its time in the vicinity of C1 and C3 ⇒ the two ends of allyl radical are
equivalent.
13.3B RESONANCE DESCRIPTION OF THE ALLYL RADICAL
- Resonance structures of the allyl radical:
A
C
C
C
H
H
H
H
H
C
C
C
H
H
H
H
H
B
C
C
C
H H
H H
H
1 2 3 1 2 3
⇒
C C C
H
HH
H^123 H
1/2 1/2
C
1) Only the electrons are moved but not the atomic nuclei.
i) In resonance theory, when two structures can be written for a chemical entity
that differ only in the positions of the electrons, the entity can not be
represented by either structure alone but is a hybrid of both.
ii) Structure C blends the features of both resonance structures A and B.
iii) The resonance theory gives the same structure of the allyl radical as in the MO
approach ⇒ the C–C bonds of the allyl radical are partial double bonds and the
unpaired electron is associated only with C1 and C3 atoms.
iv) Resonance structures A and B are equivalent ⇒ C1 and C3 are equivalent.
2) The resonance structure shown below is not a proper resonance structure
because resonance theory dictates that all resonance structures must have the
same number of unpaired electrons.
i) This structure indicates that an unpaired electron is associated with C2.
CH 2 CH CH 2 (an incorrect resonance structure)
- In resonance theory, when equivalent structures can be written for a chemical
species, the chemical species is much more stable than any resonance structure
(when taken alone) would indicate.
- Either A or B alone resembles a 1° radical.
