The Foundations of Chemistry

(Marcin) #1

Elemental analysis and determination of its molecular weight showed that the molec-
ular formula for benzene is C 6 H 6. The formula suggests that it is highly unsaturated. But
its properties are quite different from those of alkenes and alkynes.
The facts that only one monosubstitution product is obtained in many reactions and
that no addition products can be prepared show conclusively that benzene has a symmet-
rical ring structure.Stated differently, every H atom is equivalent to every other H atom,
and this is possible only in a symmetrical ring structure (a).


The debate over the structure and bonding in benzene raged for at least 30 years. In
1865, Friedrich Kekulé (1829–1896) suggested that the structure of benzene was inter-
mediate between two structures [part (b) of the preceding structures] that we now call
resonance structures. We often represent benzene as


All 12 atoms in a benzene molecule lie in a plane. This suggests sp^2 hybridization of
each carbon. The six sp^2 -hybridized C atoms lie in a plane, and the unhybridized porbitals
extend above and below the plane. Side-by-side overlap of the porbitals forms pi orbitals
(Figure 9-11). The electrons associated with the pi bonds are delocalizedover the entire
benzene ring (Figure 27-11a,b).


H

H

H

H

H

H

or, more simply The structure of benzene is described
in detail in Section 9-6 in terms of
MO theory.

27-5 Benzene 1061

C

C

C

C

C

C

H

H

H

H

H

H

(skeleton only)

(a)


C

C

C

C

C

C

H

H

H

H

H

H

C

C

C

C

C

C

H

H

H

H

H

H

(b)

A computer-generated model of a
molecule of benzene, C 6 H 6. A ball-
and-stick model is shown inside a
representation of the molecular
surface.

Figure 27-11 (a) A model of the benzene molecule, C 6 H 6 , and (b) its electron distribution.
(c) A model of toluene, C 6 H 5 CH 3. This is a derivative of benzene in which one H atom has
been replaced by an alkyl group.


(a) (c)

CH 3

(b)
Free download pdf