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Solid ap-unsaturated acids. amides. and ketones yield, on exposure to bromine vapour a t room temperature, the ' trans-adduct ' in quantitative or near-quantitative yield even where addition of bromine in solution has been reported to be difficult. Polymorphic forms of several compounds differ in their rates of bromine uptake and addition.That crystal texture plays a role is suggested by the fixation of bromine in compounds which in the pure state add bromine slowly but which do so more rapidly in the presence of small amounts of impurities (e.g. photo-dimer or saturated derivatives).PREVIOUS publications concerned with solid-state reactions have stressed the high degree of topochemical control in photochemical pr0cesses.l We now explore the range and limitation of the topochemical concept in these two-phase, two-component reactions. Schmitt showed in 1863 that solid cinnamic acid reacts with bromine vapour to give cinnamic acid dibromide [re-this type of reaction have since a~peared."~ In view of the experimental simplicity of the reaction it was of interest to establish its general applicability; we also hoped to find examples suitable for more detailed analysis of the various processes involved in reaction (l), such as diffusion of bromine into the lattice, formation of charge-transfer complexes,* &.

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