Role of molecular packing and structural defects in reactions of gases with organic solids: ozonolysis of trans-stilbene and αβ-diethyl-4,4′-dihydroxystilbene

Abstract

Optical microscopic examination of partially ozonolysed and wet-etched single-crystal faces of trans-stilbene (1a) and αβ-diethyl-4,4′-dihydroxystilbene-(1b) reveal the importance of line defects (dislocations) in governing the rate of the (anisotropic) oxidative attack of the ethylenic double bond in these two solids. In the regions of enhanced reactivity at dislocations emergent on the (100) face of (1b) the rate of ozonolysis is considerably slower than at corresponding regions of the (001) face of (1a), a direct consequence of the protection afforded to the double bond in molecules of the former. As reaction product [benzaldehyde and p-hydroxypropiophenone for (1a and b) respectively] accumulates within each solid reactant, stresses are set up which generate more dislocations on well-defined slip planes (which are identified); these, in turn, function as new centres of preferred attack so that the reaction accelerates as ozonolysis proceeds. Very many new slip systems in crystals of (1a), and relatively few new ones in crystals of (1b), are activated by the accumulation of reaction product. This is explicable in terms of the crystallographic differences between the two solids. It is directly demonstrated that numerous new families of dislocations may be introduced during the course of a gas–solid reaction; and many of the slip systems identified here for (1a) have not hitherto been detected in other aromatic solids (such as naphthalene and anthracene) which also belong to the space group P2₁/a.