1. Chapter 4 -- Introduction to Alkenes
4.1 – Structure and Bonding in Alkenes
4.3 – Unsaturation Number
4.2 – Stereocenters and Alkene Nomenclature
4.4 – Physical Properties of Alkenes (low emphasis)
4.5 – Relative Stabilities of Alkene Isomers
4.6 – Alkene Addition Reactions (more in later chapters)
4.7 – Adding Hydrogen Halides to Alkenes: Carbocations
4.8 – Reaction Rates
4.9 – Catalysis

2. Alkenes
Alkenes are hydrocarbons that contain one or more C=C double bonds.
They are classified as unsaturated hydrocarbons (alkanes are classified as saturated).
Also known as olefins.
Ethylene (ethene) is the simplest alkene.
4.1 Structure and Bonding in Alkenes

3. Actual Ethene (Ethylene) geometry
Geometry is typical of other alkenes.
Ethylene is a planar molecule (trigonal planar geometry). (perfect sp2 would 120o all around)
4.1 Structure and Bonding in Alkenes

4. Bond Length
Bonds with more s character are shorter and stronger (harder to break).
4.1 Structure and Bonding in Alkenes

5. Carbon Hybridization in Alkenes
Alkene carbon orbitals are hybridized differently than alkane carbon orbitals.
Alkene carbon – sp2 hybridized.
4.1 Structure and Bonding in Alkenes

6. Orbital Picture of Ethylene – the sp2 bonds
Trigonal planar geometry → sp2-hybridization
~33% s character, ~66% p character
More s character → electrons are held closer to the nucleus.
4.1 Structure and Bonding in Alkenes

7. The bonds of ethene ( s versus p )
s bond: head-to-head overlap of orbitals
p bond: side-to-side overlap of p orbitals
4.1 Structure and Bonding in Alkenes

8. Pi Orbital Interaction Diagram for Ethylene
4.1 Structure and Bonding in Alkenes

9. Electron Distribution in Ethylene
There is a local negative charge associated above and below the C=C double bond nodal plane.
Most of the important reactions of alkenes involve the electrons in the filled  bonding orbital acting as an electron donor (nucleophile).
4.1 Structure and Bonding in Alkenes

10. Unsaturation Number 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 degree of unsaturation
4.3 Unsaturation Number

11. Unsaturation Number: these eqns do work
Oxygen can be ignored (CH4 CH3OH )
Halogens are counted as 1 H: (CH4 CH3X)
Nitrogen increases H count by 1: ( CH3NH2 )
C = # of carbons, N = # of nitrogens,
H and X similarly the number H’s and halogens in the formula.
4.3 Unsaturation Number

12. Rotation is restricted in alkenes
4.1 Structure and Bonding in Alkenes

13. Double-Bonds can lead to Stereoisomers
Stereoisomers: Molecules with the same atom connectivity, but different spatial arrangement.
Stereoisomers of 2-butene:
Each stereoisomer of 2-butene has its own characteristic properties (i.e. different boiling points, different polarities, etc.) and they do not interconvert without a “catalyst”.
4.1 Structure and Bonding in Alkenes

14. A new concept: Stereocenters
When an alkene can exist as double-bond stereoisomers, both carbons of the double bond are stereocenters.
Stereocenter – an atom at which the interchange of two bonded groups gives a stereoisomer.
4.1 Structure and Bonding in Alkenes

15. IUPAC Nomenclature for Alkenes
Alkene nomenclature is derived by modifying alkane nomenclature.
Find principle chain with greatest number of double bonds.
The alkene carbons get lower numbers.
4.2 Nomenclature of Alkenes

16. More Than One Pi Bond
If the principal chain has two double bonds, change the –ane ending to –adiene.
Three double bonds –atriene and so on.
4.2 Nomenclature of Alkenes

17. Substituents Containing Alkenes
Some common groups that contain double bonds:
4.2 Nomenclature of Alkenes

18. Substituents Containing Alkenes
Substituent groups are numbered from the point of attachment to the principal chain or ring.
4.2 Nomenclature of Alkenes

