Electrophilic addition: e.g. addition of HCl
H Cl
H3C
CH3
H3C
CH3
H
H3C
H3C
Attack of HCl on
π-electrons with
formation of a
carbocation
Cl
H
Cl
CH3
CH3
H3C
H3C
Carbocation
(carbenium ion)
CH3
CH3
Attack of Cl- on
the carbocation to
give product (Lewis base, Clattacks Lewis acid R+)
empty p-orbital
very reactive
Addition of halogens to alkenes
Br
Bromonium ion
Br
H3C
H
H
H3C
CH3
Br
H
Br
H3C
H
H
H
CH3
Br
R
CH3
S
Br
H3C
H
Br
identical
meso
HCH
3
Br
R
S
simple electrophilic attack would give a carbocation, with an unfilled valence shell
H3C
Br
H
H
bromonium ion, with filled valence shells on all atoms, is better
CH3
there are also chloronium
H3C
Cl
H
H
and iodonium ions
CH3
H3C
I
H
H
CH3
SN2
bimolecular
nucleophilic
substitution
CH3(CH2)5 H
HO
Br
HO
CH3
H
CH3
(CH2)5CH3
Br
CH3(CH2)5
H
Br
HO
CH3
transition
state
SN1 mechanism
H7C3
CH3
C
H7C3CH3
H7C3CH3
Br
C
C2H5
Br
C
A
C2H5
B
C2H5
Nu
Nu
transition state
planar carbocation
A
B
CH3
C3H7
H3C
C3H7
C
Nu
C
C2H5
Nu
C2H5
CH3
C3H7
H7C3
HO C
CH3
C OH
C2H5
C2H5
E1 Mechanism
CH3
CH3
H3C
C
H3C
Br
C
CH2 H
CH3
EtOH
EtOH
H3C
CH3
CH3
CH3
H3C
C OCH2CH3
H3C
C OCH2CH3
CH3 H
CH3
+ EtOH2
C
CH2
EtOH2
CH3
CH3CH2
C
Br
CH3
EtOH
55ºC
CH3
CH3CH2
C OCH2CH3
CH3
65%
H
CH3
C
+
H3C
CH2
+
C
CH3
30%
more stable
CH2 C
H3C
CH3
5%
less stable
E1 and SN1 mechanisms have a common rate determining steps
The E1 mechanism can be made predominant by:
a) use of a stronger base (but this may change the mechanism)
b) higher temperature (activation energies for elimination are higher than for
substitution: more bonds are being made/broken in the transition state)
c) use of a very poorly nucleophilic solvent
The E2 mechanism
EtOH
EtO
EtO
H
H
H
CH3
CH3
H
Br
H
H
CH3
CH3
H
H
CH3
CH3
Br
Br
H
Br
Note that this requires that
overlap.
have all atoms in the same plane, to allow orbital
Carbonyl reactions
Acetal formation
CH3
CH3
C O
H
H
H
O
H
CH3
But this can continue
CH3
H
OH
C OCH
3
CH3
C O
CH3
H
O
H
H
H
H
O
CH3
CH3
OH2
H
C OCH
3
CH3
OH
C O H
H
H
OCH3
C O
H3C
H
O
CH3
H3C
C OCH
3
O
CH3
H
H3C
OH2
CH3
CH3
CH3
H
H3C
O
CH3
H
OH
C O
H
H
OCH3
C O H
H3C
H
O
CH3
H
O
H
Acetal formation is only possible in acid; in base reaction proceeds only as far as the
hemiacetal.
CH3
CH3
C O
H
CH3
H
O
O
H
O
CH3
CH3
C O
H
H3C
OH
O
C O
CH3
H3C
Imine formation
O
O
R NH2
OH
R NH2
C
R NH
aldehyde or
ketone
carbinolamine
OH
R N
R NH
H2O
+
imine
(or Schiff base)
The loss of water is acid catalyzed:
H A
OH
R NH
OH2
A
R NH
+ H2O
A
H
Note that the N is the
more basic site, but
protonation on O leads to
reaction
N
N
R
an iminium ion
R
an imine
Ester Hydrolysis: acid
O
C
O
OH
OH
CH3
-H
C O CH3
OH H
H
OH
C O
OH
OH
OH
OH
OH
C
C
OH
C
OH
OH
H2SO4
C O CH3
OH
OH2
CH3
H
OCH3
C O CH3
OH
C
OH
OCH3
OCH3
C
C
-H
O
OH
Ester hydrolysis: base
O
H3C C
O
OH
OCH3
H3C
C OH
OCH3
O
O
H3C
H3C C
C OH
OCH3
OCH3
OH
O
H3C
C
O
OCH3
OH
H3C
C
HOCH3
O
Aromatic substitution mechanisms:
Electrophilic aromatic substitution
H
E
E
E
-H
Nucleophilic aromatic substitution
Lv
Lv
Nu
NO 2
Nu
Nu
-Lv
NO 2
NO 2