We can use bonding theories to
explain reactions and reactivity
• Lewis structures allow us to “push electrons” but
explain little
• Valence bond theory allows us to use Lewis
structures in a natural way, bringing in
“resonance,” “hybridization” and “orbital
overlap.”
• Molecular orbital theory allows us to explain
how bonds are formed and broken by donation of
electrons from a HOMO to a LUMO
The nucleophile
attacks the alkyl
halide 180o away
from the halogen
H 3C
HO
Transition state for the SN2 reaction can
be explained with valence-bond theory
δ−
HO
The configuration
at carbon is inverted
CH3
CH3
HO C H
CH2CH3
δ−
C
Br
H CH2CH3
+
Br
(S)-2-butanol
Transition state with
simultaneous bond breaking
and bond forming
H
N C
H
C Cl
H
H
N C C H
H
Cl
Frontier molecular orbitals in the
SN2 reaction
H
H
C
H C Br
CH3CH2
(R)-2-bromobutane
• Lewis/Valence-bond theory does not allow
an explanation but does allow us to “push
the electrons” to provide a convenient
picture.
• “Frontier MO theory” allows us to explain
this in terms of the donation of electrons
from the HOMO of the nucleophile to the
LUMO of the alkyl halide electrophile.
N C
+
Molecular orbitals: Lewis picture
of the SN2 reaction
How do we explain this?
• Bond to chlorine
breaks as the bond to
cyanide forms.
• Notice that negative
charge is spread
“evenly” between the
nucleophile and the
leaving group.
• But why must attack
be from the backside?
The SN2 Mechanism
N C
Cl
H
C Cl
H
H H
HOMO of cyanide
interacts with the
LUMO of chloromethane
1
Molecular orbitals are required to
explain backside attack!
H3C
HO
+
H
Br
H
H
δ−
HO
H
H
The E2 Mechanism
(R)-2-bromobutane
C Cl
H 3C
N C
The proton and the
leaving group must
be anti to each other.
H CH3
H
N C
H
C Cl
H
H
δ−
Br
H3C
HOH
H CH3
+
H
H
CH3
trans-2-butene
+
Br
Transition state with
simultaneous bond breaking
and bond forming
H
Geometry of E2 elimination
“anti-periplanar”
Geometry of E2 elimination
H
H C
H
H
H
C H
X
H
H
H C C
H
X
H C C H
H
H
H C C H
H
H
Geometry of E2 elimination:
Frontier Molecular Orbitals
LUMO of
chloroethane
HOMO of
hydroxide
2