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ALKENES: STRUCTURE AND
REACTIVITY
Alkene: Hydrocarbon with
Carbon-Carbon Double Bond
2
Also called an olefin but alkene is better
Includes many naturally occurring materials
Flavors, fragrances, vitamins
Why this Chapter?
3
C-C double bonds are present in most organic and
biological molecules.
To examine consequences of alkene stereoisomerism.
To focus on general alkene reaction:
electrophilic addition
Industrial Preparation and Use of Alkenes
4
Ethylene and propylene are the most important organic chemicals
produced.
Calculating Degree of Unsaturation
5
Relates molecular formula to possible structures
Degree of unsaturation: number of multiple bonds or
rings
Formula for a saturated acyclic compound is CnH2n+2
Each ring or multiple bond replaces 2 H's
Degree of Unsaturation With Other
Elements
6
Organohalogens (X: F, Cl, Br, I)
Halogen replaces hydrogen
C4H6Br2 and C4H8 have one degree of unsaturation
Organooxygen compounds (C,H,O) - if connected by single bonds
These don't affect the total count of H's
Organonitrogen Compounds
7
Nitrogen has three bonds
So if it connects where H was, it adds a connection point
Subtract one H for equivalent degree of unsaturation in
hydrocarbon
Summary - Degree of Unsaturation
8
Count pairs of H's below CnH2n+2
Add number of halogens to number of H's
(X equivalent to H)
Ignore oxygens (oxygen links H)
Subtract N's - they have two connections
Naming of Alkenes
9
Name the parent hydrocarbon
Number carbons in chain so that double bond carbons have lowest
possible numbers
Rings have cyclo prefix
Many Alkenes are Known by Common
Names
10
Cis-Trans Isomerism in Alkenes
11
Carbon atoms in a double bond are sp2-hybridized.
Three equivalent orbitals at 120 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.
Subtractive interaction creates a anti-bonding orbital.
Occupied orbital prevents rotation about -bond.
Rotation prevented by bond - high barrier, about 268
kJ/mole in ethylene.
Rotation of Bond is Prohibitive
12
This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond).
Creates possible alternative structures.
13
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.
Each carbon must have two different groups for these isomers to occur.
Cis, Trans Isomers Require That End
Groups Must Differ in Pairs
14
180rotation superposes
Bottom pair cannot be superposed without
breaking C=C
Sequence Rules: The E,Z Designation
15
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
E,Z Stereochemical Nomenclature
16
Priority rules of Cahn, Ingold, and Prelog
Compare where higher priority groups are with respect to bond
and designate as prefix
E - opposite sides
Z - together on the same side
Ranking Priorities: Cahn-Ingold-Prelog
Rules
17
RULE 1
Must rank atoms that are connected at comparison point.
Higher atomic number gets higher priority.
Br > Cl > S > P > O > N > C > H
Extended Comparison
18
RULE 2
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.
Dealing With Multiple Bonds
19
RULE 3
Substituent is drawn with connections shown and no double or
triple bonds.
Added atoms are valued with 0 ligands themselves.
Stability of Alkenes
20
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
hyperconjugation stabilizes
Comparing Stabilities of Alkenes
21
Evaluate heat given off when C=C is converted to C-C
More stable alkene gives off less heat
trans-Butene generates 4 kJ less heat than cis-butene
Hyperconjugation
22
Stabilizing interaction between an unfilled orbital and a
neighbouring filled the C-H bond on a substituent.
More substituents, greater stabilisation.
Alkyl groups are better than H.
Electrophilic Addition of Alkenes
23
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.
Electrophilic Addition Energy Path
24
Two step process.
First transition state is high energy point.
Electrophilic Addition for Preparations
25
The reaction is successful with HCl and with HI as well as HBr.
HI is generated from KI and phosphoric acid.
Orientation of Electrophilic Addition:
Markovnikovs Rule
26
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 Hs and X attaches to the other end (to the one with the most alkyl substituents).
This is Markovnikovs Rule
Example of Markovnikovs Rule
27
Addition of HCl to
2-methylpropene.
Regiospecific one product forms where two are possible.
If both ends have similar substitution, then not regiospecific.
Markovnikovs Rule (restated)
28
More highly substituted carbocation forms as intermediate rather
than less highly substituted one.
