1. Alkenes and Alkynes
REFERENCE BOOK:
MCMURRY ORGANIC CHEMISTRY 6TH EDITION CHAPTER 6 (C) 2003
1
■By:
Dr. Muhammad Javed Iqbal
2. Alkene - Hydrocarbon
With Carbon- Carbon Double Bond
MCMURRY ORGANIC CHEMISTRY 6TH EDITION CHAPTER 6 (C) 2003 2
■Also called an olefin but alkene is better
■Includes many naturally occurring
materials
■Flavors, fragrances, vitamins
■Important industrial products
■These are feedstocks for industrial
processes
3. Degree of Unsaturation
MCMURRY ORGANIC CHEMISTRY 6TH EDITION CHAPTER 6 (C) 2003 3
■ Relates molecular formula to possible structures
■ Degree of unsaturation: number of multiple bonds or rings
■ Formula for saturated a acyclic compound is CnH2n+2
■ Each ring or multiple bond replaces 2 H's
4. Example: C6H10
4
■ Saturated is C6H14
■
Therefore 4 H's are not present.
■
■
■
■
Two double bonds?
or triple bond?
or two rings
or ring and double bond
H3C C
C CH3
H
H
C
H H
H H
C
H H
This has two degrees
of unsaturation
5. Degree of Unsaturation With Other
Elements
5
■ Organohalogens (X: F, Cl, Br, I)
■ Halogen replaces hydrogen
■ C4H6Br2 and C4H8 have one degree of unsaturation
■ Oxygen atoms - if connected by single bonds
■ These don't affect the total count of H's
6. Naming of Alkenes
6
■ Find longest continuous carbon chain for root
■ Number carbons in chain so that double bond carbons
have lowest possible numbers
■ Rings have “cyclo” prefix
7. Many Alkenes Are Known by
Common Names
7
■ Ethylene = ethene
■ Propylene = propene
■ Isobutylene = 2-
methylpropene
■ Isoprene = 2-methyl-1,3-
butadiene
8. Electronic Structure of Alkenes
8
■ 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
9. Rotation of π Bond Is Prohibitive
9
■ This prevents rotation about a carbon-carbon double
bond (unlike a carbon-carbon single bond).
■ Creates possible alternative structures
10. Cis-Trans Isomerism in Alkenes
10
■ 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
these isomers to occur
11. Cis, Trans Isomers Require That End
Groups Must Differ in Pairs
11
■ 180°rotation superposes
■ Bottom pair cannot be
superposed without
breaking C=C
X
12. Sequence Rules: The E,Z Designation
12
■ Neither compound is
clearly “cis” or “trans”
■ Substituents on C1 are
different than those on C2
■ We need to define
“similarity” in a precise way
to distinguish the two
stereoisomers
■ Cis, trans nomenclature only
works for disubstituted
double bonds
13. Develop a System for Comparison of
Priority of Substituents
13
■ 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”
14. E,Z Stereochemical Nomenclature
14
■ 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
15. Ranking Priorities: Cahn-Ingold- Prelog
Rules
15
■ Must rank atoms that are connected at comparison point
■ Higher atomic number gets higher priority
■ Br > Cl > O > N > C > H
In this case,The higher priority
groups are opposite:
(E )-2-bromo-2-chloro-propene
16. ■ 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
Extended Comparison
16
17. ■ Substituent is drawn with connections shown and no
double or triple bonds
■ Added atoms are valued with ligands themselves
Dealing With Multiple Bonds
17
18. Alkene Stability
18
■ Cis alkenes are less stable than trans alkenes
■ Compare heat given off on hydrogenation: Ho
■ Less stable isomer is higher in energy
■ And gives off more heat
■ tetrasubstituted > trisubstituted > disubstituted >
monosusbtituted
19. Comparing Stabilities of Alkenes
19
■ Evaluate heat given off when C=C is converted to C-C
■ More stable alkene gives off less heat
■ Trans butene generates 5 kJ less heat than cis-butene
20. Electrophilic Addition of HX to Alkenes
20
■ 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
21. ■ Two step process
■ First transition state is
high energy point
Electrophilic Addition Energy Path
21
22. Example of Electrophilic Addition
22
■ 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
23. Energy Diagram for Electrophilic Addition
23
■ Rate determining
(slowest) step has highest
energy transition state
■ Independent of
direction
■ In this case it is the
first step in forward
direction
24. Electrophilic Addition for preparations
24
■ The reaction is successful with HCl and with HI as well as
HBr
25. Orientation of Electrophilic Addition:
Markovnikov’s Rule
25
■ 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 alkene, the H attaches to the carbon with the most
H’s and X attaches to the other end (to the one with the
most alkyl substituents)
■ This is Markovnikov’s rule
26. ■ Addition of HCl to 2-methylpropene
■ Regiospecific – one product forms where two are possible
■ If both ends have similar substitution, then not
regiospecific
Example of Markovnikov’s Rule
26
27. Mechanistic Source of Regiospecificity
in Addition Reactions
27
■ If addition involves a
carbocation intermediate
■ and there are two
possible ways to add
■ the route producing the
more alkyl substituted
cationic center is lower
in energy
■ alkyl groups stabilize
carbocation
28. Carbocation Stability
28
■ The stability of the carbocation (measured by energy
needed to form it from R-X) is increased by the presence
