Molecules that are photostable in the solid state can be rendered photoactive if, by burial within crystals of closely analogous monomers, they can be made to mimic the molecular conformation of the latter; this fact is demonstrated for a pair of cyclopentanone derivatives, and the general significance of this result in solid-state synthesis is discussed.

The effect of exposure of Wyoming bentonite to glacial acetic acid has been studied by F.t.i.r., X.p.s, SEM and acidity measurements, in order to investigate the possibility of using such a non-aqueous technique as an alternative activation process. Treatment with glacial acetic acid, results in an overall increase in the concentration of acid sites. However, the number of Brønsted sites is reduced, whereas the population of Lewis sites increases. This change is accompanied by the leaching out of Al³⁺, but with no extensive loss of crystallinity. Treatment with mineral acids results in an increase in Brønsted acidity.

When 2,5-dibenzylidenecyclopentanone (DBCP)is irradiated by u.v. light in the crystalline state the principal product formed by a [2 + 2]dimerisation is 2,9-dibenzylidene-6, 12-diphenyldispiro[4.1.4.1]-dodecene-1,8-dione (DDBCP); a subsidiary dimeric species is formed in substantial amounts at low conversion but is consumed during the course of further reaction and is absent when the total conversion approaches 90%. Crystalline (+)-2,5-dibenzylidene-3-methylcyclopentanone [DB(+)3MeCP] under the same condition is photostable. The contrasting photoreactivity of these two solids is interpreted in terms of the mutual disposition of potentially reactive olefinnic bonds on neighbouring molecules. Even when such bonds are not parallel (and for DBCP they are inclined at an angle of 56°) reaction may ensue since the distances separating the carbon atoms are approximately 3.7 Å. It is argued that the photostability of DB(+)3MeCP, where the separation distances of the potentially reactive olefinic bonds on adjacent molecules are comparable to those in DBCP and where the bonds are again not parallel, is attributable to the non-coplanarity of the two π-bonds.