Tertiary cations and associated transition states are more stable
than primary cations.
Carbocation Structure and Stability
29
Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2 orbitals.
The fourth orbital on carbon is a vacant p-orbital.
The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents.
30
A plot of dissociation enthalpy versus substitution pattern for the
gas-phase dissociation of alkyl chlorides to yield carbocations.
More highly substituted alkyl halides dissociate more easily than
less highly substituted ones.
Inductive Stabilization of Cation Species
31
inductive effect is simply the shifting of electrons in a bond in
response to the electronegativity of nearby atoms
The Hammond Postulate
32
If carbocation intermediate is more stable than another, why is
the reaction through the more stable one faster?
The relative stability of the intermediate is related to an
equilibrium constant (G)
The relative stability of the transition state (which describes
the size of the rate constant) is the activation energy (G)
The transition state is transient and cannot be examined
Transition State Structures
33
A transition state is the highest energy species in a reaction step.
By definition, its structure is not stable enough to exist for one vibration.
But the structure controls the rate of reaction.
So we need to be able to guess about its properties in an informed way.
We classify them in general ways and look for trends in reactivity the conclusions are in the Hammond Postulate.
Examination of the Hammond Postulate
34
A transition state should be similar to an intermediate that is close in energy
Sequential states on a reaction path that are close in energy are likely to be close in
structure - G. S. Hammond
Competing Reactions and the Hammond
Postulate
35
Normal Expectation: Faster reaction gives more stable intermediate.
Intermediate resembles transition state.
Mechanism of Electrophilic Addition:
Rearrangements of Carbocations
36
Carbocations undergo structural rearrangements following set patterns.
1,2-H and 1,2-alkyl shifts occur
Goes to give more stable carbocation
Can go through less stable ions as intermediates
Hydride Shifts in Biological Molecules
37
ALKENES: REACTIONS AND
SYNTHESIS
Diverse Reactions of Alkenes
39
Alkenes react with many electrophiles to give useful products by
addition (often through special reagents).
Why this Chapter?
40
To begin a systematic description of major functional
groups.
Begin to focus on general principles and patterns of
reactivity that tie organic chemistry.
Preparation of Alkenes:
A Preview of Elimination Reactions
41
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 C)
Addition of Halogens to Alkenes
42
Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially
important process
F2 is too reactive and I2 does not add
Cl2 reacts as Cl+ Cl-
Br2 is similar
Addition of Br2 to Cyclopentene
43
Addition is exclusively trans
Mechanism of Bromine Addition
44
Br+ adds to an alkene producing a cyclic ion.
Bromonium ion, bromine shares charge with carbon.
Gives trans addition
Bromonium Ion Mechanism
45
Electrophilic addition of
bromine to give a cation is
followed by cyclization to
give a bromonium ion
This bromoniun ion is a
reactive electrophile and
bromide ion is a good
nucleophile
The Reality of Bromonium Ions
46
Bromonium ions were postulated more than 60 years ago to
explain the stereochemical course of the addition (to give the
trans-dibromide from a cyclic alkene.
Olah showed that bromonium ions are stable in liquid SO2
with SbF5 and can be studied directly.
Addition of Hypohalous Acids to
Alkenes: Halohydrin Formation
47
This is formally the addition of HO-X to an alkene to give a
1,2-halo alcohol, called a halohydrin.
The actual reagent is the dihalogen (Br2 or Cl2 in the presence
of water).
Mechanism of Formation of a Bromohydrin
48
Br2 forms bromonium
ion, then water adds
Orientation toward
stable C+ species
Aromatic rings do not
react
An Alternative to Bromine
49
Bromine is a difficult reagent to use for this reaction.
N-Bromosuccinimide (NBS) produces bromine in organic
solvents and is a safer source.
Addition of Water to Alkenes:
Oxymercuration
50
Hydration of an alkene is the addition of H-OH to give
an alcohol.
Acid catalysts are used in high temperature industrial
processes: ethylene is converted to ethanol.
Oxymercuration Intermediates
51
For laboratory-scale hydration of an alkene.
Use mercuric acetate in THF followed by sodium borohydride.
Markovnikov orientation via mercurinium ion.
