Organic Chemistry

1
Organic Chemistry
William H. Brown Christopher S. Foote
2
Alkenes II

• Chapter 6

3
Characteristic Reactions
4
Characteristic Reactions
5
Reaction Mechanisms

• A reaction mechanism describes details of how a
  reaction occurs
• which bonds are broken and which new ones are
  formed
• the order and relative rates of the various
  bond-breaking and bond-forming steps
• if in solution, the role of the solvent
• if there is a catalyst, the role of a catalyst
• the position of all atoms and energy of the
  entire system during the reaction

6
Energy Diagrams

• Energy diagram a graph showing the changes in
  energy that occur during a chemical reaction
• Reaction coordinate a measure in the change in
  positions of atoms during a reaction

7
Gibbs Free Energy

• Gibbs free energy a thermodynamic function
  relating enthalpy, entropy, and temperature
• exergonic reaction a reaction in which the Gibbs
  free energy of the products is lower than that of
  the reactants an exergonic reaction is
  spontaneous
• endergonic reaction a reaction in which the
  Gibbs free energy of the products is higher than
  that of the reactants an endergonic reaction is
  never spontaneous.

8
Gibbs Free Energy
9
Energy Diagrams

• Heat of reaction the difference in energy
  between reactants and products
• exothermic reaction a reaction in which the
  enthalpy of the products is lower than that of
  the reactants a reaction in which heat is
  released
• endothermic reaction a reaction in which the
  enthalpy of the products is higher than that of
  the reactants a reaction in which heat is
  absorbed

10
Activation Energy

• Transition state
• an unstable species of maximum energy formed
  during the course of a reaction
• a maximum on an energy diagram
• Activation Energy, ?G the difference in energy
  between reactants and a transition state
• if large, only a few collisions occur with
  sufficient energy to reach the transition state
  reaction is slow
• if small, many collisions occur with sufficient
  energy to reach the transition state reaction is
  fast

11
Energy Diagram
A one-step reaction with no intermediate
12
Energy Diagram
A two-step reaction with one intermediate
13
Activation Energy Rate

• The relationship between a rate constant, k, and
  activation energy is given by the equation
• C a constant (units s-1) that depends on the
  reaction
• R gas constant, 8.315 x 10-3 kJ (1.987 x 10-3
  kcal)mol-1K -1
• T temperature in Kelvins

14
Activation Energy Rate

• Example What is the activation energy for a
  reaction whose rate at 35C is twice that at
  25C?
• Solution
• the ratio of rate constants k2 and k1 for the
  reaction at temperatures T2 and T1 is
• taking the log of both sides and rearranging
  gives

15
Activation Energy Rate

• substituting values and solving gives

16
Developing a Mechanism

• How it is done
• design experiments to reveal details of a
  particular chemical reaction
• propose a set or sets of steps that might account
  for the overall transformation
• a mechanism becomes established when it is shown
  to be consistent with every test that can be
  devised
• this does mean that the mechanism is correct,
  only that it is the best explanation we are able
  to devise

17
Why Mechanisms?

• framework within which to organize descriptive
  chemistry
• intellectual satisfaction derived from
  constructing models that accurately reflect the
  behavior of chemical systems
• a tool with which to search for new information
  and new understanding

18
Electrophilic Additions

• hydrohalogenation using HCl, HBr, HI
• hydration using H2O, H2SO4
• halogenation using Cl2, Br2
• halohydrination using HOCl, HOBr
• oxymercuration using Hg(OAc)2, H2O

19
Addition of HX

• Carried out with pure reagents or in a polar
  solvent such as acetic acid
• Addition is regioselective
• regioselective reaction a reaction in which one
  direction of bond forming or breaking occurs in
  preference to all other directions of bond
  forming or breaking
• regiospecific reaction a reaction in which one
  direction of bond forming or breaking occurs to
  the exclusion of all other directions of bond
  forming or breaking

20
Addition of HX

• H adds to the less substituted carbon
• Markovnikovs rule in the addition of HX, H2O,
  or ROH to an alkene, H adds to the carbon of the
  double bond having the greater number of hydrogens

21
HCl 2-Butene

• A two-step mechanism
• Step 1 formation of sec-butyl cation, a 2
  carbocation intermediate
• Step 2 reaction of the sec-butyl cation (a Lewis
  acid) with chloride ion (a Lewis base) completes
  the reaction

