Sublimation and Decomposition Reactions

The thermal decomposition of a solid, which necessarily (on the above definition) incorporates a chemical step, is sometimes associated with the physical transformations to which passing reference was made above melting, sublimation, and recrystallization. Aspects of the relationships between physical transitions and decomposition reactions of solids are discussed in a book by Budnikov and Ginstling [1]. Since, in general, phase changes exert significant influence upon concurrent or subsequent chemical processes, it is appropriate to preface the main survey of the latter phenomena with a brief account of those features of melting, sublimation, and recrystallization which are relevant to the consideration of thermal decomposition reactions. 

The UV-absorption spectrum has been recorded by a number of investigators (152, 153, 175, 176), and the infrared spectrum has also been determined (177). The free base and its salts fluoresce in ultraviolet light (157, 158), and this property has been recommended as an assay method in biological material (179), but it has been indicated that this fluorescence is due to decomposition products of the alkaloid (180, 181). There is extensive literature on the identification and quantitative determination of hydrastine (118, 120, 121, 166, 167, 182-197), the methods depending upon sublimation and color reactions for identification and titrimetry for assay. 

Hexamethylenetetramine. Pure hexamethylenetetramine [100-97-0] (also called hexamine and HMTA) is a colorless, odorless, crystalline sohd of adamantane-like stmcture (141). It sublimes with decomposition at >200° C but does not melt. Its solubiUty in water varies Htde with temperature, and at 25°C it is 46.5% in the saturated solution. It is a weak monobase aqueous solutions are in the pH 8—8.5 range (142). Hexamethylenetetramine is readily prepared by treating aqueous formaldehyde with ammonia followed by evaporation and crystallisation of the soHd product. The reaction is fast and essentially quantitative (142). 

Conceptually, the simplest pair to discuss is that of p-benzoquinone oxime and 4-nitrosophenol and their substituted derivatives, here all denoted by p-OCgH4=NOH/4-HOC6H4NO . In practice, this comparison is complicated by the fact that this species cannot be sublimed without decomposition and so we are prevented from analyzing the energetics of gas phase isodesmic reactions such as equations 38 and 39. 

The desorption peak of 16 amu around 200 K is due to methane, which is produced in the titanium sublimation pump during methanol exposure, and adsorbed on the sample holder. The main desorption products are H2 around 400 K and CO above 800 K. The H2 desorption peak is much larger than that observed upon the H2 adsorption up to saturation on the clean surface. The H2 desorption peaks seem to consist of three components two relatively small components at 350-400 K and around 500 K and a sharp peak at 410 K. For the HD desorption trace, only the peak at 350-400 K is seen. These results suggests that the methanol decomposition reaction on the clean Mo(l 12) surface proceeds as follows. 

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