Photochemistry - Green Chemistry and Technology - page 417

Fig. 1 8 .1 Three examples of industrial
photoprocesses in chemicals
manufacturing. (Data from Refs 4
and 5.)

light source in use. Therefore, all the energy supplied
to the reaction system goes into the reacting
molecules. Photoreactions often can be carried out
without heating the reaction mixture and the reac-
tion vessel, thus saving energy and reducing thermal
decomposition of the products.

reactant. Familiar examples are: 2p+2p cycloaddi-
tions, e.g. the key construction of a strained cyclobu-
tane ring in the synthesis of cubane [6] (Fig. 18.2);
reactions involving singlet oxygen, e.g. the synthesis
of ascaridole from a-terpinene [7,8] (Fig. 18.3); and
the di-p-methane reaction—a method for producing
cyclopropane derivatives—an example of which [9]
is given in Fig. 18.4. In all these cases the photo-
chemical products are not generated at all if the
reaction mixtures are merely heated. In these and
numerous other examples of the photochemical syn-

1.3 Control of selectivity

It is well known that certain synthetic reactions can
be accomplished only by electronic excitation of a