22
HCl 2-Butene
23
Carbocations

• Carbocation a species in which a carbon atom has
  only six electrons in its valence shell and bears
  positive charge
• Carbocations are
• classified as 1, 2, or 3 depending on the
  number of carbons bonded to the carbon bearing
  the positive charge
• electrophiles that is, they are electron-loving
• Lewis acids

24
Carbocation Structure

• bond angles about a positively charged carbon are
  approx. 120
• carbon uses sp2 hybrid orbitals to form sigma
  bonds to the three attached groups
• the unhybridized 2p orbital lies perpendicular to
  the sigma bond framework and contains no electrons

25
Carbocation Stability

• a 3 carbocation is more stable than a 2
  carbocation, and requires a lower activation
  energy for its formation
• a 2 carbocation is, in turn, more stable than a
  1 carbocation, and requires a lower activation
  energy for its formation
• methyl and primary carbocations are so unstable
  that they are never observed in solution

26
Carbocation Stability

• relative stability
• methyl and primary carbocations are so unstable
  that they are never observed in solution

27
Carbocation Stability

• we can account for the relative stability of
  carbocations if we assume that alkyl groups
  attached to the positively charged carbon are
  electron-releasing and thereby help delocalize
  the positive charge of the cation
• we account for this electron-releasing ability of
  alkyl groups by (1) the inductive effect, and (2)
  hyperconjugation

28
Carbocation Stability

• The inductive effect
• the electron-deficient carbon bearing the
  positive charge polarizes electrons of the
  adjacent sigma bonds toward it
• the positive charge on the cation is not
  localized on the trivalent carbon, but
  delocalized over nearby atoms
• the larger the volume over which the positive
  charge is delocalized, the greater the stability
  of the cation

29
Carbocation Stability

• Hyperconjugation
• partial overlap of the ? bonding orbital of an
  adjacent C-H bond with the vacant 2p orbital of
  the cationic carbon delocalizes the positive
  charge and also the electrons of the adjacent ?
  bond
• replacing a C-H bond with a C-C bond increases
  the possibility for hyperconjugation

30
Addition of H2O

• addition of water is called hydration
• acid-catalyzed hydration of an alkene is
  regioselective hydrogen adds preferentially to
  the less substituted carbon of the double bond

31
Addition of H2O

• Step 1 proton transfer from solvent to the
  alkene
• Step 2 a Lewis acid/base reaction
• Step 3 proton transfer to solvent

32
C Rearrangements

• In electrophilic addition to alkenes, there is
  the possibility for rearrangement
• Rearrangement a change in connectivity of the
  atoms in a product compared with the connectivity
  of the same atoms in the starting material

33
C Rearrangements

• in addition of HCl to an alkene
• in acid-catalyzed hydration of an alkene

34
C Rearrangements

• driving force is rearrangement of a less stable
  carbocation to a more stable one
• the less stable 2 carbocation rearranges to a
  more stable 3 one by 1,2-shift of a hydride ion

35
C Rearrangements

• reaction of the more stable carbocation (a Lewis
  acid) with chloride ion (a Lewis base) completes
  the reaction

36
Addition of Cl2 and Br2

• carried out with either the pure reagents or in
  an inert solvent such as CH2Cl2
• addition is stereoselective
• Stereoselective reaction a reaction in which one
  stereoisomer is formed or destroyed in preference
  to all others
• Stereospecific reaction a reaction in which one
  stereoisomer is formed or destroyed to the
  exclusion to all others

37
Addition of Cl2 and Br2
38
Addition of Cl2 and Br2

• Step 1 formation of a bridged bromonium ion
  intermediate
• Step 2 attack of halide ion from the opposite
  side of the three-membered ring

39
Addition of Cl2 and Br2

• For a cyclohexene, anti coplanar addition
  corresponds to trans diaxial addition

40
Addition of HOCl and HOBr

• Treatment of an alkene with Br2 or Cl2 in water
  forms a halohydrin
• Halohydrin a compound containing -OH and -X on
  adjacent carbons

41
Addition of HOCl and HOBr

• reaction is both regiospecific (anti addition)
  and stereoselective (OH to the more substituted
  carbon)

42
Addition of HOCl and HOBr

• Step 1 formation of a bridged halonium ion
  intermediate
• Step 2 attack of H2O on the more substituted
  carbon opens the three-membered ring

43
Oxymercuration/Reduction

• oxymercuration followed by reduction results in
  hydration of a carbon-carbon double bond

