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1 Reactions of Alkenes: Addition Reactions Disparlure: sex attractant of the female gypsy moth. (A type of pheromone.)

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Presentation on theme: "1 Reactions of Alkenes: Addition Reactions Disparlure: sex attractant of the female gypsy moth. (A type of pheromone.)"— Presentation transcript:

1 1 Reactions of Alkenes: Addition Reactions Disparlure: sex attractant of the female gypsy moth. (A type of pheromone.)

2 2 I. Hydrogenation of Alkenes Addition Reactions oleic acid (unsaturated) stearic acid (saturated)

3 3 I. Hydrogenation of Alkenes A. Heats of hydrogenation  Hº ~ -30 kcal/mol  Hº kcal More substituted  more stable.

4 4 I. Hydrogenation of Alkenes A. Heats of hydrogenation less stable more stable

5 5 I. Hydrogenation of Alkenes B. Stereochemistry of hydrogenation anti additionsyn addition Catalytic hydrogenation proceeds by syn addition:

6 6 I. Hydrogenation of Alkenes Question 6-1. Give the product of the following reaction. Click on the arrow to check your answer. Check Answer

7 7 I. Hydrogenation of Alkenes Answer 6-1. Give the product of the following reaction. Click on the arrow to check your answer.

8 8 II. Electrophilic Addition “Loose”  electrons are nucleophilic (Lewis bases), react with electrophiles (Lewis acids).

9 9 II. Electrophilic Addition A. Addition of hydrogen halides (X = Cl, Br, I) Reactivity: HI > HBr > HCl >> HF (stronger acid = better electrophile)

10 10 II. Electrophilic Addition A. Addition of hydrogen halides 1. Markovnikov’s rule In the addition of HX to an alkene, the H goes to the carbon with more H’s. Question 6-2. Draw the products. Click on the arrow to check answers. Check Answer

11 11 II. Electrophilic Addition A. Addition of hydrogen halides 1. Markovnikov’s rule In the addition of HX to an alkene, the H goes to the carbon with more H’s. Answer 6-2.

12 12 II. Electrophilic Addition A. Addition of hydrogen halides 2. mechanism Mechanistic interpretation of Markovnikov’s rule: The reaction proceeds through the more stable carbocation intermediate.

13 13 II. Electrophilic Addition A. Addition of hydrogen halides 2. mechanism lower E a  faster rate of formation

14 14 II. Electrophilic Addition A. Addition of hydrogen halides 3. carbocation rearrangements

15 15 II. Electrophilic Addition A. Addition of hydrogen halides 3. carbocation rearrangements Question 6-3. Give the expected major product for each reaction. Click on the arrow to check answers. Check Answer

16 16 II. Electrophilic Addition A. Addition of hydrogen halides 3. carbocation rearrangements Answer 6-3. Give the expected major product for each reaction. Click on the arrow to check answers. forms tertiary carbocation- no need for rearrangement. forms secondary carbocation, rearranges to a tertiary carbocation via a hydride shift forms a secondary carbocation, rearranges to a tertiary carbocation via a methide shift

17 17 II. Electrophilic Addition A. Addition of hydrogen halides 4. free-radical addition of HBr Markovnikov orientation antiMarkovnikov orientation (peroxide effect)

18 18 II. Electrophilic Addition A. Addition of hydrogen halides 4. free-radical addition of HBr Free radical chain mechanism: Initiation Propagation

19 19 II. Electrophilic Addition A. Addition of hydrogen halides 4. free-radical addition of HBr Reaction proceeds through more stable radical intermediate.

20 20 II. Electrophilic Addition A. Addition of hydrogen halides 4. free-radical addition of HBr Compare: addition of HBr with and without peroxides More stable intermediate in both cases. Markovnikov orientation antiMarkovnikov orientation Regiochemical control

21 21 II. Electrophilic Addition A. Addition of hydrogen halides Question 6-5. Give the major products of the following reactions. Click on the arrow to check your answer. Check Answer

22 22 II. Electrophilic Addition A. Addition of hydrogen halides Answer 6-5. Give the major products of the following reactions. goes Markovnikov via electrophilic reaction goes anti-Markovnikov via radical reaction rearranges to more stable carbocation goes anti-Mark via free radical, no rearrangement

23 23 II. Electrophilic Addition B. Addition of sulfuric acid (industrial) alkyl hydrogen sulfate overall: hydration Markovnikov orientation

24 24 II. Electrophilic Addition C. Acid-catalyzed hydration reverse of dehydration (Le Châtelier) Markovnikov Principle of microscopic reversibility

25 25 II. Electrophilic Addition D. Addition of halogens (X = Cl or Br) a vicinal dihalide Stereoselective anti addition: trans only

26 26 II. Electrophilic Addition D. Addition of halogens Mechanism: halonium ions cyclic bromonium ion anti addition trans product

27 27 II. Electrophilic Addition E. Formation of halohydrins vicinal halohydrin anti addition

28 28 II. Electrophilic Addition E. Formation of halohydrins Regioselective:

29 29 II. Electrophilic Addition E. Formation of halohydrins resonance hybrid: more substituted C carries greater  +, has stronger attraction for nucleophile

30 30 II. Electrophilic Addition E. Formation of halohydrins Other nucleophiles:

31 31 II. Electrophilic Addition E. Formation of halohydrins Question 6-8. Give the products, showing stereochemistry where applicable. Click on the arrow to check your answers. Check Answer

32 32 II. Electrophilic Addition E. Formation of halohydrins Answer 6-8. Give the products, showing stereochemistry where applicable. Click on the arrow to check your answers. Br - is nucleophile H 2 O is nucleophile Ethanol is nucleophile Interesting! Br + is electrophile, since it is less electronegative; Cl - is nucleophile.

