1. Chapter 4 Introduction to Alkenes. Structure and Reactivity

2. Alkenes Hydrocarbons containing one or more carbon-carbon double bonds
Sometimes called olefins
Classified as unsaturated hydrocarbons
Ethylene is the simplest alkene

5. Physical Properties Very similar to corresponding alkanes: Flammable
Non-polar
Insoluble in Water
Low melting and boiling points
Low density compared to water 5

7. Bond Length
Carbon-carbon multiple bonds are shorter than their carbon-carbon single bond counterparts
133.9 pm
153.6 pm

8. Dipole moments and polarization

9. Structure and Bonding in Alkenes
Carbons involved in the carbon-carbon double bond exhibit trigonal planar geometry

10. Carbon Hybridization in Alkenes
sp2-hybridization  Trigonal planar geometry

11. sp2 = 33% s character, 66% p character
More s character → electrons are held closer to the nucleus

12. Carbon Hybridization in Alkanes

13. Carbon-carbon single bond in propene = 150 pm
Carbon-carbon single bond in propane = 154 pm
Due to sp2 vs. sp3 hybridization
Bonds with more s character are shorter

14. Hybrid Orbital Picture of Ethylene
4.1 Structure and Bonding in Alkenes

15. s bond: head-to-head overlap of orbitals
Cylindrically symmetric

16. bond: side-to-side overlap of p orbitals

17. p Orbital System in Ethylene
4.1 Structure and Bonding in Alkenes

18. The filled  molecular orbital is the  bond
EPM of Ethylene
The filled  molecular orbital is the  bond
Most of the important reactions of alkenes involve the electrons of the  bond
4.1 Structure and Bonding in Alkenes

19. Rules for Naming Alkenes
Find the longest chain that includes the double bond and name this as the parent chain by changing the ending from “-ane” to “-ene”
Hexane  Hexene
If there are two possibilities, choose the principle chain to be the one with the greatest number of double bonds
Number the chain so that the double bond has the lowest number possible and place that number before the parent chain
The double bond takes precidence over the substituents when numbering the chain
Identify the substituents with number and name

20. Problems
Name the following:

21. If there are more than one double bonds in the parent chain, identify each double bond’s position by number and indicate how many are present using Greek prefixes preceded by the letter “a.”
Example: -adiene, -atriene, etc.

22. Problems
Name the following:

23. Substituents Containing Alkenes
Some common groups with alkenes
Nonsystematic traditional names
4.2 Nomenclature of Alkenes

24. Problems
Name the following compounds:

25. Name the following compounds:
Problems
Name the following compounds:
CH3CH=CHCH3

26. Stereoisomers: Molecules that have the same molecular formula, same atom connectivity, but a different spatial arrangement
Cis-trans isomers:
Recall: Constitutional/Structural Isomers: same molecular formula, different atom connectivity

27. Restricted Rotation of Alkenes
4.1 Structure and Bonding in Alkenes

28. Properties of 2-butene Cis-2-Butene Trans-2-Butene M.P. = -139°C
B.P. = 3.7°C
Density = g/mL
Trans-2-Butene
M.P. = - 105°C
B.P. = 1°C
Density = g/mL

29. Requirements for Cis-Trans Isomerism
Only when both carbons are bonded to two different groups are cis-trans isomers possible
Compounds that have one of their carbons bonded to two identical groups can’t exist as cis-trans isomers

30. Problems
Which of the following compounds can exist as pairs of cis-trans isomers? Draw each cis-trans pair.
CH3CH=CH2
CH3CH2CH=CHCH3
(CH3)2C=CHCH3
ClCH=CHCl
There are two isomers for 3-methyl-2-pentene. Draw both of them. Which one is cis and which one is trans?

31. E,Z Nomenclature System
cis vs trans is not always clear
Cahn-Ingold-Prelog system: Assign priorities
Z (zusammen): “together”
On Zee Zame Zide
E (entgegen): “across”
E = Enemies = On Opposite Sides
4.2 Nomenclature of Alkenes

33. Priority Assignment Rules
Examine each of the double bond carbons separately. Identify the two atoms directly attached to the C and rank them according to atomic #.
Higher atomic # = higher priority
Higher isotopic mass = higher priority

34. If the first atoms connected to the double bond carbon are the same, continue moving outward until the first point of difference

35. Multiple-bonded atoms are equivalent to the same number of single bonded atoms

36. Problems
Using the Cahn-Ingold-Prelog rules, identify whether the following molecules are E or Z:

37. Problems
Give the IUPAC names for each of the following molecules, including the E, Z designation.

39. What’s the Structure of C6H10?
Saturated is C6H14
Therefore 4 H's are not present
So, what does it look like?
Double bond(s)?
Triple bond(s)?
Ring(s)?
Ring and double bond?

40. Degree of Unsaturation
Gives info on number of rings and/or p bonds
Maximum # of H’s in a hydrocarbon: CnH2n+2
Each ring and/or p bond reduces number of hydrogens by 2
aka: unsaturation number

41. For C6H10 the unsaturation # is 2
Two double bond
Two rings
One double bond and a ring
One triple bond

42. Halogens are counted as 1 H:
Calculating Degree of Unsaturation for Compounds Containing Elements Other than Carbon and Hydrogen
Halogens are counted as 1 H:
4.3 Unsaturation Number

43. Nitrogen increases H count by 1:

44. Ignore Oxygens
O’s form two bonds, don’t affect the formula of an equivalent hydrocarbon
No change in number of H atoms if insert an O into a hydrocarbon compound

