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

33. Synthesis of 1-butene from 1-butanol: • CH3CH2CH2CH2-OH + HBr  CH3CH2CH2CH2-Br • SN2 • E2  KOH(alc) • CH3CH2CH=CH2 • only! • To avoid the rearrangement in the dehydration of the alcohol the alcohol is first converted into an alkyl halide.