Basic Organic Chemistry
Course code: CHEM 12162
(Pre-requisites : CHEM 11122)
Chapter – 03
Chemistry of Alkenes
Dr. Dinesh R. Pandithavidana
Office: B1 222/3
Phone: (+94)777-745-720 (Mobile)
Email: dinesh@kln.ac.lk
1
Bond Lengths and Angles
• Hybrid orbitals have more s character.
• Pi overlap brings carbon atoms closer.
• Bond angle with pi orbitals increases.
Angle C=C-H is 121.7°
Angle H-C-H is 116. 6°
=>
2
Pi Bond
• Sideways overlap of parallel p orbitals.
• No rotation is possible without breaking
the pi bond (63 kcal/mole).
• Cis isomer cannot become trans without
a chemical reaction occurring.
=>
3
IUPAC Nomenclature
• Parent is longest chain containing the double
bond.
• -ane changes to -ene. (or -diene, -triene)
• Number the chain so that the double bond
has the lowest possible number.
• In a ring, the double bond is assumed to be
between carbon 1 and carbon 2.
4
=>
Name These Alkenes
CH2
CH CH2
CH3
1-butene
CHCH2CH3
H3C
CH3
C CH CH3
2-sec-butyl-1,3-cyclohexadiene
CH3
2-methyl-2-butene
CH3
3-n-propyl-1-heptene
3-methylcyclopentene
5
E-Z Nomenclature
• Use the Cahn-Ingold-Prelog rules to assign
priorities to groups attached to each carbon
in the double bond.
• If high priority groups are on the same side,
the name is Z (for zusammen).
• If high priority groups are on opposite sides,
the name is E (for entgegen).
=> 6
Example, E-Z
1
H3C
1
Cl
C C
H
2
2Z
Cl
1
CH CH3
2
H
CH2
2 1
C C
H
2
5E
(2Z, 5E)-3,7-dichloro-2,5-octadiene
7
Commercial Uses: Ethylene
=>
8
Commercial Uses: Propylene
=>
9
Other Polymers
=>
10
Stability of Alkenes
• Measured by heat of hydrogenation:
Alkene + H2 → Alkane + energy
• More heat released, higher energy alkene.
30.3 kcal
27.6 kcal
=>
11
Alkene Synthesis- Overview
• E2 dehydrohalogenation (-HX)
• E1 dehydrohalogenation (-HX)
• Dehalogenation of vicinal dibromides (-X2)
• Dehydration of alcohols (-H2O)
12
Removing HX via E2
• Strong base abstracts H+ as X- leaves from
the adjacent carbon.
• Tertiary and hindered secondary alkyl
halides give good yields.
• Use a bulky base if the alkyl halide usually
forms substitution products.
13
E2: Diastereomers
Ph
Br
H
Br
CH3
H
H
Ph
Ph Ph
PhPh
Ph
CH3
H
Ph
Ph
CH3
H
H
CH3
Ph
H
≡
Br
=>
Stereospecific reaction: (S, R) produces only
trans product, (R, R) produces only cis.
14
E2: Cyclohexanes
Leaving groups must be trans diaxial. =>
15
E2: Vicinal Dibromides
• Remove Br2 from adjacent carbons.
• Bromines must be anti-coplanar (E2).
• Use NaI in acetone, or Zn in acetic acid.
-
I
Br
CH3
H
CH3
Br
H
H
H3C
C C
CH3
H
=>
16
Removing HX via E1
•
•
•
•
Secondary or tertiary halides
Formation of carbocation intermediate
Weak nucleophile
Usually have substitution products too
(Refer Chapter-01 for E1 mechanism)
17
Dehydration of Alcohols
• Reversible reaction
• Use concentrated sulfuric or phosphoric
acid, remove low-boiling alkene as it forms.
• Protonation of OH converts it to a good
leaving group, HOH
• Carbocation intermediate, like E1
• Protic solvent removes adjacent H+
18
Dehydration Mechanism
H
H O H
C C
H O H
O
H O S
_
C C
O H
HSO4
O
H
H O H
H
C C
C C
H2O:
C C
+
H3O
19
Reactivity of C=C
• Electrons in pi bond are loosely held.