of alkyl substituents
■ Therefore stability of carbocations: 3º > 2º > 1º > +CH3
■ More stable carbocation forms faster
Give Reason: Home work
29. Diverse Reactions of Alkenes
29
■ Alkenes react with many electrophiles to give useful
products by addition (often through special reagents)
■ alcohols (add H-OH)
■ alkanes (add H-H)
■ halohydrins (add HO-X)
■ dihalides (add X-X)
■ halides (add H-X)
■ diols (add HO-OH)
30. Preparation of Alkenes: A Preview of
Elimination Reactions
30
■ Alkenes are commonly made by
■ elimination of HX from alkyl halide
(dehydrohalogenation)
■ Uses heat and KOH
■ elimination of H-OH from an alcohol (dehydration)
■ require strong acids (sulfuric acid, 50 ºC)
31. Addition of Halogens to Alkenes
31
■ Bromine and chlorine add to alkenes to give 1,2-dihaldes,
an industrially important process
■ F2is too reactive and I2 does not add
■ Cl2 reacts as Cl+ Cl-
■ Br2 is similar
32. Addition of Br2 to Cyclopentene
32
■ Addition is exclusively trans
+
33. Halohydrin Formation
33
■ This is formally the addition of HO-X to an alkene (with +OH
as the electrophile) to give a 1,2-halo alcohol, called a
halohydrin
■ The actual reagent is the dihalogen (Br2 or Cl2in water in
an organic solvent)
34. Addition of Water to Alkenes
34
■ Hydration of an alkene is the addition of H-OH to to give
an alcohol
■ Acid catalysts are used in high temperature industrial
processes: ethylene is converted to ethanol
35. Reduction of Alkenes: Hydrogenation
35
■ Addition of H-H across C=C
■ Reduction in general is addition of H2or its equivalent
■ Requires Pt or Pd as powders on carbon and H2
■ Hydrogen is first adsorbed on catalyst
■ Reaction is heterogeneous (process is not in solution)
37. Oxidation of Alkenes: Hydroxylation and
Cleavage
37
■ Hydroxylation adds OH to each end of C=C
■ Catalyzed by osmium tetroxide
■ Product is a 1,2-dialcohol or diol (also called a glycol)
38. Alkene Cleavage: Ozone
38
■ Ozonolysis is a widely used method for locating the double
bond
■ Ozone, O3, adds to alkenes to form ketones and/or
aldehydes
39. Examples of Ozonolysis of Alkenes
39
■ Used in determination of structure of an unknown alkene
41. Permangante Oxidation of Alkenes
41
■ Oxidizing reagents other than ozone also cleave alkenes
■ Potassium permanganate (KMnO4) can produce carboxylic
acids and carbon dioxide if H’s are present on C=C
42. Addition of Radicals to Alkenes: Polymers
42
■ A polymer is a very large molecule consisting of repeating
units of simpler molecules, formed by polymerization
■ Alkenes react with radical catalysts to undergo radical
polymerization
■ Ethylene is polymerized to poyethylene, for example
43. Free Radical Polymerization of Alkenes
43
■ Alkenes combine many times to give polymer
■ Reactivity induced by formation of free radicals
45. Alkynes
45
■ Hydrocarbons that contain carbon-carbon
triple bonds
■ Acetylene, the simplest alkyne is produced
industrially from methane and steam at high
temperature
■ Our study of alkynes provides an introduction
to organic synthesis, the preparation of
organic molecules from simpler organic
molecules
46. Electronic Structure of Alkynes
46
■ Carbon-carbon triple bond result from sp orbital on
each C forming a sigma bond and unhybridized pX
and py orbitals forming a π bond
■ The remaining sp orbitals form bonds to other atoms
at 180º to C-C triple bond.
■ The bond is shorter and stronger than single or
double
■ Breaking a π bond in acetylene (HCCH) requires
318 kJ/mole (in ethylene it is 268 kJ/mole)
47. Naming Alkynes
47
■ General hydrocarbon rules apply wuith “-yne”
as a suffix indicating an alkyne
■ Numbering of chain with triple bond is set so
that the smallest number possible include the
triple bond
48. Reactions of Alkynes: Addition of HX
and X2
48
■ Addition reactions of alkynes are similar to
those of alkenes
■ Intermediate alkene reacts further with
excess reagent
■ Regiospecificity according to Markovnikov
49. Reduction of Alkynes
49
■ Addition of H2 over a metal catalyst (such as
palladium on carbon, Pd/C) converts alkynes to
alkanes (complete reduction)
■ The addition of the first equivalent of H2produces an
alkene, which is more reactive than the alkyne so
the alkene is not observed
50. Conversion of Alkynes to cis-Alkenes
50
■ Addition of H2 using chemically deactivated
palladium on calcium carbonate as a catalyst
(the Lindlar catalyst) produces a cis alkene
■ The two hydrogens add syn (from the same
side of the triple bond)
51. Conversion of Alkynes to trans- Alkenes
51
■ Anhydrous ammonia (NH3) is a liquid below -33 ºC
■ Alkali metals dissolve in liquid ammonia and function
as reducing agents
■ Alkynes are reduced to trans alkenes with sodium or
lithium in liquid ammonia
52. Oxidative Cleavage of Alkynes
■ Strong oxidizing reagents (O3or KMnO4) cleave
internal alkynes, producing two carboxylic acids
■ Terminal alkynes are oxidized to a carboxylic acid and
carbon dioxide
■ Neither process is useful in modern synthesis – were
used to elucidate structures because the products
indicate the structure of the alkyne precursor
52
