Nuclei autocatalytic

Several benzyl derivatives exhibit potentially hazardous properties arising from the activation by the adjacent phenyl group, either of the substituent or of a hydrogen atom. Halides, in particular, are prone to autocatalytic Friedel Crafts polymerisation if the aromatic nucleus is not deactivated by electron withdrawing substituents. 

The great instability of 2-bromomethyl- and 2-chloromethyl-furans, often manifest as violent or explosive decomposition on attempted distillation, is reviewed. The furan nucleus is sensitive to traces of halogen acids, and decomposition becomes autocatalytic. Individually indexed compounds are ... 

A series of studies described in the present chapter has been performed on the assumption that only high explosives are the chemicals of the true AC type. In this regard, however, it is very probable that there exist chemicals, other than high explosives, which decompose in accordance with the autocatalytic reaction mechanism. Besides, although it is quite certain that nitrate esters, such as nitroglycerine and nitrocellulose, decompose in accordance with the autocatalytic reaction mechanism, it is beyond the scope of the present document to discuss whether nucleus-substituted polynitro compounds, such as tetryl, TNT and picric acid, also decompose in accordance with the same mechanism or not. Some other possibilities are, therefore, also suggested herein for the decomposition reaction mechanisms of a few high explosives, respectively. 

Finally the oligomers form a crystalline nucleus and the reaction becomes now exothermic due to the added heat of crystallization during simultaneous or successive polymerization and crystallization. The crystals of polyoxymethylene observed are hexagonal with the polymer chain peuallel to the c-axis (2,82). They increase with time in lateral dimensions as well as in thickness, taking a p5mmidal shape 82). The addition of new oligomers on the lateral surface would explain an autocatalytic acceleration of the reaction with time as well as the pyramidal shape 82). 

Electron Transfer Mechanism. Thus, the first oxidation step of the hydroquinone occurs, provided the nuclearity is supercritical, n> nc, which means when the potential °(Ag, + i-Ag +i) becomes higher than the threshold °(Q "-QH2). Then the supercritical cluster acts as a nucleus for its own growth through an autocatalytic electron transfer (reactions 5,6) according to the mechanism that was summarized by reaction 12 ... 

If the concentration of is high, the reactions depicted by Eqs. (29)-(31) are faster than the coalescence reactions (Eqs. 10 and 11) with a fixed total concentration of atoms and the clusters now grow mostly by successive additions of supplementary reduced atoms (electron plus ion). It has been shown that once formed, a critical cluster, of silver for example,indeed behaves as a growth nucleus. Alternate reactions of electron transfer (Eqs. 32 and 34) and adsorption of surrounding metal ions (Eq. 33) make its redox potential more and more favorable to the transfer (Fig. 8), and autocatalytic growth is observed.The observation of an effective transfer therefore implies that the potential of the critical cluster is at least slightly more positive than that of the electron-donor system, i.e. °(M +/M ) > °(S/S-). 

The authors attribute the observed selectivity to an autocatalytic effect arising from the rapid production of secondary nuclei from a single primary nucleus in the stirred system. In essence, they propose that the first crystal to form will be either an L-crystal or o-crystal, each with 50% probability. When the solution is stirred, this ur-crystal (i.e., original crystal) serves as the template or the source for the formation of other crystals from solute brought to it by the stirring-induced transport in the supersaturated solution. In the absence of stirring, crystals form independently—there is no communication between crystals already formed and the rest of the solution, except for that provided by the relatively slow process of diffusion. 

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