19. Traditional (common) names
Some alkenes have nonsystematic traditional names that are recognized by IUPAC.
For now you don’t need to know styrene, but you are expected to know isoprene
4.2 Nomenclature of Alkenes

20. The E, Z Nomenclature System
cis vs trans is not always clear
Cahn-Ingold-Prelog system: Assign relative priorities to each of the two groups attached to each alkene carbon.
Z (zusammen): “together” – Higher priority groups on the same side.
E (entgegen): “across” – Higher Priority groups on opposite sides.
4.2 Nomenclature of Alkenes

21. Priority Sequence Rules
Sequence Rules are used to assign relative priorities.
Examine atoms directly attached to each carbon of the double bond (level 1).
Higher atomic # = higher priority
Higher isotopic mass = higher priority
4.2 Nomenclature of Alkenes

22. Sequence Rules
If level 1 atoms are the same, then work outwards until you find the first point of difference.
Arrange atoms in each set by decreasing priority order.
Higher atomic # (or mass isotope) at the first point of difference = higher priority
4.2 Nomenclature of Alkenes

23. Sequence Rules Dealing with double and triple bonds:
Double bonds – rewritten as single bonds and the atoms at each end of the double bond are duplicated.
Triple bonds – rewrite and triplicate
4.2 Nomenclature of Alkenes

24. DO SOME then skip through the next few

25. Sequence Rules
If level 2 sets in the two group are identical, then, within each set, choose the atom of highest priority. Identify the level 3 set of atoms attached to it. Compare these level 3 sets.
4.2 Nomenclature of Alkenes

26. Sequence Rules
3a. If no decision at step 3, choose the atoms of next highest priority in the level 2 set and repeat the process in step 3.
4.2 Nomenclature of Alkenes

27. Sequence Rules
3b. If two or more atoms in any set are the same, decide on their relative priorities by continuing outward from each. 3c. If no decision is possible, move away from the double bond within each group to atoms at the next level and repeat step 3.
4.2 Nomenclature of Alkenes

28. Physical Properties (won’t be tested)
Very similar to corresponding alkanes
Dipole moments of some alkenes, though small, are greater than those of the corresponding alkanes.
4.4 Physical Properties of Alkenes

29. Relative Stabilities of Alkene Isomers
More alkyl substituents → more stable alkene
trans places larger groups farther apart.
Could be obtained from heats of formation (DHf ˚), as in the text, but can also use heats of combustion or (just for alkenes) heat of hydrogenation.
4.5 Relative Stabilities of Alkene Isomers

30. Relative Stabilities more stable = lower potential energy
Standard enthalpy change (DH˚) for a reaction is given by
I’ll derive relative stabilities of alkene isomers from this relationship, a different manner than used in the text.
Exothermic reactions: DH˚ < 0. If two reactants produce the same product, the difference in heat released reflects the relative Ho’s of the reactants.
Endothermic reactions: DH˚ > 0 – not spontaneous.
4.5 Relative Stabilities of Alkene Isomers

31. The general method is illustrated by isomeric alkane combustion

32. Heats of combustion, some examples –
n-C8H O2 → 8CO H2O kcal/mol
n-C9H O2 → 9CO H2O kcal/mol
D = kcal/CH2
Cyclo-alkanes are nothing but (CH2)n, how do they compare ?

