Question

Q 3(b) [50 Marks]
Ozone is a 1,3-dipole, and ozonolysis (reaction of ozone and an alkene) has been
used extensively in identification of organic structure, prior to development of modern
spectroscopic techniques.
(i) Draw the 1,2- and 1,3- canonical forms of ozone.
(ii) Draw the molecular orbitals for a 1,3-dipole, and identify the HOMO and
LUMO of ozone.

Answer 1

(i) The two canonical form of ozone is shown below

:0: :0: 1,3-dipole 1,2-dipole :o :

(ii)

Each oxygen atom utilizes its 2s, 2pz and 2py orbitals to form three 2sp2 hybrid orbitals.

• O-1 utilizes one 2sp2 orbital which then overlaps with one 2p2 orbital of O-2, forming first sigma bonding and sigma antibonding orbital

• O-3 utilizes one 2sp2 orbital which overlaps with a 2nd 2sp2 orbital of O-2, forming second sigma bonding and sigma antibonding orbital

• Now two 2sp2 orbitals which is left on O-1, one 2sp2 orbital on O-2, and remaining two 2sp2 orbitals on O3 that are essentially non-bonding

Now the 2px orbital on O-1, 2px orbital on O-2, & 2px orbital on O-3 overlaps to generate three pi symmetry orbitals

The p orbitals which is in the image below indicate electron density which is present in the given orbitals.

• π1, bonding which is present along all the 3 atoms

• π2, non-bonding, which essentially has zero pi electron density available on the second atom

• π3, antibonding, which is nothing but the the mathematical sign of the ozone wavefunction that changes with every atom, represent repulsive interaction between them

888 8-8 200 - T3 12 O 2px 02 2px O' 2px al 000

The complete molecular orbital diagram is given below

O, Molecular Orbitals Oxygen Atomic Orbitals -12 O' 2p, O 2Px O' 2sp? 0° 2sp? O* 2sp? n.b. ENERGY·