• Electrophiles are attracted to the pielectrons.
• Carbocation intermediate forms.
• Nucleophile adds to the carbocation.
• Net result is addition to the double bond.
20
Electrophilic Addition
• Step 1: Pi electrons attack the electrophile.
E
C
C
+
E
+
C
C +
• Step 2: Nucleophile attacks the carbocation.
E
C C+
+
_
Nuc:
E Nuc
C C
=>
21
Types of Additions
=>
22
Addition of HX
Protonation of double bond yields the most stable
carbocation. Positive charge goes to the carbon
that was not protonated.
CH3
CH3 C CH CH3
+
H
CH3
CH3 C CH CH3
H Br
+ Br
X
_
CH3
CH3 C CH CH3
+
H
=>
23
Addition of HX
CH3
CH3 C CH CH3
H Br
CH3
CH3 C CH CH3
+
H
_
Br
CH3
CH3 C CH CH3
+
H
+ Br
CH3
CH3 C CH CH3
=>
Br H
24
_
Regiospecificity
• Markovnikov’s Rule: The proton of an acid adds
to the carbon in the double bond that already has
the most H’s. “Rich get richer.”
• More general Markovnikov’s Rule: In an
electrophilic addition to an alkene, the electrophile
adds in such a way as to form the most stable
intermediate.
• HCl, HBr, and HI add to alkenes to form
Markovnikov products.
25
Free-Radical Addition of HBr
• In the presence of peroxides, HBr adds to
an alkene to form the “anti-Markovnikov”
product.
• Only HBr has the right bond energy.
• HCl bond is too strong.
• HI bond tends to break heterolytically to
form ions.
26
Free Radical Initiation
• Peroxide O-O bond breaks easily to
form free radicals.
R O O R
heat
R O
+
O R
• Hydrogen is abstracted from HBr.
R O
+ H Br
R O H
+ Br
=>
Electrophile
27
Propagation Steps
• Bromine adds to the double bond.
Br
+
C C
C C
Br
• Hydrogen is abstracted from HBr.
C C
Br
+
H Br
C C
+
Br
Br H
Electrophile =>
28
Anti-Markovnikov ??
CH3
CH3 C CH CH3
CH3
CH3 C CH CH3
Br
+
Br
X
CH3
CH3 C CH CH3
Br
• Tertiary radical is more stable, so that
intermediate forms faster.
=>
29
Hydration of Alkenes
+
H
C C
alkene
+ H2O
H OH
C C
alcohol
• Reverse of dehydration of alcohol
• Use very dilute solutions of H2SO4 or
H3PO4 to drive equilibrium toward
hydration.
=>
30
Mechanism for Hydration
H
H
C C
+
H O H
+
+
H2O
H
+
H O H
H
+
C C
+
H2O
H
+
H O H
C C
+
C C
C C
H
H O
+
H2O
C C
+
H3O
+
31
Orientation for Hydration
• Markovnikov product is formed.
H
CH3
CH3 C CH CH3 + H O+ H
CH3
CH3
CH3 C CH CH3
H
CH3
CH3 C CH CH3 + H2O
+
H
H2O
CH3 C CH CH3
+O H
H
H
H2O
O H
32
Indirect Hydration
• Oxymercuration-Demercuration
Markovnikov product formed
Anti addition of H-OH
No rearrangements
• Hydroboration
Anti-Markovnikov product formed
Syn addition of H-OH
33
Oxymercuration
• Reagent is mercury(II) acetate which
dissociates slightly to form +Hg(OAc).
• +Hg(OAc) is the electrophile that attacks the
pi bond.
O
CH3
O
O
C O Hg O C CH3
CH3
_
C O
O
+
Hg O C CH3
34
Oxymercuration
The intermediate is a cyclic mercurinium ion,
a three-membered ring with a positive
charge.