33. What makes a reaction go?
Producing more stable materials more stable = lower energy
Enthalpy changes (DH˚) for a reaction are one way to track this
Favorable reactions are Exothermic: DH˚ < 0
and A mechanism with an achievable transition state barrier must exist !
In the case of hydrogenation a catalyst is required
4.5 Relative Stabilities of Alkene Isomers

34. Catalytic hydrogenation (adding H2 to an alkene) is exothermic
Catalytic hydrogenation (adding H2 to an alkene) is exothermic. If two alkenes afford the same alkane, differences in the heat of reaction are due to differences in the potential energies of the alkenes.
Shown diagram here

35. RE: Stabilities of Alkene Isomers
More alkyl substituents → more stable alkene
trans places larger groups farther apart.
4.5 Relative Stabilities of Alkene Isomers

36. Addition Reactions A characteristic reaction of alkenes
The p bond is electron rich = nucleophilic
HF, HCl, HBr, HI add to alkenes → alkyl halides
4.6 Addition Reactions of Alkenes

37. Regioselectivity of Addition
Unsymmetrical alkenes can give regioisomers.
Generally, the halogen goes to the most substituted carbon and H goes to the carbon with the most H.
4.7 Addition of Hydrogen Halides to Alkenes

38. Carbocation Intermediates
Addition occurs in two successive steps:
1. e- pair in alkene is donated to the proton in H-X. The resulting species is called a carbocation (carbenium ion).
Carbocations are reactive intermediates
4.7 Addition of Hydrogen Halides to Alkenes

39. Carbocation Intermediates
Addition occurs in two successive steps:
2. The halide ion (X-) is a nucleophile and donates a lone pair of electrons to the electron deficient carbocation forming a new bond.
The complete description of a reaction pathway, including any reactive intermediates, is called the mechanism of the reaction.
4.7 Addition of Hydrogen Halides to Alkenes

40. Back to the Regioselectivity
The reaction mechanism can be used to explain regioselectivity.
The tert-butyl cation is the only one that is formed, thus it leads to t-butylbromide as the only reaction product.
4.7 Addition of Hydrogen Halides to Alkenes

41. Structure and Stability of Carbenium Ions
Carbocations are trigonal planar → sp2-hybridized.
Electron deficient = Lewis acidic
Stabilized by alkyl groups
Stability: primary < secondary < tertiary
4.7 Addition of Hydrogen Halides to Alkenes

42. Hyperconjugation
Hyperconjugation is an additional factor in stabilization for a carbocation (carbenium ion).
The overlap of bonding electrons from the adjacent s- bond with the unoccupied p-orbital of the carbocation.
4.7 Addition of Hydrogen Halides to Alkenes

43. Hyperconjugation-2 (low emphasis)
Hyperconjugation is sometimes shown with “no- bond” resonance structures. I am not a fan of such depictions.
4.7 Addition of Hydrogen Halides to Alkenes

44. Carbocation Rearrangements
Can be identified by a change in carbon skeleton
Rearrangement is favored if a more stable carbocation can form
4.7 Addition of Hydrogen Halides to Alkenes

45. Another example

46. How a shift actually happens

47. Carbocation Rearrangements
Carbocation rearrangements take place in order to form a more stable carbenium ion.
4.7 Addition of Hydrogen Halides to Alkenes

48. Carbocation Rearrangements
Carbocation rearrangements are not limited to alkyl shifts.
Hydride shift - a hydrogen atom along with its two bonding electrons migrates.
4.7 Addition of Hydrogen Halides to Alkenes

49. RE: What makes a reaction go?
Producing more stable materials more stable = lower energy
Enthalpy changes (DH˚) for a reaction are one way to track this
Favorable reactions are Exothermic: DH˚ < 0
Free energies (DG˚) are a better measure.
and A mechanism with an achievable transition state barrier (DGǂ) must exist !
4.5 Relative Stabilities of Alkene Isomers

50. Relative Reaction Rates
The reactions below have the same DG˚ between reactants and products but occur at very different rates. The same considerations apply when a single reaction can yield alternative products: two reactions are in competition.
4.8 Reaction Rates

51. Reaction Rates
Two factors govern intrinsic reaction rates and the outcome when competing reactions are possible.
The sizes of the energy barriers.
The temperature: reactions are faster at higher temperatures.
As a result many reaction are more selective at lower temperatures.
4.8 Reaction Rates

52. Molecular Energy Distributions
Molecules must possess enough energy to get to the transition state.
Maxwell-Boltzmann distribution – characterizes the energy of a collection of molecules.
4.8 Reaction Rates