44
Oxymercuration/Reduction

• addition of Hg(II) and oxygen is anti coplanar
  stereoselective

45
Oxymercuration/Reduction

• Step 1 dissociation of mercury (II) acetate
  gives AcOHg, a Lewis acid
• Step 2 attack of AcOHg on the double bond gives
  a bridged mercurinium ion intermediate in which
  the two electrons of the pi bond form a two-atom
  three-center bond

46
Oxymercuration/Reduction
47
Oxymercuration/Reduction

• Step 3 stereospecific and regioselective attack
  of H2O on the bridged intermediate opens the
  mercurinium ion ring
• Step 4 reduction of the C-HgOAc bond

48
Oxymercuration/Reduction

• the fact that oxymercuration occurs without
  rearrangement indicates that the intermediate is
  not a true carbocation, but rather a resonance
  hybrid closely resembling a bridged mercurinium
  ion intermediate
• regioselectivity is accounted for by at least
  some carbocation character in the bridged
  intermediate
• stereospecificity is accounted for by anti attack
  on the bridged intermediate

49
Hydroboration/Oxidation

• Hydroboration the addition of borane, BH3, to an
  alkene to form a trialkylborane
• Borane dimerizes to diborane, B2H6

50
Hydroboration/Oxidation

• borane forms a stable complex with ethers such as
  THF
• the reagent is used most often as a commercially
  available solution of BH3 in THF

51
Hydroboration/Oxidation

• Hydroboration is both regioselective (boron to
  the less hindered carbon) and stereospecific (syn
  addition)

52
Hydroboration/Oxidation

• mechanism involves concerted regioselective and
  stereospecific addition of B and H to the
  carbon-carbon double bond

53
Hydroboration/Oxidation

• trialkylboranes are rarely isolated
• oxidation with alkaline hydrogen peroxide gives
  an alcohol and sodium borate
• The result of hydroboration/oxidation is
  regioselective and stereospecific hydration of a
  carbon-carbon double bond

54
Hydroboration/Oxidation
55
Oxidation/Reduction

• Oxidation the loss of electrons
• or the loss of H, the gain of O, or both
• Reduction the gain of electrons
• or the gain of H, the loss of O, or both
• Recognize using a balanced half-reaction
• 1. write a half-reaction showing one reactant and
  its product(s)
• 2. complete a material balance. Use H2O and H in
  acid solution use H2O and OH- in basic solution
• 3. complete a charge balance using electrons, e-

56
Oxidation/Reduction

• three balanced half-reactions

57
Oxidation with OsO4

• Oxidation by OsO4 converts an alkene to a glycol,
  a compound with -OH groups on adjacent carbons
• oxidation is syn stereospecific

58
Oxidation with OsO4

• OsO4 is both expensive and highly toxic
• it is used in catalytic amounts with another
  oxidizing agent to reoxidize its reduced forms
  and, thus, recycle OsO4

59
Oxidation with O3

• Treatment of an alkene with ozone followed by a
  weak reducing agent cleaves the CC and forms two
  carbonyl groups in its place

60
Oxidation with O3

• the initial product is a molozinide which
  rearranges to an isomeric ozonide

61
Reduction of Alkenes

• Most alkenes react with H2 in the presence of a
  transition metal catalyst to give alkanes
• commonly used catalysts are Pt, Pd, Ru, and Ni
• The process is called catalytic reduction or,
  alternatively, catalytic hydrogenation

62
Reduction of Alkenes

• Most common pattern is syn stereoselectivity

63
Reduction of Alkenes

• Mechanism of catalytic hydrogenation
• H2 is absorbed on the metal surface with
  formation of metal-hydrogen bonds
• the alkene is also absorbed with formation of
  metal-carbon bonds
• a hydrogen atom is transferred to the alkene
  forming one new C-H bond
• a second hydrogen atom is transferred forming the
  second C-H bond

64
?H of Hydrogenation

• Reduction of an alkene to an alkane is exothermic
• there is net conversion of one pi bond to one
  sigma bond
• ?H depends on the degree of substitution
• the greater the substitution, the lower the value
  of ?H
• ?H for a trans alkene is lower than that of an
  isomeric cis alkene

65
?H of Hydrogenation
66
?H of Hydrogenation

• The greater the degree of substitution of a
  double bond, the lower its heat of hydrogenation
• the greater the degree of substitution, the more
  stable the double bond
• The heat of hydrogenation of a trans alkene is
  lower than that of the isomeric cis alkene
• a trans alkene is more stable than its isomeric
  cis alkene
• the difference is due to nonbonded interaction
  strain in the cis alkene

67
?H of Hydrogenation
68
Reaction Stereochemistry

• In several of the reactions presented in this
  chapter, stereocenters are created
• Where one or more stereocenters are created, is
  the product
• one enantiomer and, if so, which one?
• a pair of enantiomers?
• a meso compound?
• a mixture of stereoisomers?
• or what?

