Radical stability

1
Radical stability
Stabilized by both electron WITHDRAWING and
DONATING groups
e.g.
.
All are stabilized compared to CH3
But remember
2
Electron withdrawing groups are electron
Withdrawing because they have a low energy empty
p
LUMO
p
SOMO
Single electron in SOMO of lower energy
3
e.g. AIBN fragment
CN is electron withdrawing so LUMO-SOMO
Interaction is relevant
NEW LUMO
SOMO
LUMO
NEW SOMO
4
Electron donating groups are electron donating
because they have electron pairs in higher
energy non-bonding orbitals
New SOMO
SOMO
n-orbital
Single electron in higher energy SOMO But lone
pair is now in a lower energy orbital
5
e.g. Methoxy substituents
OMe is electron donating so HOMO-SOMO
Interaction is relevant
NEW SOMO
HOMO
SOMO
HOMO
6
Electrophilic/nucleophilic radicals
Electron withdrawing groups are electron
Withdrawing because they have a low energy
empty p
p
SOMO
Single electron in SOMO of lower energy
So low energy SOMO tends to accept electrons
IE IT IS ELECTROPHILIC
7
Electron donating groups are electron donating
because they have electron pairs in higher
energy non-bonding orbitals
New SOMO
n-orbital
So high energy SOMO tends to release the electron
IE IT IS NUCLEOPHILIC
8
Electrophilic radical
Electron rich alkene
9
Ie the polymerization etc.leads to a perfectly
alternating structure VA-MA-VA-MA-VA-MA etc.
Why? VA radical is nucleophilic adds to the
electron deficient MA in pref. to VA .
Electrophilic MA radical then adds to electron
rich VA monomer.
10
The molecular orbitals
Electron rich-high energy SOMO on VA perturbs
LUMO on MA
SOMO next to Electron donating grp.
LUMO
HOMO
MA
VA rad.
New SOMO Nxt to on MA-at lower energy
CO
11
Electron deficient-low energy SOMO on MA perturbs
HOMO on VA
SOMO next to Electron withdrawing grp.
New SOMO-high energy-electron rich
LUMO
HOMO
VA
MA rad.
12
The MOs
VA radical SOMO
VA LUMO
MA radical SOMO
MA LUMO