1. Chapter 4 Reactions of Alkenes and Alkynes
Suggested Problems: 21-2,27,33,35-39,41-3,45-8,53

2. Diverse Reactions of Alkenes
Alkenes react with many electrophiles to give useful products by addition (often through special reagents)

3. 4.1 Addition of HX to Alkenes: Markovnikov’s Rule
In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other
If one orientation predominates, the reaction is regioselective
Markovnikov observed in the 19th century that in the addition of HX to an alkene, the H attaches to the carbon with more H’s and X attaches to the carbon with fewer H’s (to the carbon with more alkyl substituents)
This is Markovnikov’s rule

4. Example of Markovnikov’s Rule
Addition of HCl to 2-methylpropene
Regiospecific – one product forms where two are possible
If both ends have similar substitution, then not regiospecific

5. Markovnikov’s Rule (restated)
More highly substituted carbocation favored as an intermediate rather than less highly substituted one
Tertiary cations and associated transition states are more stable than primary cations (3o > 2o >1o)

6. Markovnikov’s Rule (restated)

7. 4.2 Carbocation Structure and Stability
Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2 orbitals
the fourth orbital on carbon is a vacant p-orbital
the stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents

8. 4.3 Addition of Water to Alkenes
Hydration of an alkene is the addition of H-OH to give an alcohol
Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

9. 4.3 Addition of Water to Alkenes
Hydration of an alkene is the addition of H-OH to give an alcohol
Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

10. 4.3 Addition of Water to Alkenes Oxymercuration-demercuration
In the laboratory, alkenes are often hydrated by the oxymercuration–demercuration procedure
Reaction is initiated by electrophilic addition of Hg2+ ion to the alkene (affords mercurinium ion)
Regiochemistry - Markovnikov addition of H2O

11. Worked Examples
What products would you expect from oxymercuration–demercuration of the following alkenes? a) b)
Solution:
Oxymercuration is equivalent to Markovnikov addition of H2O to an alkene
Hydration of Alkenes: Addition of H2O by oxymercuration

12. Worked Examples b)
Hydration of Alkenes: Addition of H2O by oxymercuration

13. 4.3 Addition of Water to Alkenes Hydroboration-Oxidation
Hydroboration–oxidation affords alternate regiochemistry – Anti-Markovnikov
Hydroboration involves addition of B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate
Boron is oxidized to afford an OH in place of BH2

14. 4.3 Addition of Water to Alkenes Hydroboration-Oxidation
Hydroboration–oxidation occurs with syn stereochemistry – B, H, and OH add to same face of double bond.

15. Worked Example
What alkene might be used to prepare the following alcohol by hydroboration–oxidation?
Solution:
The products result from hydroboration/oxidation of a double bond
The –OH group is bonded to the less substituted carbon of the double bond in the starting material
Hydration of Alkenes: Addition of H2O by hydroboration

16. 4.3 Addition of Water to Alkenes
Hydration of carbon-carbon double bonds also occurs in various biological pathways but not by the carbocation mechanism

17. 4.4 Addition of Halogens to Alkenes
Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process
F2 is too reactive and I2 does not add
Cl2 reacts as Cl+ Cl-
Br2 is similar

18. Addition of Br2 to Cyclopentene
Addition is exclusively trans

19. Mechanism of Bromine Addition
Br+ adds to an alkene producing a cyclic ion
Bromonium ion, bromine shares charge with carbon
Gives trans addition

20. Bromonium Ion Mechanism
Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion
This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile

21. 4.5 Reduction of Alkenes: Hydrogenation
Addition of H-H across C=C
Reduction in general is addition of H2 or its equivalent
Requires Pt or Pd as powders on carbon and H2
Hydrogen is first adsorbed on catalyst
Reaction is heterogeneous (process is not in solution)

22. Hydrogen Addition - Selectivity
Selective for C=C. No reaction with C=O, C=N
Polyunsaturated liquid oils become solids
If one side is blocked, hydrogen adds to other

23. Mechanism of Catalytic Hydrogenation
Heterogeneous – reaction between phases
Addition of H-H is syn

24. 4.6 Oxidation of Alkenes: Epoxidation, Hydroxylation, and Cleavage
Epoxidation results in a cyclic ether with an oxygen atom
Epoxides undergo acid catalyzed ring-opening .
Hydroxylation is the net of the two-step reaction.

25. Epoxide Ring-Opening Mechanism
Protonation of epoxide
Nucleophilic addition of water
Anti (trans) addition of nucleophile to epoxide

26. Permangate Hydroxylation of Alkenes
Hydroxylation of an alkene can occur in a single step via the reaction with potassium permanganate (KMnO4)
Requires a basic solution
Note – cis diol formed.

