1. Essential Organic Chemistry
Paula Yurkanis Bruice
Chapter 4
Alkenes: Structure, Nomenclature, Stability, and an Introduction to Reactivity

2. 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

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

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

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

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

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

8. 4.1 Molecular Formulas Alkane: CnH2n+2 Alkene: CnH2n or CnH2n+2- 2P
P = number of double bonds
+ 2H

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

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

12. 4.2 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.

13. 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.

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

15. 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.

16. 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.

17. 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.

18. 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]

19. Nomenclature of Alkenes

21. 4.3 The Structure of Alkenes
All six atoms of the double bond system are in the same plane.

23. 4.4 Cis-Trans Isomerism
Because rotation about a double bond does not readily occur, an alkene such as 2-butene can exist in two distinct forms.

24. cis/trans Isomers sp2-sp2  bond p-p  bond side view p-p  bond
front view
The p-p  bond restricts free rotation.

25. cis/trans Isomers Upon rotation we lose p-p overlap, thus
rotation doesn’t happen (easily).
Consequently, geometrical isomers exist.

26. cis/trans Isomers cis trans All substituents are All substituents are
on one side
of  bond
All substituents are
on different sides
of  bond

27. Figure: UN
Title:
Diagrams of cis- and trans-2-Butene
Caption:
The electrostatic potential maps, ball-and-stick models, and structures of the two isomers of 2-butene. These isomers are called geometric isomers. They have the same molecular geometry; they differ in their relative arrangement in space.
Notes:
The cis isomer is the one where the same substituents are on the same side of the plane or the carbon-carbon double bond. The trans isomer is where they are on opposite sides.

28. Figure: UN
Title:
Cis-Trans Isomers
Caption:
The different isomers are not possible for these two examples since they have the same substituent on the same carbon.
Notes:
The criterion is to have the same substituent on adjacent carbons, not on the same carbon.

29. cis/trans Isomers 3 2 5 4 1
cis-2-pentene 3 2 3 5 5 2 4 4 1 1

30. cis/trans Isomers 1 2 3 4 5 6 7
trans-3-heptene 2 6 4 1 2 3 6 4 7 5 3 1 7 5

32. 4.5 The E,Z System of Nomenclature
For more than two substituents the cis/trans
system cannot be used.
A new system, the E/Z system is introduced.
To use the E/Z system we need to assign
priorities to each substituent on each carbon.

33. E/Z System In case high priorities are on the same side,
we assign a
Z configuration.
In case high priorities are on opposite sides,
we assign an
E configuration.

34. E/Z System- Rule 1
The relative priorities of the two groups depend on the atomic numbers of the atoms bonded directly to the sp2 carbon. The greater the atomic number, the higher is the priority of the group.

35. E/Z System Priorities are first assigned based on atomic numbers.1 2
F > H
E-configuration
I > C 1 2
F > H
Z-configuration

36. E/Z System- Rule 2
If the two substituents attached to the sp2 carbon start with the same atom, you must move outward and consider the atomic numbers that are attached to the “tied” atoms.

37. E/Z System
If you can’t decide using the first atoms attached, go out to the next atoms attached. If there are nonequivalent paths, always follow the path with atoms of higher atomic number.
path goes to O,
not H 1 2 C H O
comparison
stops here
path goes to C,
not H
Z-configuration

38. E/Z System- Rule 3
If an atom is doubly (or triply) bonded to another atom, the priority system treats it as if it were singly bonded to two (or three) of those atoms.

39. E/Z System
path goes to C,
not H
Atoms in double bonds are “replicated” at either end of the double bond. 1 2
E-configuration

43. 4.6 The 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.

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

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

48. 4.7 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!

49. 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 Nuclophile reacts with an electrophile!

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

52. Reactions Alkenes are similar in structure and do similar reactions.
All contain a double bond
All contain the same functional group
Reactions are categorized through different types of mechanisms.

53. Reactions Typical for unsaturated systems
is the addition reaction: A+B  C

54. Reactions
A LOOK AT THE
REACTANTS

55. Reactions
WHAT IS THE NATURE
OF THIS REAGENT?

56. Reactions Hydrogen bromide is a strong acid and forms
electrophile
Hydrogen bromide is a strong acid and forms
hydronium ions, H3O+, and bromide, Br–, when
dissolved in water.
H3O+ is positively charged,
thus it is electron deficient
it is electrophilic “electron loving”

57. Reactions In the presence of an electron-rich species
the hydronium ion reacts:
electrophile
A new positively charged species is formed.

58. Reactions The newly formed species, a carbocation, is
again electron deficient, thus it is electrophilic.
electrophile

59. Reactions One species present that is rich in electrons is Br–.
Since Br– bears a negative charge it seeks for
neutralization.
It is nucleophilic (nuclei are positively charged).

60. Reactions The two species, electrophile and nucleophile, combine
and form a new compound.
electrophile
nucleophile

61. Mechanism Summarizing our reaction, we realize
it is a 2-step mechanism
STEP 1
STEP 2

62. Figure: UN
Title:
Electrophilic Addition of HBr to 2-Butene
Caption:
The two-step reaction of the addition of HBr to 2-butene. In the first step a carbocation is formed, followed by the second step, which is the addition of the bromide ion.
Notes:
The alkene acts like a nucleophile and takes the hydrogen from HBr. The bromide ion that forms can act like a nucleophile and add to the carbocation that formed.

63. Mechanism Step 1 reaches a carbocation “intermediate.”
One new bond is formed.
Intermediates are species with a very short lifetime.
However, their stability (energy) often determines
the outcome of a reaction.
Step 2 completes the reaction by forming a second
bond. Again, it is the interplay between positively charged (electrophilic) and negatively charged (nucleophilic) species.

64. A Few Words about Curved Arrows
Figure: UN
Title:
Curved Arrows in Mechanisms
Caption:
Make certain that the curved arrows are drawn in the direction of the electron flow and never against the electron flow.
Notes:
This means that the arrow is always drawn away from a negative charge and towards a positive charge.

66. 4.8 Using a Reaction Coordinate Diagram (Energy Profile) to Describe a Reaction

67. Transition State

68. 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

69. Reactions
Overall reaction coordinate