1. University of California,
Chapter 5
Alkenes Structure, Nomenclature, and an introduction to Reactivity • Thermodynamics and Kinetics
Paula Yurkanis Bruice
University of California,
Santa Barbara

2. Saturated and Unsaturated Hydrocarbons
Saturated hydrocarbons have no double bonds.
Unsaturated hydrocarbons have one or more double bonds.

3. C=C double bond consists of
Introduction
Alkenes contain a C=C double bond
C=C double bond consists of
p-p  bond
sp2-sp2  bond

4. Introduction compared to alkanes, bond lengths decrease in alkenes
compared to alkanes, bond angles increase in alkenes

5. Alkenes

6. Introduction Typical representatives are
Ethene, plant growth hormone, affects seed germination, flower maturation, and fruit ripening.

7. Introduction Typical representatives are
citronellol, in rose and geranium oils O H 4 3 2 5 1 6 7 8 7 4 8 5
Geranium “Mavis Simpson” 6 1 3 2

8. Introduction Typical representatives are
limonene, in lemon and orange oils 1 6 2 6 1 2 5 5 3 4 3 4
Citrus limon

9. Introduction Typical representatives are
-phellandrene, in oil of eucalyptus 1 1 2 6 2 6 5 3 5 3 4 4
Eucalyptus globulus

10. Molecular Formulas Alkane: CnH2n+2 Alkene: CnH2n or CnH2n+2- 2P
P = number of double bonds
+ 2H

11. Molecular Formulas Alkane: CnH2n+2 Ring: CnH2n or CnH2n+2- 2R
R = number of rings
+ 2H

12. CnH2n+2- 2P-2R Molecular Formulas Alkene: P = number of double bonds
R = number of rings.

13. Nomenclature of Alkenes
The functional group is the center of reactivity in a molecule.
The IUPAC system uses a suffix to denote certain functional groups.

14. Nomenclature of Alkenes
1-1. Find the longest carbon chain.
1-2. Enumerate the carbons such that the functional group, here the double bond, gets the lowest possible number.

15. Stereoisomers of an Alkene are named using a cis or trans Prefix

16. Nomenclature of Alkenes
2. Substituents are cited before the parent longest chain, along with a number indicating its position at the chain.

17. Nomenclature of Alkenes
3. If a chain has more than one double bond, we first identify the chain by its alkane name, replacing the “ne” ending with the appropritate suffix: diene, triene, etc.
4. If a chain has more than one substitutent, substituents are cited in alphabetical order.

18. Nomenclature of Dienes
two double bonds = diene

19. Nomenclature of Alkenes
5. If the same number for alkene is obtained in both directions, the correct name is the name that contains the lowest substituent number.

20. Nomenclature of Alkenes
6. A number is not needed to denote the position of the double bond in a cyclic alkene because the double bond is always placed between carbons 1 and 2.
7. Numbers are needed if the ring has more than one double bond.

21. Nomenclature of Alkenes
Remember that the name of a substituent is stated before the name of the parent hydrocarbon, and the functional group suffix is stated after that.
[substitutent] [parent hydrocarbon] [fucntional group suffix]

22. Nomenclature of Alkenes

23. Nomenclature of Cyclic Alkenes
A number is not needed to denote the position of the functional group; it is always between C1 and C2.

24. Vinylic and Allylic Carbons
vinylic carbon: the sp2 carbon of an alkene
allylic carbon: a carbon adjacent to a vinylic carbon

25. Reactions of Organic Compounds
Organic compounds can be divided into families.
All members of a family react in the same way. The family a compound belongs to depends on its functional group. 25

26. Each family can be put in one of four Groups
The families in a group react in similar ways.

27. How Alkenes React ; Curved Arrows
The functional group is the center of reactivity of a molecule.
In essence, organic chemistry is about the interaction between electron-rich atoms or molecules and electron-deficient atoms or molecules. It is these forces of attraction that make chemical reactions happen.
A very simple rule:
Electron-rich atoms or molecules are attracted to electron-deficient atoms or molecules!

