1. 3. Alkenes: The Nature of Organic Reactions

2. Alkene - Hydrocarbon With Carbon-Carbon Double Bond
Also called an olefin but alkene is better
The characteristic feature of alkene structure is the carbon- carbon double bond. The characteristic reactions of alkenes are those that take place at the pi portion of the double bonds

3. Naming of Alkenes
Find longest continuous carbon chain that contains the double bond for parent name
Number carbons in chain so that double bond carbons have lowest possible numbers
Rings have “cyclo” prefix; the two doubly bonded carbons are C1 and C2; and the numbering directions is such that the first group has the smallest number

4. Many Alkenes Are Known by Common Names
Ethylene = ethene
Propylene = propene
Isobutylene = 2-methylpropene

5. Electronic Structure of Alkenes
. Electronic structure of alkenes: Cis - Trans Isomerism
The double bond of an alkene consists of a strong sigma bond and a weaker pi bond. Info leading to this statement was obtained through a study of the energy requirements for bond breakage.
Ethene C C bond strength (sigma +pi) 152 kcal/mole
Ethane C C bond strength kcal/mole
Subtracting these two values give a rough estimate of the amount of energy required to break the pi bond of an alkene (its bond strength).
152 kcal/mole
kcal/mole
Difference (pi) 64 kcal/mole

6. The Presence of the Double Bond in Alkenes has Numerous Consequences
One of these is the phenomenon of "restricted rotation". There is no rotation about the carbon-carbon double bond (CCDB). If we were to force rotation we would have to break the pi bond temporarily. This lack of rotation about the CCDB introduces the possibility of cis - trans isomerism

7. Cis-Trans Isomerism in Alkenes
The presence of a carbon-carbon double can create two possible structures
cis isomer - two similar groups on same side of the double bond
trans isomer similar groups on opposite sides
Each carbon must have two different groups for cis-trans isomerism to exist

8. Cis, Trans Isomers Require That The Two Groups Bonded to Each Doubly Bonded Carbon Be Different
Cis-Trans does not pertain to top pair because one of the DB Carbons is bonded to the same two groups. So, if you switched A & B you would get the same molecule.
Cis-Trans does pertain to bottom pair. Switching A with B produces a different molecule. X

9. Alkene Stability
Cis alkenes are less stable than trans alkenes because of steric crowding
Relative stabilities of various alkenes can be determined by comparing heats given off on hydrogenation to alkanes
The less stable isomer is higher in energy to begin with and therefore it gives off more heat upon being hydrogenated to the alkane
Similar 'heats of hydrogenation' studies for various alkenes have lead to the following general statement about alkene stability:
tetrasubstituted > trisubstituted > disubstituted > monosusbtituted

10. Comparing Stabilities of Alkenes
Evaluate heat given off when C=C is converted to C-C
The more stable alkene gives off less heat
Trans-2- butene generates 5 kJ less heat than cis-2-butene C 3 C 3
Energy H2 H2

11. Sequence Rules: The E,Z Designation
Neither compound is clearly “cis” or “trans”
Cis, trans nomenclature only works for disubstituted double bonds
These are tri-substituted double bonds and a new system of designation is needed. Let’s hope its E,Z.

12. Develop a System for Comparison of Priority of Substituents
Assume a valuation system
If Br has a higher “value” than Cl
If CH3 is higher than H
Then, in A, the higher value groups are on opposite sides
In B, they are on the same side
Requires a universally accepted “valuation”

13. E,Z Stereochemical Nomenclature
Priority rules of Cahn, Ingold, and Prelog
Compare where higher priority group is with respect to bond and designate as prefix
E -entgegen, opposite sides
Z - zusammen, together on the same side

14. Ranking Priorities: Cahn-Ingold-Prelog Rules
Must rank atoms that are connected at the same comparison points
Higher atomic number gets higher priority
Br > Cl > O > N > C > H
In this case,The higher priority groups are opposite:
(E )-1-bromo-1-chloro-propene

15. Extended Comparison
If atomic numbers are the same at the same point of comparison, then compare at next and then if necessary at the next connection point until a difference is determined.
Compare until something has higher atomic number

16. Dealing With Multiple Bonds
A multiple bond is equivalent to the same number of single bonds

17. Organic Reactions Can Be Categorized On Two Levels
Kinds of Reactions – this is more of a surface level of categorizing org. rxns.; it looks only at the Reactants and the final Products
How the Reactions Occur – this is a deeper level of categorizing org. rxns.; it looks at the individual steps that transform the Reactants into Products, the Reaction Mechanism

