Addition to Carbon-Carbon Pi Bonds
H2O
H2C



CH2
H3C
catalyst
OH
Need a catalyst
This is one way ethanol is manufactured
How does this reaction proceed?
 Pi Bonds
H
H
HC
H
CH
H
Alkene
Two election clouds
Alkyne
Many mechanisms that apply
alkenes also apply to alkynes.
 Generic Addition Mechanism
R
elec
R
electrophile
R
R
RDS
+
R
C
R
R
nucleophile
elec
R
nuc
R
R
R
R
Carbocation Intermediate:
Which fate?
 Rearrangement – nothing better than tertiary
 Lose proton – no protons to lose
 Capture nucleophile!
1
 Addition of H—X to a Pi Bond
X = Cl, Br, I
H3C
H
H3C
Br
+
+
CH2
or
C
CH2
H3C
H
H3C
H3C
H3C
Primary - less stable carbocation
Tertiary
3˚ is more stable carbocation:
Which fate?
 Rearrangement – won’t stabilize charge
 Lose proton – reaction will go backwards
 Capture nucleophile – Br !
H3C
H
Br
+
-
Major Product
Formed from tertiary carbocation intermediate
H3C
CH3
C
H3C
H3C
H3C
+
CH2
CH4
Br -
H3C
Br
H3C
H3C
Minor Product
Formed from primary carbocation intermediate
Br
 This noted addition of Br to the more substituted end of the alkene was discovered by Markovnikov
Markovnikov’s Rule:
When a hydrogen halide adds to an alkene, the hydrogen is added to the carbon bearing the most
hydrogen substituents (least substituted carbon), and the halide is added to the end of the alkene with the
least number of hydrogens.
X
CH2
H
X
H
H goes to the least substituted carbon
X goes to the more substituted carbon
2
 Addition of H—Br to a Triple Bond
H
H
Br
H
+
CH3
H2C
+
C
+
C
CH3
CH3
1˚ - Minor Product
2 ˚ - Major Product
Major Product:
H2C
+
C
CH3
Carbocation fate?
 Lose a proton  unproductive, would move reaction backwards
 Rearrangement  Vinyl carbocations CANNOT rearrange!
H
H2C
+
X
C x
H2C
+
H
C
H
H
 Capture a nucleophile!
H2C
Br
Br -
+
C
H
H
Br
C
H2C
CH3
H
CH3
Br
+
Br
H3C
H3C
CH3
2˚ with resonance! 
One contributor with
complete octets!
Br
+
+
C
CH3
1˚ without resonance 
CH3
Br
Br -
Br
H3C
CH3
FINAL PRODUCT
3
 Addition of H2O
H2C
+
CH2
H
H3C
OH
OH
Ethylene
Example:
No!
H
OH
+
C
+
HO
-
H

OH is not a leaving group
unless you have a very good
acid
Mechanism:
+
H
OH2
+
C
+
H2O
Carbocation Intermediate:
 Rearrangement is unproductive
 Lose proton  reaction goes backwards
 Capture nucleophile: H2O
H
H2O
H
O
+
H2O
HO
+
+
C
H3O
+
 This reaction is reversible
 It is in EQUILIBRIUM!
HO
+
H2O
 Favors alkene just a little
 How do you control the direction of equilibrium? Think of la Chatelier’s Principle: increase the
concentration on one side (reactants or products) to shift the equilibrium to the other side.
4
 Addition of H2O
OH
H2SO4 (aq)
H3O+
Ph = Phenyl = C6H5
H
H
+
OH2
Ph
H2O
+
Ph
C
+
C
Ph
H
O
+
H2O
Ph
H
Carbocation intermediate:
Resonance, but no extra stability
 Rearrange to 3˚ with resonance
HO
+
H3O
+
Ph
 Alkynes and H3O+
H
H3O+
Ph
HO
Ph
Alkene and H3O
+
Alkyne and H3O
+
CH3
CH2
H3O+
Ph
H
OH
Ph
CH
CH2
Enol
5
Mechanism:
H
H
+
OH2
Ph
Ph
O
OH2
+
C
CH
+
H
OH
+
Or
C
Ph
+
C
CH3
+
?
H
H
2˚ with resonance
O
+
OH2
CH2
CH2
OH
H
Ph
Ph
CH2
Ph
OH
OH2
1˚ without resonance 
O
OH2
H
+
Ph
Ph
CH3
Resonance contributor
with complete octets
H3O
+
CH3
Major Product
The intermediate labeled “1˚ without resonance ” can proceed to deprotonation and formation of an
enol. The keto product is favored greatly.
OH
O
Ph
Ph
H
CH3
H
Enol
Keto
These products are tautomers, which are created through
tautomerization, a process where a hydrogen atom migrates to
another carbon and a double bond shifts to an adjacent bond.
Why not?
OH
Ph
OH2
+
C
CH3
H2O
+
OH
This product will go backwards
to the happiest stage (keto form)
Ph
CH3
6
 Miscellaneous Additions
a) Catalytic Hydrogenation
H
H2
H
Pt
-
Syn-addition: new things are added on the same face
Markovnikov’s rule does not apply
No nucleophile or electrophile
Catalyst is a transition metal
Think of it as “removing” the pi bond
b) Addition of Br2
δ+ δ-
Br
Br Br
-
Br
Anti-addition
Markovnikov’s rule does not apply
+
Electrophile = Br2 (induced δ / δ when Br2 approaches the pi bond)
Nucleophile = alkene
c) Hydroboration-Oxidation
H
1. BH3
2. H2O2, HO-
OH
Syn-addition
Anti-Markovnikov
Electrophile = BH3 (Borane, because of the open octet)
Nucleophile = alkene
Addition of water on the alkene converts it into an alcohol
7
d) Ozonolysis
1. O3
2. (CH3)2S
-
H
O
+
O
H
Syn-addition
No nucleophile or electrophile
Ozone (O3) reacts with an alkene and forms carbonyls
o Imagine the pi bond splitting in half and each end bonding to oxygen
8