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ALKENE AND ALKYNE REACTIONS Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections 7.7-7.8, 7.10-7.11, 10.3-10.4, 8.2-8.8, 8.10, 8.12, 9.3-9.8, 7.1,

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Presentation on theme: "ALKENE AND ALKYNE REACTIONS Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections 7.7-7.8, 7.10-7.11, 10.3-10.4, 8.2-8.8, 8.10, 8.12, 9.3-9.8, 7.1,"— Presentation transcript:

1 ALKENE AND ALKYNE REACTIONS Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections , , , , 8.10, 8.12, , 7.1, 8.1, 9.2, 9.9

2 Outline Reactions of alkenes Reactions of alkynes Preparation of alkenes and alkynes Synthesis No reactions of alkyl halides (originally on syllabus)

3 Reaction Charts Help organize reaction details Organize charts by reaction type, starting material, product See webpage for template Example: Reaction Type Starting Material Reagent Regiochemistry/ stereochemistry Rearrangement possible? Product Oxymercuration- demercuration Alkene 1. Hg(OAc) 2, H 2 O 2. NaBH 4 Markovnikov Anti addition noAlcohol

4 Reactions of Alkenes I. Allylic halogenation II. Electrophilic addition III. Reduction IV. Oxidation V. Polymerization

5 I. Allylic Halogenation Similar to radical halogenation of alkanes Alkene react with molecular halogen in the presence of heat or light Alkyl halide is produced Substitution of –X for –H at the allylic position Most stable radical intermediate Stabilized by resonance

6 Allylic Halogenation Another set of reagents: N-bromosuccinimide (NBS), h Bromination only (no chlorination) Product is a racemic mixture (if there is a stereocenter)

7 Radical Stability

8 What is the major product of the reaction of 1-octene with NBS (in the presence of light)?

9 Reaction occurs at less sterically hindered carbon and produces the more stable C=C

10 What is the major product of the following reaction?

11 II. Electrophilic Addition Most common reaction of alkenes Examples: Break  bond of alkene Form new  bonds to each C of double bond Alkene is nucleophile; reacts with electrophile (HX, H 2 O, etc.) Forms carbocation intermediate

12 Electrophilic Addition General mechanism: Step 1: Step 2: Which step is RDS?

13 Addition of Hydrogen Halides HCl, HBr, HI Example: 2-methylpropene + HBr

14 What is the major product of the following reaction? Stereochemistry of product = racemic mixture Carbocation intermediate is planar, sp 2 hybridized Regiochemistry of reaction Which C gets the H? Which C gets the X? Reaction is regiospecific for one product

15 Regiochemistry of Electrophilic Addn. Markovnikov’s Rule: In the addition of HX (or H 2 O) to an alkene, the H will add to the carbon with the greater number of H’s already bonded to it The X (or OH) attaches to the carbon with fewer H’s (the more substituted carbon) Product = Markovnikov product Opposite product = anti-Markovnikov or non-Markovnikov Formed under specific conditions

16 Markovnikov’s Rule

17 Why is the Markovnikov product favored? Look at reaction intermediate Carbocation Markovnikov addition forms the more stable R + 3º > 2º > 1º More stable carbocation forms faster, will react to give product

18 Markovnikov’s Rule

19 Draw and name the major product of the following reaction.

20 Expected product = Actual product = What happened?

21 Carbocation Rearrangement Carbocation intermediates can rearrange to form a more stable carbocation structure Hydride shift = H: - moves from C adjacent to carbocation

22 Carbocation Rearrangement Alkyl groups can also shift Typically methyl or phenyl (Major product)

23 Anti-Markovnikov Addition of HBr In the presence of peroxides H 2 O 2 or R 2 O 2 Free radical mechanism Only HBr, not HCl or HI

24 Addition of Halogens X 2 = Br 2 or Cl 2 (F 2 too reactive, I 2 unreactive) Solvent = inert, nonaqueous Stereochemistry = anti addition Two X atoms add from opposite sides of the C=C Product = a vicinal dihalide Two X atoms on adjacent carbons

25 Mechanism

26 Addition of Halogens

27 Draw the major product of the following reaction.

28 Addition of Halogens in the Presence of Water Stereochemistry: X and OH add anti Regiochemistry: X adds to the less substituted carbon OH adds to the more substituted carbon Mechanism the same as addition of X 2, except H 2 O is the nucleophile in the second step

29 Mechanism

30 Water attacks the carbon with the largest  + Results in OH on more substituted carbon

31 Draw the major product of the following reaction.

32 Hydration Addition of water Three methods: A. Acid-catalyzed hydration B. Oxymercuration-demercuration C. Hydroboration-oxidation

