds

4. A pi bond is one in which the electrons in the p orbitals are held above and below the plane of the molecule. The sigma bond is stronger than the pi bond. A double bond is formed from a sigma bond and a pi bond, and so it is stronger than a single bond.

5. Physical Properties

6. Physical properties: • non-polar or weakly polar • no hydrogen bonding • relatively low mp/bp ~ alkanes • water insoluble • Importance: • common group in biological molecules • starting material for synthesis of many plastics

7. The Chemistry of Vision

10. The more substituted alkene will form

11. Saytzeff orientation: • In dehydrohalogenation the preferred product is the alkene that has the greater number of alkyl groups attached to the doubly bonded carbon atoms • (the more substituted alkene will form) • Ease of formation of alkenes: • R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2 • Stability of alkenes: • R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2 • CH3CH2CHCH3 + KOH(alc)  CH3CH2CH=CH2RCH=CH2 • Br 1-butene 19% • sec-butyl bromide + • CH3CH=CHCH3RCH=CHR • 2-butene 81%

12. Mechanisms of EliminationE2 with concentrated base 3>2>1second order rate = K[RX][B]

13. Mechanisms of EliminationE1 with dilute or weak base 3>2first order rate = K[RX]

15. Order of reactivity in E2: 3o > 2o > 1o • CH3CH2-X  CH2=CH2 3 adj. H’s • CH3CHCH3  CH3CH=CH26 adj. H’s & more stable • X alkene • CH3 CH3 • CH3CCH3  CH=CCH39 adj. H’s & most stable • X alkene

16. Evidence for the E2 mechanism1) second order2) No Rearrangement3) Show a large hydrogen isotope effect Primary hydrogen isotope effect: A bond to hydrogen (protium) is broken faster than a bond to deuterium (D) KH / KD = 5 - 8 This means that the breaking of hydrogen is in the rate determining step

17. Evidence for the E2 mechanismThe Absence of Hydrogen Exchange The carbanion mechanism (E1cB elimination unimolecular of the conjugate base)

18. Run the reaction until about half the substrate had been converted into alkene. Unconsumed 2-phenylethyl bromide was recovered. It contained no deuterium. So, the reaction was not acompanied by hydrogen exchange. This rules out the carbanion mechanism

19. Evidence for the E2 mechanismThe Element Effect (is the breaking of the C-X bond in the rate determining step????) Strength of the bond R-F > R-Cl > R-Br > RI Reactivity toward SN2, SN1, E2 and E1 R-I > R-Br > R-Cl > R-F So, R-X bond breaking is in the rate determining step

20. E1 Mechanism • Elimination, unimolecular E1 • a)RX: 3o > 2o > 1o • b) rearragement possible  • c) may yield mixtures  • d) Saytzeff orientation • e) element effect • f) no isotope effect • g) rate = k [RW]

24. The E1 reaction: Orientation

25. Elimination vs. substitution

27. Substitution is generally the main reaction, but, E1 Elimination occurs more with 3 > 2 >1

28. dehydration of alcohols: • ROH: 3o > 2o > 1o • acid is a catalyst • rearrangements are possible  • mixtures are possible  • Saytzeff • mechanism is E1

30. Mechanism of Dehydration (E1)

31. Dehydration (Rearrangement)

32. E1 Mechanism, Rearrangement

36. Lithium diisopropylamide Potassium tert-butoxide

37. Descriptions and explanations of the four categories. Nuc/Base Strengths • Strong/strong. In general, good bases are also good nucleophiles.  Therefore, strong bases such as negatively charged oxygens and nitrogens will also be strong nucleophiles. • Weak/weak.   In general, weak bases are also weak nucleophiles.  Therefore, weak bases such as neutral oxygens with a proton will also be weak nucleophiles. 

38. Weak/weak nuc/bases are usually also the solvent for their reactions.  This makes sense as they are so weak that you need a lot of the nuc/base to facilitie the substitution or elimination reaction.

39. Weak/strong * One exception to strong bases also being strong nucleophiles is for very bulky nuc/bases.  SN2 reactions are particularly sensitive to the size of the nuc/base because they proceed via a crowded transition state. ** Elimination reactions are less sensitive to the size of the nuc/base since the beta-hydrogen is sticking out and is easy to access. *** Therefore, a very bulky (large) nuc/base can be a weak nucleophile while still being a strong base.   

40. Strong/weak.  These nuc/bases fall into two general categories that will reduce their basicity: (i) Neutral nuc/bases that have lone pairs on less electronegative atoms such as nitrogen, sulfur, and phosphorous.  These include amines, thiols and phosphines. (ii) Negatively charged nuc/bases that are stabilized by resonance or have a negative charge on a large atom such as sulfur or iodine.

41. Examples: R-X = 1° -OH = strong nuc strong base Solvent: PAS

42. R-X  = 3°        CH3OH = weak nuc and weak base Solvent: PPS

43. R-X  = 2°    -CN = strong nuc AND weak base Solvent: PAS

44. R-X  = 2°        -OCH3 = strong nuc AND  strong base Solvent: PAS

46. References 1) http://www.chem.sc.edu/faculty/shimizu/333/Chem_333/6a.ii.html 2) http://www.freelance-teacher.com/organic_chemistry_sn2_sn1_e2_e1.pdf