A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

Me-CH—CH—CH—CH 2 —>• Me-CJ|^CH—CgpgH*

(I)

CHr-CH—CHj-CH—CHS -*• CHs-CH— CHs-CH—CH 8

In either case, overlapping of the p atomic orbitals on adjacent carbon

atoms can lead to the formation of two localised n bonds as shown,

and the compounds would be expected to behave like ethylene, only

twice as it were! This adequately represents the observed behaviour

of (II) but not of the conjugated compound (I). On looking more

closely at (I), however, it is realised that interaction is also possible

between the p atomic orbitals of titk two centre carbon atoms ojNhe

conjugated system, as well as between each of these and the p orbitals

on the outside carbon atoms of the sjetem. An alternative formula­

tion is thus a 7r orbital covering all four carbon atoms (III)

Me - CH—CH—CH—CH. CH^CH—CH=CHS

(ni)

(rv)

CHf-CH-CH—CH, ,„%

^^^^gssaa> (V) :

in which the electrons are said to be delocalised as they are now spread
over, and are held in common by, the whole of the conjugated
system. There will, of course, need to be two such delocalised orbitals
as no orbital can contain more than two electrons and four electrons
are here involved. The result is a region of negative charge above and
below the plane containing all the atoms in the molecule.
The better description that this view affords of the properties of
conjugated dienes including the possibility of adding, for example,
bromine to the ends of the system (1:4-addition) rather than merely
to one of a pair of double bonds (l':2-addition) is discussed below
(p. 150).