OAc
+
C C
+
Hg(OAc)
Hg
C C
35
Oxymercuration
• Water approaches the mercurinium ion from the
side opposite the ring (anti addition).
• Water adds to the more substituted carbon to form
the Markovnikov product.
OAc
+
Hg
C C
OAc
Hg
Hg
C C
+
H2O
OAc
H2O
C C
H O
O
H
H
=>
36
Demercuration
Sodium borohydride, a reducing agent, replaces
the mercury with hydrogen.
OAc
Hg
4
C C
H
_
+ NaBH4 + 4 OH
4
C C
O
O
H
H
+
NaB(OH)4
_
+ 4 Hg + 4 OAc
37
Predict the Product
Predict the product when the given alkene
reacts with aqueous mercuric acetate,
followed by reduction with sodium
borohydride.
CH3
D
(1) Hg(OAc)2, H2O
(2) NaBH4
anti addition
OH
CH3
D
H
=>
38
Alkoxymercuration - Demercuration
If the nucleophile is an alcohol, ROH, instead
of water, HOH, the product is an ether.
C C
(1) Hg(OAc)2,
CH3OH
Hg(OAc)
H3C
H
(2) NaBH4
C C
C C
O
O
H3C
39
Hydroboration
• Borane, BH3, adds a hydrogen to the most
substituted carbon in the double bond.
• The alkylborane is then oxidized to the alcohol
which is the anti-Mark product.
C C
-
(1) BH3
(2) H2O2, OH
C C
C C
H BH2
H OH
40
Borane Reagent
• Borane exists as a dimer, B2H6, in equilibrium with
its monomer.
• Borane is a toxic, flammable, explosive gas.
• Safe when complexed with tetrahydrofuran.
H
2
O
THF
+
B2H6
2
+
-
O B H
=>
H
THF . BH3
41
Mechanism
• The electron-deficient borane adds to
the least-substituted carbon.
• The other carbon acquires a positive charge.
• H adds to adjacent C on same side (syn).
42
Actually, Trialkyl
CH3
H
H3C C C H
H3C
H
C C
3
H3C
H
+ BH3
H
B
H
H C
H3C C
H
C
H
CH3
H
H C CH3
CH3
Borane prefers least-substituted carbon due to
steric hindrance as well as charge distribution.
43
Oxidation to Alcohol
• Oxidation of the alkyl borane with basic hydrogen
peroxide produces the alcohol.
• Orientation is anti-Markovnikov.
CH3 H
CH3
C C H
B
H
CH3 H
H2O2, NaOH
H2O
CH3
C C H
OH
H
=>
44
Predict the Product
Predict the product when the given alkene reacts with
borane in THF, followed by oxidation with basic hydrogen
peroxide.
CH3
(1) BH3, THF
-
D
(2) H2O2, OH
syn addition
H
CH3
OH
D
=>
45
Hydrogenation
•
•
•
•
Alkene + H2 → Alkane
Catalyst required, usually Pt, Pd, or Ni.
Finely divided metal, heterogeneous
Syn addition
=>
46
Addition of Carbenes
• Insertion of -CH2 group into a double bond
produces a cyclopropane ring.
• Two methods:
Diazomethane
Alpha elimination, haloform
47
Diazomethane
N N CH2
N N CH2
diazomethane
N
N
H
heat or uv light
CH 2
N2
+
C
H
carbene
C
C
H
C
H
Extremely toxic and explosive.
C
C
H
C
H
48
Alpha Elimination
• Haloform reacts with base.
• H and X taken from same carbon
CHCl3 + KOH
Cl
+-
K CCl3 + H2O
C
C Cl
Cl
+
-
Cl
Cl
Cl
CHCl3
Cl
KOH, H2O
Cl
49
Addition of Halogens
• Cl2, Br2, and sometimes I2 add to a double
bond to form a vicinal dibromide.
• Anti addition, so reaction is stereospecific.
Br
C C
+
Br2
C C
Br
50
Mechanism for Halogenation
• Pi electrons attack the bromine molecule.