18. Kinds of Organic Reactions
Addition reactions – two molecules combine to form one
A + B
Elimination reactions – one molecule splits into two A
Substitution – two molecules exchange partners
AB + CD AD + CB
Rearrangement –a molecule undergoes a reorganization of bonds and atoms to form an isomeric product
A Z C
B + C

19. How Organic Reactions Occur: Mechanisms
In a clock the hands move but the mechanism behind the face is what causes the movement
In the overall organic reaction, we see the reactants and the final products. The mechanism describes the steps behind these changes
Reactions occur in defined steps that lead from reactant to product

20. Steps in Mechanisms
A step in a reaction mechanism involves either the formation or breaking of a covalent bond

21. Different Ways of Breaking and Making a Covalent Bond
Breaking of a covalent bond
Homolytic or heterolytic
Formation of a covalent bond
Homogenic or heterogenic

22. Heterogenic Formation of a Bond
One fragment supplies two electrons
One fragment supplies no electrons
Usually involves oppositely charged ions
Common in organic chemistry

23. Homogenic Formation of a Bond
One electron comes from each fragment
No electronic charges are involved
Not common in organic chemistry

24. Homolytic Breaking of Covalent Bonds
Each product gets one electron from the bond
Not common in organic chemistry

25. Heterolytic Breaking of Covalent Bonds
Both electrons from the bond that is broken become associated with one resulting fragment
A common pattern in reaction mechanisms

26. Method of Indicating the Movement of Electrons in the Breaking & Forming of Bonds
Curved arrows indicate breaking and forming of bonds
Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’) The “fish-hook” indicates the movement of only one electron
Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’) The arrowhead indicates the movement of two electrons

27. Types of Reaction Mechanisms
Most organic reactions can be categorized in terms of two reaction mechanisms
Radical Reactions – the individual steps of this mechanism involve homolytic bond cleavage and homogenic bond formation
Polar Reactions – this is the most commonly occurring organic reaction. These reactions occur between two polar molecules or between a polar molecule and a charged polyatomic ion. The individual steps of this mechanism involve heterolytic bond cleavage and heterogenic bond formation.

28. Radical Reactions and How They Occur
Radicals are highly reactive chemical species that have an unpaired electron (odd # of electrons). These reactive species need to stabilize themselves by acquiring an extra electron from somewhere. There are two ways that a radical can do this:

29. Radical Stabilization
A radical can abstract an atom from another molecule, giving substitution in the original molecule and generating another radical in the process

30. Radical Stabilization
2. One radical can combine directly with another radical

31. Example Radical Reaction: The Radical Chlorination of Methane
The overall reaction is pictured below. What kind of reaction is this?
Substitution

32. The Mechanism for the Radical Chlorination of Methane
Initiation:
Propagation:
Termination:

33. Polar Reactions and How They Occur
These reactions occur between two polar molecules or between a polar molecule and a charged polyatomic ion
Polar organic molecules result from polar bonds within functional groups that are part of the organic molecule
Polar bonds result whenever two covalently bonded atoms have substantially different electornegativity values

34. Electronegativity of Some Common Elements
The relative electronegativity is indicated
Higher numbers indicate greater electronegativity
Carbon bonded to a more electronegative element has a partial positive charge (+)

36. The Keys to Understanding All Polar Reactions
Opposite charges attract
There should be 4 bonds to every carbon in a neutral molecule

37. The Fundamental Characteristics of all Polar Reactions are:
An electron rich reagent (nucleophile) donates a pair of e-'s to the electron poor reagent (electrophile) and subsequently forms a new bond.
Simultaneous to the formation of this new bond is the rupture of an old bond. The leaving group departs the molecule with both e-'s of the old bond.

38. An Example of a Polar Reaction: Addition of HBr to Ethylene
HBr adds to the  part of C-C double bond
The  bond is electron-rich, allowing it to function as a nucleophile
H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile S-

39. Using Curved Arrows in Polar Reaction Mechanisms
Curved arrows are a way to keep track of changes of bonds in a polar reaction
The arrows track “electron movement”
In polar reactions electrons always move in pairs
Charges change during the reaction

40. Example Polar Reaction

41. Mechanism of Addition of HBr to Ethylene
HBr electrophile is attacked by  electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide anion
Bromide anion adds to the positive center of the carbocation, which is an electrophile, forming a C-Br  bond
The result is that ethylene and HBr combine to form bromoethane
All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

42. Rules for Using Curved Arrows
The arrow goes from the nucleophilic reaction site to the electrophilic reaction site
The nucleophilic site can be neutral or negatively charged
The electrophilic site can be neutral or positively charged

43. Just as the chemistry of alkenes is dominated by addition reactions, the prep of alkenes is dominated by elimination reactions. Additions and eliminations are two sides of the same coin.