69
Reaction Stereochemistry

• Which of the three possible stereoisomers of
  2,3-dibromobutane are formed in the addition of
  bromine to trans-2-butene?
• the three possible stereoisomers for this product
  are a pair of enantiomers and a meso compound

70
Reaction Stereochemistry

• Reaction of bromine with the alkene forms a
  cyclic bromonium ion intermediate
• which is then opened by attack of bromide ion
  from the side opposite the bromine bridge

71
Reaction Stereochemistry
72
Reaction Stereochemistry

• How many and what kind of stereoisomers are
  formed in the oxidation of cis-2-butene by OsO4?

73
Reaction Stereochemistry
74
Reaction Stereochemistry

• How many and what kind of stereoisomers are
  formed in the oxidation of trans-2-butene by OsO4?

75
Reaction Stereochemistry

• Enantiomerically pure products can never be
  formed from achiral starting materials and
  reagents
• An enantiomerically pure product can be generated
  in a reaction if at least one of the reactants is
  enantiomerically pure, or if the reaction is
  carried out in an achiral environment

76
Prob 6.15

• Draw the isomeric carbocations formed on
  treatment of each alkene with HCl. Which is the
  more stable?

77
Prob 6.16

• Arrange the alkenes in each set in order of
  increasing rate of reaction with HI.

78
Prob 6.17

• Write the major product formed on treatment
  of 2-butene with each reagent.

79
Prob 6.18

• What alkene undergoes acid-catalyzed
  hydration to give each alcohol as the major
  product?

80
Prob 6.19

• Reaction of 2-methyl-2-pentene with each
  reagent is regiospecific. What is the
  regiospecificity and how it is accounted for?

81
Prob 6.21

• Draw the alkene of indicated molecular
  formula that gives the compound shown as the
  major product.

82
Prob 6.22

• Account for the fact that addition of HCl to
  1-bromopropene gives 1-bromo-1-chloropropane.

83
Prob 6.23

• Propenoic acid reacts with HCl to give
  3-chloropropanoic acid. Account for this result.

84
Prob 6.24

• Draw a structural formula for the alkene of
  molecular formula C5H10 that reacts with Br2 to
  give each product.

85
Prob 6.26

• Draw a structural formula of the cycloalkene
  of molecular formula C6H10 that reacts with Cl2
  to give each compound.

86
Prob 6.27

• Treatment of this bicycloalkene with Br2
  gives a trans dibromide. Of the two possible
  trans dibromides, only one is formed. Which is
  formed? Account for its formation to the
  exclusion of its isomer.

87
Prob 6.28

• Propose a structural formula for terpin. How
  many cis,trans isomers are possible for the
  structural formula you have proposed?

88
Prob 6.29

• Propose a mechanism for this reaction.

89
Prob 6.30

• Propose a mechanism for this reaction.

90
Prob 6.31

• Propose a mechanism for the formation of each
  product.

91
Prob 6.32

• Propose a mechanism for the formation of each
  product.

92
Prob 6.33

• Propose a mechanism for this reaction.

93
Prob 6.34

• Propose a mechanism for this reaction.

94
Prob 6.35

• Propose a mechanism for each reaction.

95
Prob 6.36

• Propose a mechanism for this reaction.

96
Prob 6.37

• Draw a structural formula for the alcohol
  formed by treatment of each alkene with B2H6 in
  THF followed by treatment with alkaline H2O2.

97
Prob 6.38

• Of the four possible cis,trans isomers
  possible for this compound, one is formed in 85
  yield. Propose a structure for this isomer.

98
Prob 6.41

• Draw a structural formula of the alkene that
  gives each set of products.

99
Prob 6.47

• State the number and kind of stereoisomers
  formed when (R)-3-methyl-1-pentene is treated
  with each reagent.

100
Prob 6.49

• For each reaction determine (1) how many
  stereoisomers are possible for the product, (2)
  which of the possible ones are formed, and (3)
  whether the product is optically active or
  inactive.

101
Prob 6.49 (contd)
102
Alkenes II

• End Chapter 6