45. Calculate the degree of unsaturation for the following molecules
Problems
Calculate the degree of unsaturation for the following molecules
C12H20
C4H6
C6H5N
C6H5NO2
C8H9Cl3
Draw as many structures as you can for #2

46. Alkene Stability
Who’s more stable?

48. Product Ratio
Interconversion between cis and trans isomers can be made to happen using a strong acid catalyst
More stable product is favored

49. Heats of combustion
ΔH°combustion = kJ/mol
ΔH°combustion = kJ/mol
The cis configuration is more strained = higher energy

50. Heats of Hydrogenation
More stable alkene gives off less heat

51. Effects of Branching on Alkene Stability

52. Relative Stabilities of Alkene Isomers
From heats of hydrogenation (DH°hydrog):
More alkyl substituents on double bond → more stable alkene
In general, the number of alkyl groups is more important than the identity

53. Hyperconjugation
A stabilizing interaction between a vacant p orbital or anti-bonding π orbital on one atom with the electrons in a neighboring σ bond (usually C-H or C-C)
In alkenes, occurs between the unfilled anti-bonding C=C π bond orbital and the electrons of a C-H σ bond of a neighboring substituent
More substituents present = more opportunities for hyperconjugation = more stable alkene

54. Bond Strength sp2-sp3 bond is stronger than sp3-sp3
More highly substituted alkenes have higher ratio of sp2-sp3 to sp3-sp3

55. Addition Reactions of Alkenes
Characteristic reaction of alkenes
The p bond of carbon-carbon double bond is:
Broken
Nucleophilic
Three main types of alkene addition rxns:
Rxn with hydrogen halides (ex: HCl)
Catalytic hydrogenation
Hydration

56. Alkene Reactions with Hydrogen Halides
Called “Electrophilic Addition of Alkenes”
HF, HCl, HBr, HI add to alkenes → alkyl halides
HF is rarely used due to its extreme toxicity
Lung and cornea damage
Eventual cardiac arrest and death

58. Electrophilic Addition Energy Path
Two step process
First transition state is high energy point

59. Problems
Draw the mechanism for the following reactions

60. Writing Organic Reactions

61. Markovnikov’s Rule In the addition of HX to an alkene:
the H attaches to the carbon with fewer alkyl substituents
the carbocation forms on the more substituted carbon of the C=C
the X attaches to the more substituted carbon
When one of the possible isomer products predominates, the reaction is said to be regiospecific or regioselective

64. Problems
Complete the following reaction showing the complete mechanism
What alkene would you start with to prepare the following alkyl halide?

65. Regioselectivity of Addition
Unsymmetrical alkenes can give regioisomers
If both carbons of the C=C have similar substitution, then reaction is not regiospecific

66. Carbocation Structure and Stability
The more highly substituted carbocation is more stable
Formation of more substituted carbocation intermediate is favored

67. Why?
Inductive Effect
Electrons from groups neighboring the carbocation can shift toward the positive charge
Larger groups can “donate” electrons more easily than H can

69. Hyperconjugation
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
Alkyl groups can share electrons with the empty neighboring p orbital

71. Problems
Predict the products of the following reactions:

72. Problems
Which alkenes would you start with to prepare the following alkyl halides?

73. Carbocation Rearrangements
Sometimes, we get an unexpected product
In a Carbocation Rearrangement, a group (alkyl, aryl, or hydride) moves to a different position so that a more stable carbocation can form

74. Alkyl Shift

75. Hydride Shift

76. Hydride shifts in biological molecules

77. Problem
On treatment with HBr, vinylcyclohexane undergoes addition and rearrangement to yield the following product. Propose a mechanism to account for this result

78. Catalysis
Catalyst: A substance that increases the reaction rate without being consumed
Lowers the Ea
Heterogeneous
Homogeneous
4.9 Catalysis

79. Catalytic Hydrogenation of Alkenes
Catalyst: Pt or Pd as
In powdered form on carbon
Heterogeneous catalyst

80. Aromatic  bonds are less reactive
Example:
Aromatic  bonds are less reactive
4.9 Catalysis

84. Problems
What product would you obtain from the catalytic hydrogenation of (CH3)2C=CHCH2CH3?
What product would you obtain from the reaction of 3,3-Dimethylcyclopentene with Palladium on Carbon in the presence of H2?

85. Hydration of Alkenes
Alkene
Acid-catalyzed addition of H2O across an double bond
Homogeneous catalyst

87. Problems
Draw out the mechanism and products for the following reactions:

88. Problems
What alkenes might the following alcohols have been prepared from?

89. Nature produces catalysts called enzymes
Enzymes Catalysis
Nature produces catalysts called enzymes
Most biological mechanisms would be too slow to be useful without enzymes
4.9 Catalysis

90. Principle of Microscopic Reversibility
Generally, the reverse reaction follows the exact reverse of forward mechanism
Dehydration is the reverse of hydration
4.9 Catalysis

91. Commonly, two or more reactions are in competition
Reaction Rates
Commonly, two or more reactions are in competition
4.8 Reaction Rates

92. Forward and reverse reaction have same transition state (‡)
The Transition State
Forward and reverse reaction have same transition state (‡)
4.8 Reaction Rates

93. Molecules must possess enough energy to get over the transition state
The Energy Barrier
Molecules must possess enough energy to get over the transition state
Maxwell-Boltzmann distribution
4.8 Reaction Rates

94. Intermediates are formed in many reactions
Multistep Reactions
Intermediates are formed in many reactions
Rate-determining step: Controls reaction rate
4.8 Reaction Rates

95. Hammond’s Postulate
The structure and energy of the transition state can be approximated by the structure and energy of the intermediate
4.8 Reaction Rates