1. Chapter 3 Alkenes and Alkynes: The Nature of Organic Reactions
Suggested Problems: 21,25,26-32,35,40-2,43,47,51,54,56,58

2. Alkene - Hydrocarbon With Carbon-Carbon Double Bond
Hydrocarbon that contains a C=C double bond
Sometimes called an olefin but alkene is better
Includes many naturally occurring materials
Flavors, fragrances, vitamins

3. Alkyne - Hydrocarbon With Carbon-Carbon Triple Bond
Hydrocarbon that contains a C≡C triple bond
Less common than alkenes
Simplest member acetylene
Alkenes & alkynes are said to be unsaturated

4. 3.1 Naming Alkenes and Alkynes
Name the parent hydrocarbon using suffix –ene in place of -ane
Number the carbons in chain so that double bonded carbons have lowest possible numbers
Note – the branch point gets the lowest number above right

5. Naming Alkenes (Continued)
Write the full name- Number substituents according to: 1) Position in chain, 2) Alphabetically
Rings have “cyclo” prefix

6. Many Alkenes Are Known by Common Names

7. Worked Example Give IUPAC names for the following compounds a) b)
Naming alkenes

8. Worked Example Solution: a) The compound is 3,4,4-Trimethyl-1-pentene
The longest chain is a pentene
Consists of three methyl groups
The methyl groups are attached to C3 and C4
Using tri- before methyl and using numbers to signify the location of the double bond
The compound is 3,4,4-Trimethyl-1-pentene
Naming alkenes

9. Worked Example b) The compound is 3-Methyl-3-hexene
The longest chain is a hexane
The methyl group is attached to C3
Using numbers to signify the location of the double bond
The compound is 3-Methyl-3-hexene
Naming alkenes

10. Naming Alkynes
General hydrocarbon rules apply with “-yne” as a suffix indicating an alkyne
Numbering of chain with triple bond is set so that the smallest number possible is assigned to the first carbon of the triple bond

11. Worked Example
Name the following alkynes: A) B)
Naming alkynes

12. Worked Example Solution: A) 2,5-Dimethyl-3-hexyne
B) 3,3-Dimethyl-1-butyne
Naming alkynes

13. 3.2 Electronic Structure of Alkenes
Rotation of  bond is prohibitive
 bond must break for rotation to occur (unlike a carbon-carbon single bond).
Creates possible alternative structures

14. 3.3 Cis-Trans Isomers of Alkenes
Carbon atoms in a double bond are sp2-hybridized
Three equivalent orbitals at 120º separation in plane
Fourth orbital is atomic p orbital
Combination of electrons in two sp2 orbitals of two atoms forms  bond between them
Additive interaction of p orbitals creates a  bonding orbital
Occupied  orbital prevents rotation about -bond
Rotation prevented by  bond - high barrier, about 268 kJ/mole in ethylene

15. Cis-Trans Isomers of Alkenes (Continued)
the presence of a carbon-carbon double bond can create two possible structures
cis isomer - two similar groups on same side of the double bond
trans isomer - similar groups on opposite sides
cis alkenes are less stable than trans alkenes

16. Worked Example Which compound in each pair is more stable a) b)
Stability of alkenes

17. Worked Example
Solution: a) b)
Stability of alkenes

18. Cis-Trans Isomers of Alkenes (Continued)
Cis-Trans Isomerization requires that end groups differ in pairs
Bottom pair cannot be superposed without breaking C=C

19. 3.4 Sequence Rules: The E,Z Designation
Cis-Trans naming system discussed thus far only works with disubstituted alkenes
Tri- and Tetra substituted double bonds require more general method
Method referred to as the E,Z system

20. Sequence Rules: The E,Z Designation (Continued): E,Z Stereochemical Nomenclature
Priority rules of Cahn, Ingold, and Prelog
Compare where higher priority groups are with respect to bond and designate as prefix
E -entgegen, opposite sides
Z - zusammen, together on the same side
Z zame zide

21. Sequence Rules: The E,Z Designation (Continued): Cahn-Ingold-Prelog Rules
Rank substituent atoms according to atomic number of first atom
Higher atomic number gets higher priority
Br > Cl > S > P > O > N > C > H

22. Sequence Rules: The E,Z Designation (Continued): Cahn-Ingold-Prelog Rules
If atomic numbers are the same, compare at next connection point at same distance
Compare until something has higher atomic number
Do not combine – always compare

23. Sequence Rules: The E,Z Designation (Continued): Cahn-Ingold-Prelog Rules
Multiple-bonded atoms are equivalent to the same number of single-bonded atoms
Substituent is drawn with connections shown and no double or triple bonds

24. Worked Example
Assign stereochemistry (E or Z) to the double bond in the following compound
Convert the drawing into a skeletal structure (red = O)
Alkene Stereochemistry and the E,Z Designation

25. Worked Example Solution:
Alkene Stereochemistry and the E,Z Designation

26. 3.5 Kinds of Organic Reactions
Common patterns describe the changes in organic reactions
Addition reactions – two molecules combine
Elimination reactions – one molecule splits into two

27. Kinds of Organic Reactions (Continued)
Substitution – parts from two molecules exchange

28. Kinds of Organic Reactions (Continued)
Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected

29. 3.6 How Reactions Occur: Mechanisms
In a clock the hands move but the mechanism behind the face is what causes the movement
In an organic reaction, we see the transformation that has occurred. The mechanism describes the steps behind the changes that we observe
Reactions occur in defined steps that lead from reactant to product – invariably this involves the flow of electrons from one atom to another

30. Steps in Mechanisms We classify the types of steps in a sequence
A step involves either the formation or breaking of a covalent bond
Steps can occur individually or in combination with other steps
When several steps occur at the same time, they are said to be concerted

31. Types of Steps in Reaction Mechanisms
Bond formation or breakage can be symmetrical or unsymmetrical
Symmetrical- homolytic
Unsymmetrical- heterolytic

32. Indicating Steps in Mechanisms
Curved arrows indicate breaking and forming of bonds
Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’) – one electron processes
Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’) – two electron processes

33. Radical Reactions (Unpaired electrons)
Not as common as polar reactions
Radicals react to complete electron octet of valence shell
A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule
A radical can add to an alkene to give a new radical, causing an addition reaction

34. Polar Reactions (paired electrons)
Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities
This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom
The more electronegative atom has the greater electron density
Elements such as O, F, N, Cl are more electronegative than carbon

35. Generalized Polar Reactions
An electrophile, an electron-poor species, combines with a nucleophile, an electron-rich species
An electrophile is a Lewis acid (+ or d+)
A nucleophile is a Lewis base (electron pair)
The combination is indicated with a curved arrow from nucleophile to electrophile

36. Some Nucleophiles and Electrophiles

37. 3.7 The Mechanism of an Organic Reaction: Addition of HBr to Ethylene
HBr (Note: The text uses HCl.) adds to the  part of a 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

38. Mechanism of Addition of HBr to Ethylene
HBr electrophile is attacked by  electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion
Bromide 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

39. 3.8 Describing a Reaction: Transition States and Intermediates
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 (Eact)

40. Describing a Reaction: Transition States and 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

41. Hydrogenation of double bond with Pd
Effect of a Catalyst
A catalyst is a substance that increases the rate of a reaction by providing an alternative mechanism.
The catalyst participates in the reaction.
It is regenerated during the reaction.
The catalyst provided a mechanism with a lower activation energy.
Example:
Hydrogenation of double bond with Pd