Addition of Water to Alkenes:
Hydroboration
52
Herbert Brown (HB) invented hydroboration (HB)
Borane (BH3) is electron deficient (Lewis acid)
Borane adds to an alkene to give an organoborane
Non-Markonikov product
Hydroboration-Oxidation Forms an
Alcohol from an Alkene
53
Addition of H-BH2 (from BH3-THF complex) to three alkenes
gives a trialkylborane.
Oxidation with alkaline hydrogen peroxide in water produces
the alcohol derived from the alkene.
Orientation in Hydration via Hydroboration
54
Regiochemistry is opposite to Markovnikov orientation.
OH is added to carbon with most Hs
H and OH add with syn stereochemistry, to the same
face of the alkene (opposite of anti addition).
Mechanism of Hydroboration
55
Borane is a Lewis acid.
Alkene is a Lewis base.
Transition state involves anionic development on B.
The components of BH3 are added across C=C.
More stable carbocation is also consistent with steric preferences.
Addition of Carbenes to Alkenes
56
The carbene functional group is half of an alkene.
Carbenes are electrically neutral with six electrons in the
outer shell.
They add symmetrically across double bonds to form
cyclopropanes.
Formation of Dichlorocarbene
57
Base removes
proton from
chloroform.
Stabilized
carbanion remains.
Unimolecular
elimination of Cl
- gives electron
deficient species,
dichlorocarbene
Reaction of Dichlorocarbene
58
Addition of dichlorocarbene is stereospecific cis
Simmons-Smith Reaction
59
Equivalent of addition of CH2:
Reaction of diiodomethane with zinc-copper alloy produces a carbenoid
species.
Forms cyclopropanes by cycloaddition.
Reduction of Alkenes: Hydrogenation
60
Addition of H-H across C=C.
Reduction in general is addition of H2 or 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).
Hydrogen Addition - Selectivity
61
Selective for C=C. No reaction with C=O, C=N
Polyunsaturated liquid oils become solids
If one side is blocked, hydrogen adds to other
Sensitive to steric environment
Mechanism of Catalytic Hydrogenation
62
Heterogeneous
reaction between
phases
Addition of H-H is
syn
Oxidation of Alkenes:
Epoxidation and Hydroxylation
63
Epoxidation results in a cyclic ether with an oxygen atom
Stereochemistry of addition is syn
Osmium Tetroxide Catalyzed Formation
of Diols
64
Hydroxylation - converts to syn-diol.
Osmium tetroxide, then sodium bisulfate.
Via cyclic osmate di-ester.
Oxidation of Alkenes:
Cleavage to Carbonyl Compounds
65
Ozone, O3, adds to alkenes to form molozonide.
Reduce molozonide to obtain ketones and/or aldehydes.
Ozonolysis of Alkenes
66
Result of ozonolysis : C=C bond cleaved and oxygen is doubly
bonded to each of the original alkene carbons.
Permangante Oxidation of Alkenes
67
Oxidizing reagents other than ozone also cleave alkenes.
Potassium permanganate (KMnO4) can produce carboxylic
acids and carbon dioxide if Hs are present on C=C
Cleavage of 1,2-diols
68
Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4, cleaves
the diol into two carbonyl compounds.
Sequence of diol formation with OsO4 followed by diol cleavage is a
good alternative to ozonolysis.
Addition of Radicals to Alkenes:
Polymers
69
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.
For example, ethylene is polymerized to polyethylene.
Free Radical Polymerization: Initiation
70
Initiation - a few radicals are generated by the reaction of a
molecule that readily forms radicals from a nonradical molecule.
A bond is broken homolytically.
Polymerization: Propagation
71
Radical from initiation adds to alkene to generate alkene
derived radical.
This radical adds to another alkene, and so on many
times.
Polymerization: Termination
72
Chain propagation ends when two radical chains
combine.
Not controlled specifically but affected by reactivity and
concentration.
Other Polymers
73
Other alkenes give other common polymers
Biological Additions of Radicals to
Alkenes
74
Severe limitations to the usefulness of radical addition reactions in the lab.
In contrast to electrophilic additions, reactive intermediate is not
quenched so it reacts again and again uncontrollably.
Biological Reactions
75
Biological reactions different than in the laboratory.
One substrate molecule at a time is present in the active
site of an enzyme.
Biological reactions are more controlled, more specific
than other reactions.