33 A. Acid-catalyzed hydration Regiochemistry = Markovnikov Acid catalyst typically H 2 SO 4 or H 3 PO 4 (or just H 3 O + ) Carbocation intermediate, so rearrangement can occur

34 Mechanism

35 Draw the major product of the following reaction.

36 B. Oxymercuration-demercuration Step 1: Alkene reacts with mercuric acetate Step 2: Reduction with sodium borohydride Regiochemistry =Markovnikov Stereochemistry = anti addition of OH and H No rearrangements Milder conditions than H 3 O + Electrophile is + HgOAc Formed by dissociation of AcO-Hg-Oac Intermediate is bridged mercurinium ion (similar to bromonium)

37 Oxymercuration-demercuration

38 Draw the major product for each of the following reactions.

39 C. Hydroboration-oxidation Anti-Markovnikov product Syn addition of H and OH (add on same side of C=C) No rearrangements THF stabilize highly reactive BH 3

40 Hydroboration-oxidation Mechanism of first step: BH 2 on the right because less steric hindrance Leads to anti-Markovnikov product Second step: H 2 O 2 /NaOH replace –BH 2 with –OH Keep same stereochemistry (syn)

41 Draw the major product of the following reaction.

42 Draw the major product formed when the following alkene undergoes (a) acid-catalyzed hydration, (b) oxymercuration- demercuration, and (c) hydroboration-oxidation.

43 Oxidation and Reduction What is oxidation? What is reduction? Classify these reactions as oxidation or reduction: CH 3 ─CH═CH 2 → CH 3 ─CH 2 ─CH 3 CH 3 ─CH 2 ─OH → CH 3 ─CO 2 H

44 III. Reduction Catalytic hydrogenation Seen before with heat of hydrogenation (alkene stability) Catalyst = metal, usually Pd, Pt, or Ni Reaction takes place on metal surface Stereochemistry = syn (both H’s add to same side of C=C)

45 Mechanism

46 Catalytic Hydrogenation This reduction does not work with C=O, C=N, or benzene except at very high P or T, or with a special catalyst

47 IV. Oxidation Three types A. Epoxidation B. Hydroxylation C. Oxidative cleavage

48 A. Epoxidation Formation of epoxide Cyclic ether Example: Reagent is peroxy acid (RCO 3 H) Stereochemistry = syn Another method: treat halohydrin with base:

49 B. Hydroxylation Formation of a 1,2-diol/glycol/vicinal diol Methods: 1. Opening of epoxide using aqueous acid Product is trans diol Mechanism:

50 Hydroxylation 2. Addition of osmium tetroxide (OsO 4 ) or potassium permanganate (KMnO 4 ) How do you know these are both oxidizing agents? Reaction includes some appropriate work-up H 2 O 2 or NaHSO 3, H 2 O for OsO 4 HO - (aq) for KMnO 4 Stereochemistry = syn

51 Draw the major product of the following reaction.

52 C. Oxidative Cleavage Oxidize and alkene and split the C=C Results in formation of 2 carbonyls Type of carbonyls depends on alkene structure and the oxidizing agent used Three types of oxidizing agents 1. Ozone 2. Potassium permanganate 3. Periodic acid

53 Oxidative Cleavage 1. Ozone Ozonolysis Reagents: 1. O 3 2. (CH 3 ) 2 S or Zn, H 3 O + Products = 2 carbonyls (ketones or aldehydes) Terminal alkenes give CO 2

54 Oxidative Cleavage 2. KMnO 4 Reagents: KMnO 4 (excess or concentrated) and heat or acid Use heat and excess KMnO 4 to split intermediate glycol Products = 2 carbonyls (ketones or carboxylic acids) Aldehydes oxidize to carboxylic acids in KMnO 4 Terminal alkenes still give CO 2

55 Oxidative Cleavage 2. HIO 4 Specifically used to split glycol

56 Draw the major product for each of the following reactions.

57 V. Polymerization Polymer = large molecule synthesized by covalently linking single parts (monomers) Biological polymers: proteins, cellulose, nucleic acids Organic polymers: plastics Chain-growth polymers: made from alkene monomers Radical reaction

58 Chain-growth Polymerization Initiation by peroxides: Propagation: Termination: R─CH 2 CH 2 + CH 2 CH 2 ─R → R─CH 2 CH 2 CH 2 CH 2 ─R

59 Chain-growth Polymers

60 Draw the structure of poly(vinyl chloride).


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