Chemistry Reference
In-Depth Information
Fig. 11.5
Correlation
diagram of molecular orbitals
for the cycloaddition of
ethene at butadiene
the conjugated double bonds of butadiene are easily polarizable, thus offering it
the capacity of being reduced with sodium or sodium amalgam as electron donors,
and alcohol, water or acids as proton donors;
the enhanced reactivity of the 1 and 4 positions with higher free valence is also
attested by the diene synthesis. Applying the Woodward-Hoffmann rule of conser-
vation of the orbital symmetry with respect to the symmetry plane, which passes
through the centre of the butadiene molecule between C
2
-C
3
, the correlation
diagram is obtained; see Fig.
11.5
(Woodward and Hoffmann
1970
, pp. 52-75).
In this cycloaddition (4
2), the binding levels of the reactants are correlated with
the binding levels of the product. There are no correlations intersecting high-energy
intervals between the bonding and antibonding levels. The ground state levels are
directly correlated with one another. The diagram shows that, from the symmetry
point of view, the thermal process should take place without activation energy. Con-
sequently, there is no barrier conditioned by symmetry, even though experimentally
there is found E
+
20 kcal mol
−
1
. The activation energy is not directly related to
the conservation of the orbital symmetry, but it is determined by rehybridization,
increase or decrease of the bond length, or deformation of the valence angles. There-
fore, the reaction of butadiene with ethene is slightly thermic, because there are no
=