28. Electrophiles vs Nucleophiles
Electrophile: electron-deficient atom or molecule that can accept a pair of electrons.
Nucleophile: electron-rich atom or molecule that has a pair of electrons to share.
A very simple rule restated:
A nucleophile reacts with an electrophile!

29. Electrophiles vs Nucleophiles
Organic molecules with double bonds
(alkenes, alkynes) are also nucleophilic.
Examples:

30. Electrophiles

31. Nucleophiles A nucleophile has a negative charge a lone pair
a double bond

32. A Nucleophile reacts with an Electrophile
Your first organic reaction is a two-step reaction.

33. Curved Arrows first step of the reaction
Curved arrows are used to show the mechanism of a reaction.
The mechanism of a reaction is the step-by-step description of the process by which reactants are converted into products.

34. Curved Arrows second step of the reaction
The curved arrow shows where the electrons start from
and where they end up.

35. How to draw Curved Arrows

36. How to draw Curved Arrows

37. How to draw Curved Arrows

38. How to draw Curved Arrows

39. A Reaction Coordinate Diagram
A reaction coordinate diagram shows the energy changes that take place in each step of a reaction.

40. Thermodynamics and Kinetics

41. The Equilibrium Constant
The equilibrium constant gives the concentration of reactants and products at equilibrium.

42. Exergonic and Endergonic Reactions

43. Gibbs Free-Energy Change (∆G°)
ΔG° = free energy of the products –
free energy of the reactants
ΔH° = heat required to break bones –
heat released from forming bonds
ΔS° = freedom of motion of the products –
freedom of motion of the reactants

44. A Reduction Reaction

45. Catalytic Hydrogenation

46. Using ∆H° Values to determine the Relative Stabilities of Alkenes

47. Using ∆H° Values to determine the Relative Stabilities of Alkenes
The relative energies (stabilities) of three alkenes that can be catalytically hydrogenated to 2-methylbutane. The most stable alkene has the smallest heat of hydrogenation. (Notice that when a reaction coordinate diagram shows ∆H° values, the y-axis is potential energy; when it shows ∆G° values, the y-axis is free energy [Figure 5.2].)

48. Relative Stabilities of Alkenes
Alkyl substituents that are bonded to the sp2 carbons of an alkene have a stabilizing effect on the alkene.
The more alkyl substituents bonded to the sp2 carbons of an alkene, the greater is its stability.

49. Stability The more substituents, the more stable
The stability of alkenes depends
upon number of substituents
The more substituents, the more stable

50. Relative Stabilities of Cis and Trans Alkenes

51. Relative Stabilities of Dialkyl-Substituted Alkenes

52. Stability
Steric repulsion (Steric strain) is responsible for energy differences among the disubstituted alkenes

53. Kinetics: How fast is the product formed?
∆G‡ = free energy of the transition state - free energy of the reactants
The greater the energy barrier, the slower the reaction.

54. Rate of a Chemical Reaction
Increasing the concentration increases the rate.
Increasing the temperature increases the rate.
The rate can also be increased by a catalyst.

55. An Electrophilic Addition Reaction
Transition states have partially formed bonds.

56. Reaction Coordinate Diagram for each step
of the addition of HBr to 2-Butene

57. Reaction Coordinate Diagram for the addition of HBr to 2-Butene
The rate-limiting step of the reaction is the step that has its transition state at the highest point of the reaction coordinate diagram.

58. Transition State

59. Transition state TS 1 The chemical species that exists
bond breaking
bond
forming
TS 1
The chemical species that exists
at the transition state, with old
bonds in the process of breaking
and new bonds in the process of
forming:
bond
forming
TS 2

60. A Catalyst
A catalyst provides a pathway for a reaction with a lower energy barrier.
A catalyst does not change the energy of the starting point (the reactants) or the energy of the end point (the products).

61. Enzymes Most biological reactions require a catalyst.
Most biological catalysts are proteins called enzymes.
The reactant of a biological reaction is called a substrate.

62. The Active Site of an Enzyme
An enzyme binds its substrate at its active site.

63. Enzyme Side Chains that
bind the Substrate
Some enzyme side chains bind the substrate.

64. Enzyme Side Chains that
catalyze the Reaction
Some enzyme side chains are acids, bases, and
nucleophiles that catalyze the reaction.