Addition to Conjugated Dienesthe actual addition of ozone is usually followed by reductive cleavage
of the products with Pd/Ha. This ensures that the carbonyl com
pounds, especially aldehydes, do not undergo further oxidation as
tends to happen on simple hydrolytic cleavage due to the hydro
peroxides (cf. p. 252) that are then formed. This is important as one
of the advantages of ozonolysis as a preparative or a diagnostic
method is the ease of isolation and characterisation of the carbonyl
compounds that it yields as end-products.
The presence of delocalised n orbitals in conjugated dienes, and their
effect in transmitting reactivity over the whole of the system, has
already been referred to (p. 8). Conjugated dienes are somewhat more
stable than otherwise similar dienes in which the double bonds are not
conjugated, as is revealed by a study of their respective heats of
hydrogenation (cf. p. 11), the delocalisation energy consequent on the
extended ir orbital system probably being of the order of 6 kcal/mole.
Conjugated dienes tend nevertheless to undergo addition reactions
somewhat more readily than non-conjugated dienes because the
transition state in such reactions, whether the addition is proceeding
by a polar or a radical mechanism, is allylic in^ifure and thus more
readily formed (cf. pp. 83,234) than that from arffsolated douMBVond:
ADDITION TO CONJUGATED DIENES
X©CH 2 =CH 2X