53. What is the “transition state”?
The transition state is the structural and dynamic features present at the energy barrier to the interconversion between reactants and products.
The forward and reverse of any single step of a reaction have same transition state (‡).
But in the case of HX addition to an alkene, this free energy course diagram doesn’t apply.
The “product” of going over the energy barrier is not the product that isolated; rather, it’s carbocation intermediate that lies at higher energy than the reactants.
4.8 Reaction Rates

54. What is the “transition state”?
The transition state represents the structure present at the highest energy point along a free energy course diagram. For
it would be something like
4.8 Reaction Rates

55. Hydration of Alkenes Hydration = the addition of H2O to a molecule.
Alkene hydration is acid-catalyzed (shown as 1M HONO2 in this case, really H3O+).
H-OH is added across the alkene.
4.9 Catalysis

56. End of coverage for MQ1

57. Multistep Reactions
Multistep reactions take place with the formation of reactive intermediates.
Rate-determining step: The slowest step in a chemical reaction; controls the reaction rate.
4.8 Reaction Rates

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

59. Hammond’s Postulate
Two possible modes of HBr addition to 2- methylpropene:
4.8 Reaction Rates

60. RE: What makes a reaction go?
Producing more stable materials more stable = lower energy
Enthalpy changes (DH˚) for a reaction are one way to track this
Favorable reactions are Exothermic: DH˚ < 0
Free energies (DG˚) are a better measure.
and A mechanism with an achievable transition state barrier (DGǂ) must exist ! In some cases “catalysts” are required.
4.5 Relative Stabilities of Alkene Isomers

61. Catalysis
Catalyst: A substance that increases the reaction rate without being consumed.
Catalyzed vs un-catalyzed reaction:
4.9 Catalysis

62. Catalysis
A catalyst increases the reaction rate. This means that it lowers the standard free energy of activation for a reaction.
A catalyst is not consumed. It may be consumed in one step of a catalyzed reaction, but if so, it is regenerated in a subsequent step.
A catalyst does not affect the energies of reactants and products.
A catalyst accelerates both the forward and reverse reactions by the same factor.
4.9 Catalysis

63. Hydration of Alkenes Hydration = the addition of H2O to a molecule.
Alkene hydration is Acid-catalyzed. (Homogeneous)
H-OH is added across the alkene.
4.9 Catalysis

64. Hydration of Alkenes Mechanism
Protonation of the double bond to give the more stable carbocation.
H3O+ is acting as the acid catalyst.
4.9 Catalysis

65. Hydration of Alkenes Mechanism
Water, the nucleophile, combines with the carbocation in a Lewis Acid-Base association reaction.
4.9 Catalysis

66. Hydration of Alkenes Mechanism
A proton is lost to solvent in another acid-base reaction to give the alcohol.
The acid catalyst (H3O+) is regenerated.
4.9 Catalysis

67. Acid catalyzed interconversion of alkene isomers
cis- and trans-2-butene can be equilibrated by adding a few drop of a strong acid.
1-butene can be converted to the equilibrium mixture of cis- and trans-2-butene by adding a catalytic amount of a proton donor.

68. Catalysis
Heterogeneous Catalyst: the catalyst and the reactants exist in separate phases.
Homogeneous Catalyst: a catalyst that is soluble in the reaction mixture.
4.9 Catalysis

69. Catalytic Hydrogenation of Alkenes
Metal catalyst allows for addition of H2.
Heterogeneous
Benzene  bonds are less reactive.
4.9 Catalysis

70. Principle of Microscopic Reversibility
If a reaction occurs by a certain mechanism, the reverse reaction under the same conditions occurs by the same exact reverse of that mechanism.
Dehydration is the reverse of hydration.
4.9 Catalysis

71. Enzymes Catalysis Biological catalysts are called enzymes.
Most biological mechanisms would be too slow to be useful without enzymes.
4.9 Catalysis