44. 7.1 Preparation of Alkenes: A Preview of Elimination Reactions: A B + C
Alkenes are commonly made by
elimination of H,X from alkyl halide (dehydrohalogenation)
Uses heat and KOH
elimination of H,OH from an alcohol (dehydration)
require strong acids (sulfuric acid, 50 ºC) H

45. Elimination of H-X from an Alkyl Halide to produce an Alkene
The Mechanism for this reaction involves the base pulling off a proton (H+) from a carbon adjacent to the halogen bearing carbon. When using KOH then the base is OH-, The electron pair that is left behind after the proton leaves, becomes the new alkene pi bond and the halogen departs as a halide anion (:X-) with the two electrons from the C-X bond. 
Transition State Species
OH- K+ OH -
HO-H K+

46. Zaitsev’s Rule for Elimination Reactions
Elimination reactions almost always give a mixture of alkene products as the 1st step in the mechanism involves the removal of a proton from a carbon adjacent to the halogen bearing carbon, and most alkyl halides have more than one carbon adjacent to the halogen bearing carbon. In these cases the more substituted alkene predominates because it is the more stable (Zaitsev’s Rule) 
Then this product forms
KOH
If the H+ is pulled off this carbon
KOH

47. Dehydration of Alcohols to Yield Alkenes
This reaction involves forming an alkene from an alcohol through loss of OH- from one carbon and H+ from an adjacent carbon (hence dehydration: loss of HOH.
Unsymmetrical alcohols give a mix of products. Zaitsev’s Rule applies.
H+ Cat

48. Diverse Reactions of Alkenes
Alkenes react with many electrophiles to give useful products by electrophilic addition reactions (often through special reagents)
Alcohols; (add H,OH) across the double bond
Alkanes; (add H,H) across the double bond
Dihalides; (add X,X) across the double bond
Halides; (add H,X) across the double bond
Halohydrins; (add HO,X) across the double bond
Diols;(add HO,OH) across the double bond
Cyclopropane; (add :CH2) across the double bond

49. Reactions of Alkenes

50. 7.1 Preparation of Alkenes: A Preview of Elimination Reactions: A B + C
Alkenes are commonly made by
elimination of H,X from alkyl halide (dehydrohalogenation)
Uses heat and KOH
elimination of H,OH from an alcohol (dehydration)
require strong acids (sulfuric acid, 50 ºC)

51. Elimination of H-X from an Alkyl Halide to produce an Alkene
The Mechanism for this reaction involves the base pulling off a proton (H+) from a carbon adjacent to the halogen bearing carbon. When using KOH then the base is OH-, The electron pair that is left behind after the proton leaves, becomes the new alkene pi bond and the halogen departs as a halide anion (:X-) with the two electrons from the C-X bond. 
Transition State Species
OH-
OH-
HO-H

52. Zaitsev’s Rule for Elimination Reactions
Elimination reactions almost always give a mixture of alkene products as the 1st step in the mechanism involves the removal of a proton from a carbon adjacent to the halogen bearing carbon, and most alkyl halides have more than one carbon adjacent to the halogen bearing carbon. In these cases the more substituted alkene predominates because it is the more stable (Zaitsev’s Rule) 
Then this product forms
KOH
If the H is pulled off this carbon
KOH

53. Describing a Reaction: Equilibria, Rates, and Energy Changes
Any chemical reaction may be thought of as an equilibrium process.
The exact position of equilibrium is expressed by the equilibrium constant
If the Keq is large then the product concentratioins [C] [D] are larger than the reactant [A] [B] and the reaction proceeds far to the right.If the Keq value is small then the reaction does not takeplace at all.
Keq = [Products]/[Reactants] = [C]c [D]d / [A]a[B]b

