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. Contents of Chapter 5 General Formulae and Nomenclature of Alkenes
Reactivity Considerations
Curved Arrow Mechanisms
Thermodynamics and Kinetics
Chapter 4

3. General Molecular Formula for Alkenes
General molecular formula for acyclic alkanes is CnH2n+2
CH3CH2CH2CH2CH3
an alkane
C5H12
CnH2n+2
Chapter 4

4. General Molecular Formula for Alkenes
Each  bond introduced, reduces the H content by 2
CH3CH2CH2CH=CH2
an alkene
C5H10
CnH2n
Chapter 4

5. General Molecular Formula for Alkenes
Each ring also reduces the H content by 2
a cyclic alkane
C5H10
CnH2n
Chapter 4

6. General Molecular Formula for Alkenes
Generalization: The molecular formula for a hydrocarbon is CnH2n+2 minus 2 hydrogens for every  bond and/or ring present in the molecule
Each  bond or ring is considered a unit of unsaturation.
C5H8
CnH2n-2
a cyclic alkene with 2 units of unsaturation
Chapter 4

7. Saturated and Unsaturated Hydrocarbons
Alkanes or saturated hydrocarbons contain the maximum number of carbon-hydrogen bonds
CH3CH2CH2CH2CH3
a saturated hydrocarbon
Chapter 4

8. Saturated and Unsaturated Hydrocarbons
Alkenes contain fewer than the maximum number of carbon-hydrogen bonds and are therefore referred to as unsaturated hydrocarbons
CH3CH2CH2CH=CH2
an unsaturated hydrocarbon
Chapter 4

9. Nomenclature of Alkenes
IUPAC names of alkenes are based on the corresponding alkane with “ane” replaced by “ene”
Chapter 4

10. IUPAC Rules for Alkene Nomenclature
The longest chain containing the functional group (the double bond) is numbered such that the double bond is the lowest possible number
Chapter 4

11. IUPAC Rules for Alkene Nomenclature
If there are substituents, the chain is still numbered in a direction that gives the double bond the lowest number
Chapter 4

12. IUPAC Rules for Alkene Nomenclature
If chain has more than one substituent, they are cited in alphabetical (not numerical) order.
Rules for alphabetizing are the same as for alkanes
Chapter 4

13. IUPAC Rules for Alkene Nomenclature
If the same number for the double bond is obtained in both directions, number in the direction that gives lowest number to a substituent.
Chapter 4

14. IUPAC Rules for Alkene Nomenclature
In cyclic compounds, a number is not needed to denote the position of the functional group
The double bond is assumed to be between carbons 1 and 2
Chapter 4

15. IUPAC Rules for Alkene Nomenclature
If both directions yield same low number for a functional group and for one substituent, number in the direction which yields the lower number for one of the remaining substituents
Chapter 4

16. IUPAC Rules for Alkene Nomenclature
Two groups containing double bonds that are used as names for substituents are the vinyl group and the allyl group
Chapter 4

17. IUPAC Rules for Alkene Nomenclature
The sp2 carbons of an alkene are called vinylic
An sp3 adjacent carbon is called allylic
Chapter 4

18. IUPAC Nomenclature of Dienes
Find the longest chain containing both double bonds 1 2 3 4 5
3-butyl-1,4-pentadiene
Chapter 4

19. IUPAC Nomenclature of Dienes
Use corresponding alkane name but replace the “ne” ending with “diene”
3-butyl-1,4-pentadiene
“pentane” changed to “pentadiene”
Chapter 4

20. IUPAC Nomenclature of Dienes
Number in the direction that gives the lowest number to a double bond
1,5-heptadiene
not 2,6-heptadiene
Chapter 4

21. IUPAC Nomenclature of Dienes
List substituents in alphabetical order
5-ethyl-2-methyl-2,4-heptadiene
Chapter 4

22. IUPAC Nomenclature of Dienes
Place numbers indicating the double bond positions either in front of the parent compound or in the middle of the name immediately before the diene suffix
5-ethyl-2-methyl-2,4-heptadiene
or 5-ethyl-2-methyl-hepta-2,4-diene
Chapter 4

23. The E, Z System of Nomenclature
Left: Z-1-bromo-2-chloropropene
Right: E-1-bromo-2-chloropropene
Chapter 4

24. Relative Stabilities of Alkenes
Chapter 4

25. Relative Stabilities of Alkenes
The more alkyl substituents attached to a double bond the more stable the double bond.
Trans alkenes more stable than cis alkenes
Not difficult concepts but should be learned now in order to understand Chapter 9 later.
Chapter 4

26. Reactivity Considerations
Electrophiles react with nucleophiles
An alkene has electron density above and below the  bond making it electron-rich and therefore a nucleophile
Therefore alkenes react with electrophiles
Chapter 4

27. Reaction Mechanisms
We use curved arrows to indicate the movement of pairs of electrons as two molecules, ions or atoms interact
Chapter 4

28. Reaction Mechanisms
Curved arrows are drawn only from the electron-rich site to the electron deficient site
Chapter 4

29. Thermodynamics When G° is negative the reaction is exergonic
Chapter 4

30. Thermodynamics When G° is positive the reaction is endergonic
Chapter 4

31. Kinetics
Knowing the G° of a reaction will not tell us how fast it will occur or if it will occur at all
We need to know the rate of reaction
The rate of a reaction is related to the height of the energy barrier for the reaction, G‡, the free energy of activation
Chapter 4

32. Free Energy of Activation
Chapter 4

33. Rate-Determining Step
Formation of the carbocation intermediate is the slower of the two steps
It is the rate-determining step
Chapter 4

34. Rate-Determining Step
Carbocation intermediates are consumed by bromide ions as fast as they are formed
The rate of the overall reaction is determined by the slow first step
Chapter 4

35. Transition States and Intermediates
It is important to distinguish between a transition state and a reaction intermediate
A transition state
is a local maximum in the reaction coordinate diagram
has partially formed and partially broken bonds
has only fleeting existence
Chapter 4

36. Transition States and Intermediates
An intermediate
is at a local minimum energy in the reaction coordinate diagram
may be isolated in some cases
Chapter 4