27. Permangate Oxidation of Alkenes
Potassium permanganate (KMnO4) can cleave the double bond and produce carboxylic acids and carbon dioxide if H’s are present on C=C
If hydrogens are not present on the double bond, carbonyl containing products are produced.
Requires an acid solution.

28. Permangate Oxidation of Alkenes
The type of product produced varies with the number of hydrogens on the double bond.

29. 4.7 Addition of Radicals to Alkenes: Polymers
A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization
Alkenes react with radical catalysts to undergo radical polymerization
Ethylene is polymerized to polyethylene, for example
Examples on this slide result from condensation rxns

30. Free Radical Polymerization: Initiation
Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a nonradical molecule
A bond is broken homolytically

31. Polymerization: Propagation
Radical from initiation adds to alkene to generate alkene derived radical
This radical adds to another alkene, and so on many times
Note: BzO represents a benzoyloxy moiety

32. Polymerization: Termination
Chain propagation ends when two radical chains combine
Not controlled specifically but affected by reactivity and concentration

33. Other Polymers
Other alkenes give other common polymers

34. 4.8 Conjugated Dienes
Compounds can have more than one double or triple bond
If they are separated by only one single bond they are conjugated and their orbitals interact
The conjugated diene 1,3-butadiene has properties that are very different from those of the nonconjugated diene, 1,4-pentadiene.

35. Orbital Interaction Orbital view of buta-1,3-diene
Each of the four carbon atoms has a p orbital
Allowing for an electronic interaction across the C2-C3 single bond

36. Addition of HX and X2
Conjugated diene reactions are similar to that of alkenes
But, distinct differences in the addition of HX and X2

37. Carbocations from Conjugated Dienes
Addition of H+ leads to delocalized secondary allylic carbocation

38. 4.9 Stability of Allylic Carbocations: Resonance
Some molecules have structures that cannot be shown with a single representation
In these cases we draw structures that contribute to the final structure but which differ in the position of the  bond(s) or lone pair(s)
Such a structure is delocalized and is represented by resonance forms
The resonance forms are connected by a double-headed arrow

39. Allylic Carbocations
Three electrons are delocalized over three carbons
Spin density surface shows single electron is dispersed

40. Allylic Carbocations and Resonance
A structure with resonance forms does not alternate between the forms
Instead, it is a hybrid of the resonance forms, so the structure is called a resonance hybrid

41. Products of Addition to Delocalized Carbocation
Nucleophile can add to either cationic site
The transition states for the two possible products are not equal in energy

42. 4.10 Drawing and Interpreting Resonance Forms
Resonance is an extremely useful explanatory concept for a variety of chemical phenomena
In the acetate ion, both C—O bonds are identical
Acetate is a resonance hybrid of two structures with both oxygen atoms sharing the p electrons and the negative charge equally.

43. Resonance Hybrids
Another example, benzene (C6H6) has two resonance forms with alternating double and single bonds
In the resonance hybrid, the actual structure, all its C-C bonds are equivalent, midway between double and single

44. Rules for Resonance Forms
Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure)
Resonance forms differ only in the placement of their  or nonbonding electrons
Different resonance forms of a substance do not have to be equivalent
Resonance forms must be valid Lewis structures: the octet rule generally applies
The resonance hybrid is more stable than any individual resonance form would be

45. 4.11 Alkynes and Their Reactions
Reduction accomplished by addition of H2 over a metal catalyst
Is a two-step reaction using an alkene intermediate
Reduction of alkynes

46. 4.11 Alkynes and Their Reactions
Addition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene
The two hydrogens add syn (on the same side of the triple bond)

47. Addition of HX to Alkynes Involves Vinylic Carbocations
Addition of H-X to alkyne should produce a vinylic carbocation intermediate
Secondary vinyl carbocations form less readily than primary alkyl carbocations
Primary vinyl carbocations probably do not form at all
Nonetheless, H-Br can add to an alkyne to give a vinyl bromide if the Br does not end up on a primary carbon
Markovnikov’s Rule applies

48. Addition of Bromine and Chlorine
Initial addition gives trans intermediate
Product with excess reagent is tetrahalide

49. Addition of H2O Addition of H-OH as in alkenes
Mercury (II) catalyzes Markovnikov oriented addition
Hydroboration-oxidation gives the anti-Markovnikov product

50. Formation of Acetylide Anions
Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic (pKa ~ 25. See Table 9.1 for comparisons))
Reaction of strong anhydrous bases with a terminal alkyne produces an acetylide ion (NaNH2 is base of choice)
The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus

51. Formation of Acetylide Anions
Acetylide ions can react as nucleophiles as well as bases
Reaction with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes

52. Let’s Work a Problem
Prepare n-octane from 1-pentyne.

53. Let’s Work a Problem
Prepare n-octane from 1-pentyne. The best strategy to approach this problem is to use acetylide coupling: