very fourth of July, chemists once again turn their imaginations to thermal decomposition, reactions with large negative delta H values, and the kinetics of rapidly-expanding gas-phase products. This holiday, it seems, was created with chemists in mind. Indeed, many chemists celebrate the Fourth of July periodically throughout the year in small ways in their laboratories. Therefore, it's particularly appropriate to recommend as some fascinating holiday reading this English translation of a Russian monograph on the topic of thermal decomposition, written by researchers at the Semenov Institute of Chemical Physics in the former USSR, where scientists for decades studied the chemical kinetics and chemical physics of high-energy compounds. These molecules are thermodynamically unstable and participate in complex series of elimination, rearrangement, and monomolecular decay reactions.
Although these decompositions are too rapid to allow bimolecular reactions to proceed, most of the reactions are reversible, and the rate constants of rapid homolytic reactions observed under high-vacuum pyrolysis will be greatly underestimated if this reversibility was not taken into account. The main competition for the reverse reaction, say the authors, is disproportionation. Relatively small differences in measured rates caused by factors such as termination of the reaction by the chamber wall can lead researchers to make erroneous conclusions about the mechanism.
Another critical point emphasized by the authors is the role of acid-base autocatalytic processes. The decomposition of difluoroamines, for example, is catalyzed by HF, and nitric oxides formed during the reaction, despite being free radical inhibitors, are efficient oxidizers. These and other intermediate products act as catalysts that can greatly accelerate the reaction.
Although oxidizers can act as catalysts, many nonspecialists don't realize that, in general, these reactions are not oxidations. In fact, oxygen is actually created by the decomposition of many compounds, including HNO3 and HClO4. (Another good example is TATP, an organic peroxide which rapidly decomposes into ozone, oxygen, acetone, and CO2.) Even in such nominally ionic compounds, the counterion often participates in a free radical reaction rather than an ionic one. The first step in NaN3 decomposition, for example, is NaN3 → Na· + N3· and not NaN3 → Na+ + N3-, as would occur in aqueous solution. According to the authors, combination of the sodium radicals to form sodium metal accounts for an abrupt decrease in the activation energy that greatly accelerates the decomposition.
That doesn't mean ionic interactions are unimportant. Careful experiments showed that the rate of decomposition of dinitramide by nitric acid in anhydrous acetic acid was increased by the pH-dependent formation of an unstable mixed anhydride. Similarly, say the authors, formation of an anhydride explains the higher rate of reaction of perchlorate in liquid than in the vapor phase.
The emphasis in most chapters is on factors that influence reaction rates. There are numerous equations, graphs, tables, and chemical structures. The chapters are not a collection of random articles on overlapping topics written by random scientists who will most likely never even read each other's article, as is so often the case, but are written collaboratively by the authors and are well-integrated into the book's theme. Topics include aliphatic nitro compounds, aromatics, difluoroamines, and heterocyclics. The discussion focuses almost exclusively on decomposition and combustion of well-characterized organonitrates and inorganic nitrates like dinitramide, nitrobenzenes and RDX, using them as model compounds to uncover general scientific principles. Synthetic routes, C-O and O-O compounds, and exotic inorganics are not covered. There are also large chapters on structurally simple inorganics, focusing principally on the commercially-important nitrate and perchlorate.