• A bromide ion splits off.
• Intermediate is a cyclic bromonium ion.
C C
+ Br Br
Br
C C
+ Br
Halide ion approaches from side opposite the three-membered
ring.
Br
Br
C C
C C
Br
Br
51
Examples of Stereospecificity
52
Formation of Halohydrins
• If a halogen is added in the presence of
water, a halohydrin is formed.
• Water is the nucleophile, instead of halide.
• Product is Markovnikov and anti.
Br
C C
Br
Br
C C
C C
H2O
H
O
H
H
+
+ H3O
O
H2O
53
Regiospecificity
The most highly substituted carbon has the
most positive charge, so nucleophile attacks
there.
=>
54
Predict the Product
Predict the product when the given alkene reacts
with chlorine in water.
CH3
Cl2, H2O
D
OH
CH3
D
Cl
55
Epoxidation
• Alkene reacts with a peroxyacid to form an
epoxide (also called oxirane).
• Usual reagent is peroxybenzoic acid.
O
C C
+R C O O H
O
C C
O
+ R C O H
56
Mechanism
One-step concerted reaction. Several
bonds break and form simultaneously.
O
C
O
C
R
C
H O
O
C
O
C
R
C
+
H
O
Since there is no opportunity for rotation around the doublebonded carbons, cis or trans stereochemistry is maintained
57
Opening the Epoxide Ring
• Acid catalyzed.
• Water attacks the protonated epoxide.
• Trans diol is formed.
OH
C C
H
+
H3O
O
O
C C
C C
O
OH
H
C C
=>
O
H2O
H
H
H2O
58
One-Step Reaction
• To synthesize the glycol without isolating the
epoxide, use aqueous peroxyacetic acid or
peroxyformic acid.
• The reaction is stereospecific.
O
OH
CH3COOH
H
H
=>
OH
59
Syn Hydroxylation of Alkenes
• Alkene is converted to a cis-1,2-diol,
• Two reagents:
Osmium tetroxide (expensive!), followed by
hydrogen peroxide
or
Cold, dilute aqueous potassium
permanganate, followed by hydrolysis with
base
60
=>
Mechanism with OsO4
Concerted syn addition of two oxygens to
form a cyclic ester.
O
C
O
O
C
Os
C
O
Os
C
O
O
H2O2
O
O
C OH
C OH
+ OsO4
=>
61
Stereospecificity
If a chiral carbon is formed, only one
stereoisomer will be produced (or a pair of
enantiomers).
H
CH 2 CH 3
(1) Os O 4
C
(2) H 2 O 2
C
H
CH 2 CH 3
cis -3-hexene
CH 2 CH 3
H
C
OH
H
C
OH
CH 2 CH 3
meso -3,4-hexanediol
62
Oxidative Cleavage
• Both the pi and sigma bonds break.
• C=C becomes C=O.
• Two methods:
Warm or concentrated or acidic KMnO4.
Ozonolysis
• Used to determine the position of a double
bond in an unknown.
63
Cleavage with MnO4• Permanganate is a strong oxidizing agent.
• Glycol initially formed is further oxidized.
• Disubstituted carbons become ketones.
• Monosubstituted carbons become carboxylic
acids.
64
• Terminal =CH2 becomes CO2.
Example
H
CH3
C C
CH3
CH3
H CH3
KMnO4
H3C C C CH3
(warm, conc.)
OH OH
H
H3C C
CH3
+
O
C CH3
O
H3C C
O
OH
65
Ozonolysis
• Reaction with ozone forms an ozonide.
• Ozonides are not isolated, but are treated
with a mild reducing agent like Zn or
dimethyl sulfide.
• Milder oxidation than permanganate.
• Products formed are ketones or aldehydes.
66
Example of Ozonolysis
H
CH3
C C
CH3
O
H
O3
C
C
CH3
H3C
O O
CH3
CH3
Ozonide
(CH3)2S
H
C O
H3C
O C
O
CH3
+
CH3
CH3
S CH3
DMSO
67