Advices For Studying Organic Chemistry

Figure 13.3 The π molecular orbitals of the allyl cation. The allyl cation, like the
allyl radical, is a conjugated unsaturated system. The shapes of
molecular orbitals for the allyl cation calculated using quantum
mechanical principles are shown alongside the schematic orbitals.

1. The bonding π molecular orbital of the allyl cation contains two spin-paired
   electrons.


2. The nonbonding π molecular orbital of the allyl cation is empty.


3. Removal of an electron from an allyl radical gives the allyl cation ⇒ the electron
   is removed from the nonbonding π molecular orbital.
   CH 2 CHCH 2 CH 2 CHCH 2 +
   i) Removal of an electron from a nonbonding orbital requires less energy than
   removal of an electron from a bonding orbital.
   ii) The positive charge on the allyl cation is effectively delocalized between C1
   and C3.
   iii) The ease of removal of a nonbonding electron and the delocalization of
   charge account for the unusual stability of the allyl cation in MO theory.

2. Resonance theory depicts the allyl cation as a hybrid of structures D and E:

D

C

C

C

H

H

H

H

H

C

C

C

H

H

H

H

H

E

(^123123)
++ 1/2++C C1/2
C
H
HH
H 1 2 3 H
F

1. D and E are equivalent resonance structures ⇒ the allyl cation should be
   unusually stable.


2. The positive charge is located on C3 in D and on C1 in E ⇒ the positive charge
   is delocalized over both carbon atoms and C2 carries none of the positive charge.


3. The hybrid structure F includes charge and bond features of both D and E.