54. Describing a Reaction: Equilibria, Rates, and Energy Changes
The position of any equilibrium always favors the more stable(lower in energy) species, reactant or product
If a reaction has a large Keq value, then the energy of the products must be lower (more stable) than the energy of the reactants and the reaction must be exothermic(exergonic).
If a reaction has a small Keq value, then the energy of the reactants must be lower (more stable) than the energy of the products and the reaction must be endothermic (endergonic)

55. Magnitudes of Equilibrium Constants
If the value of Keq is greater than 1, this indicates that at equilibrium most of the material is present as products
If Keq is 10, then the concentration of the product is ten times that of the reactant
A value of Keq less than one indicates that at equilibrium most of the material is present as the reactant
If Keq is 0.10, then the concentration of the reactant is ten times that of the product

56. Describing a Reaction: Energy Diagrams and Transition States
The highest energy point in a reaction step is called the transition state
The energy needed to go from reactant to transition state is the activation energy (DG‡)

57. First Step in Addition
In the addition of HBr the (conceptual) transition-state structure for the first step
The  bond between carbons begins to break
The C–H bond begins to form
The H–Br bond begins to break

58. Describing a Reaction: Intermediates
If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product
These are called reaction intermediates or simply “intermediates”
Each step has its own free energy of activation
The complete diagram for the reaction shows the free energy changes associated with an intermediate

59. Formation of a Carbocation Intermediate
HBr, a Lewis acid, adds to the  bond
This produces an intermediate with a positive charge on carbon - a carbocation
This is ready to react with bromide

60. Carbocation Intermediate Reactions with Anion
Bromide ion adds an electron pair to the carbocation
An alkyl halide produced
The carbocation is a reactive intermediate

61. Reaction Diagram for Addition of HBr to Ethylene
Two separate steps, each with a own transition state
Energy minimum between the steps belongs to the carbocation reaction intermediate.

62. Reactions of Alkenes Please recall that:
The carbon-carbon double bond consists of a strong sigma and a weaker pi bond and that the e-'s of the pi bond are very accessible as they are located above and below the plane of the bond.
Understanding these characteristics of the carbon-carbon double bond, one would correctly assume that the loosely held, highly exposed pi e-'s would be particularly vulnerable to chemical species that are seeking electrons.

63. Electrophiles and Nucleophiles
Chemical species that seek e-'s are usually electron deficient. These electron seeking reagents are collectively referred to as electrophiles. (Greek for electron loving).
The alkenes themselves are referred to as nucleophiles. (Greek for nucleus loving). Nucleophiles are reagents with electron-rich sites which form a bond by donating a pair of e-'s to an electron-poor reagent (electrophile)
Alkene chemistry should, therefore, be dominated by reaction of the electron rich carbon-carbon double bond (nucleophile) with electron-poor species (electrophiles). This is exactly what we find. *The most important reaction of alkenes is their reaction with electrophiles.

64. Electrophilic Addition of HX to Alkenes
General reaction mechanism: electrophilic addition
Attack of electrophile (such as HBr) on  bond of alkene
Produces carbocation and bromide ion
Carbocation is an electrophile, reacting with nucleophilic bromide ion

65. Example of Electrophilic Addition
Addition of hydrogen bromide to 2-Methyl-propene
H-Br transfers proton to C=C
Forms carbocation intermediate
More stable cation forms
Bromide adds to carbocation

66. Energy Diagram for Electrophilic Addition

67. Writing Organic Reactions
No established convention – shorthand
Not necessarily balanced
Reactants can be before or on arrow
Solvent, temperature, details, on arrow

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

69. Electrophilic Addition for preparations
The reaction is successful with HCl and with HI as well as HBr
Note that HI is generated from KI and phosphoric acid

70. Orientation of Electrophilic Addition: Markovnikov’s Rule
In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other
If one orientation predominates, the reaction is regiospecific
Markovnikov observed in the 19th century that in the addition of HX to an alkene, the H always attaches to the carbon with the most H’s to begin with and the X attaches to the other Carbon (to the one with the most alkyl substituents)
This is Markovnikov’s rule

71. Example of Markovnikov’s Rule
Addition of HCl to 2-methylpropene
Regiospecific – only one product forms where two are possible

72. Energy of Carbocations and Markovnikov’s Rule
The more stable carbocation intermediate always forms faster
The stability order for carbocations is as follows:
R3C+ > R2CH+ > RCH2+ > CH3 +

73. Markovnikov’s Rule Restated
All electrophillic addition reactions proceed by way of the more stable (more highly substituted) carbocation intermediate