33 33 III. Other Reactions of Alkenes A. Hydroboration-oxidation a boron hydride an organoborane antiMarkovnikov orientation stereoselective syn addition no rearrangements )3)3 anti-Markovnikov syn addition

34 34 III. Other Reactions of Alkenes A. Hydroboration-oxidation Mechanism:

35 35 III. Other Reactions of Alkenes A. Hydroboration-oxidation Regiochemical control in synthesis: Markovnikov antiMarkovnikov

36 36 III. Other Reactions of Alkenes A. Hydroboration-oxidation Question 6-9. Give the products of the following reactions. Click on the arrow to check your answers. Check Answer

37 37 III. Other Reactions of Alkenes A. Hydroboration-oxidation Answer 6-9. Give the products of the following reactions. Click on the arrow to check your answers. anti-Markovnikov Markovnikov

38 38 III. Other Reactions of Alkenes B. Epoxidation epoxyethane (ethylene oxide) 1,2-epoxypropane (propylene oxide) 1,2-epoxycyclohexane (cyclohexene oxide)

39 39 III. Other Reactions of Alkenes B. Epoxidation Stereospecific syn addition: cis-2,3-epoxybutane trans-2,3-epoxybutane

40 40 III. Other Reactions of Alkenes C. Ozonolysis oxidative cleavage

41 41 III. Other Reactions of Alkenes C. Ozonolysis Synthesis: Analysis:Question What is the structure of the alkene if ozonolysis produced the following: Check Answer

42 42 III. Other Reactions of Alkenes C. Ozonolysis Answer What is the structure of the unknown alkene if ozonolysis produced the following: The fragments are colored to make it easier to see how they go together. The fragment with two carbonyls must contain two double bonds. The alkene is 2,7-dimethyl-2,4-octadiene. It is impossible to determine whether the diene is E or Z. + +

43 43 III. Other Reactions of Alkenes D. Polymerization monomerpolymer n = 1000’s – 10,000’s radical initiators:O 2 di-t-butyl peroxide benzoyl peroxide

44 44 III. Other Reactions of Alkenes D. Polymerization monomerpolymer polyethylene polypropylene (“poly”) poly(vinyl chloride) (PVC, “vinyl”) polystyrene

45 45 III. Other Reactions of Alkenes D. Polymerization monomerpolymer “Saran” “Orlon” polytetrafluoroethylene (PTFE, “Teflon”)

46 46 III. Other Reactions of Alkenes D. Polymerization Free-radical chain polymerization: Initiation Propagation “head to tail” polymerization

47 47 Summary: Reactions of Alkenes Question Draw the structures of the products in the following reactions, including stereochemistry where appropriate. Check Answer

48 48 Summary: Reactions of Alkenes Answer Draw the structures of the products in the following reactions, including stereochemistry where appropriate.

49 49 Summary: Reactions of Alkenes Question What reagents would be used to carry out the following conversions? Check Answer

50 50 Summary: Reactions of Alkenes Answer What reagents would be used to carry out the following conversions?

51 51 IV. Introduction to Organic Synthesis What kinds of reactions will A undergo? What kinds of reactions will produce Z? How do I find a path between them?

52 52 IV. Introduction to Organic Synthesis Work backwards! And know the reactions! What different methods do you know for making an alkene? 1. Dehydrating an alcohol with strong acid; OR 2. Dehydrohalogenating an alkyl halide with a strong base. So, you could either make an alcohol and dehydrate it, or you could make an alkyl halide and dehydrohalogenate it. Which seems easier to make? It will be easier to make an alkyl halide since you know how to convert an alkane to an alkyl halide with free radical halogenation. Since free radical bromination is more selective for tertiary positions, you should use Br 2 and heat or light rather than free radical chlorination, which would give mixtures. Solution:

53 53 IV. Introduction to Organic Synthesis Question Convert the starting material to the indicated product. Show all necessary reagents. Write the product of each step. Then check your answers. Check Answer

54 54 IV. Introduction to Organic Synthesis Answer Convert the starting material to the indicated product. Show all necessary reagents. Write the product of each step. Then check your answers. 1. Cl 2, hv (or Br 2, heat); 2. KOH, ethanol 1. Br 2, heat; 2. KOH, EtOH; 3. a) B 2 H 6 ; b)H 2 O 2, OH - 1. KOH, EtOH; 2. HBr, peroxides 1. Cl 2, hv; 2. KOH, EtOH; 3. Cl 2, CCl 4

55 55 IV. Introduction to Organic Synthesis Answer Convert the starting material to the indicated product. Show all necessary reagents. Write the product of each step. Then check your answers. 1. H 2 SO 4, heat; 2. Br 2, H 2 O 1. Br 2, CH 3 OH; 2. KOH, EtOH 1. H 2 SO 4, heat; 2. A) O 3 ; b) H 2 O, Zn 1. KOH, EtOH; 2. CH 3 CO 3 H


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