Engineering Chemistry S Datta

VALENCY AND CHEMICAL BONDING 33
[Energy level diagram for heteronuclear diatomic molecule]
Table 2.5: Electron Distribution in Molecular Orbitals of some
Simple Molecules shown in above M.O. diagrams
Molecule σ(sp) 1
σ*(sp) 1
π(2p) σ(sp) 2
π*(2p) σ*(sp) 2
unpaired
electrons
B 2
↑↓ ↑↓ ↑ ↓ 2
C 2
↑↓ ↑↓ ↑↓ ↑↓ 0
N 2
↑↓ ↑↓ ↑↓ ↑↓ ↑↓ 0
O 2
↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑ ↑ 2
CO ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ 0
NO ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑ 1
Aromatic Character
Organic compounds have been classified into two broad subdivisions: (i) aliphatic and
(ii) aromatic.
The word aliphatic came from ‘fatty’ and the word aromatic came from ‘fragrant’, have
no longer their significance. Aliphatic compounds are mainly open chain compounds. They
may be cyclic also but they resemble the open chain compounds in their properties. Aromatic
compounds a2e benzene or the compounds that resemble benzene in chemical behaviour.
Kekule established the structure of benzene as follows:
or
too:
According to the Kekule’s structure there should be four isomeric di-substituted benzenes,
Br
Br
Br
Br
Br
Br
Br
Br
;
;
Br
Br
Kekule’s structure fails to explain certain characteristics of benzene viz:
(i) Benzene undergoes substitution reaction rather than addition.
(ii) Heat of hydrogenation of benzene is lower than that expected.
(iii) The C—C bond lengths in benzene are equal and intermediate between a single and
a double bond.
The idea of resonance could explain all the three above anomalies and established the
structure of benzene as put forward by Kekule.
Br
Br