Perchlorate, esters

Several trialkyl- or triaryl-silyl perchlorates explode on heating [1], A syringe used to inject a sample of the title perchlorate ester into a reaction flask exploded soon after addition was complete [2],... 

Contact of cellulose acetate with 1200 1 of uncooled anhydrous acid in acetic anhydride led to a particularly disastrous explosion [1], and interaction of benzyl cellulose with boiling 72% acid was also explosive [3], Perchlorate esters of cellulose may have been involved in all these incidents. 

Dining maintenance work on casings of fans used to extract perchloric acid fumes, seven violent explosions occurred when flanges sealed with lead oxide-glycerol cement were disturbed. The explosions, attributed to formation of explosive compounds by interaction of the cement with perchloric acid, may have involved perchlorate esters and/or lead salts. Use of an alternative inorganic silicate-hexafluorosilicate cement is recommended. 

Glycols and their ethers undergo violent decomposition in contact with 70% perchloric acid. This seems likely to involve formation of the glycol perchlorate esters (after scission of ethers) which are explosive, those of ethylene glycol... 

Electrophilic additions to olefins, especially of XC1 species, can give substantial quantities of perchlorate esters as by-products if performed in the presence of lithium perchlorate. These esters are extremely explosive, and may concentrate during distillation of crude products. 

The explosively unstable behaviour of stored nitronium perchlorate is attributed to the formation of small equilibrium concentrations of the isomeric covalent nitryl perchlorate ester (below). 

This very thorough study led the authors to the conclusion that propagation by unpaired and paired ions and by perchlorate ester can co-exist, and that their relative importance is affected by all the reaction parameters, including the ratio w/[HA]0 (where HA = HC104), which of course falls as a reaction progresses. The reaction patterns were all complicated and sensitive to conditions, but somewhat simpler at -60 °C and -80 °C, at which temperatures the pseudo-cationic contribution is not important. Pepper paid particular attention to the contribution from non-ionic propagation. The rate, Ru when [P ] = [P +] + [P +A ] is at its maximum [P ]M, was given by... 

A further advantage of our treatment of the styrene results is that we can obtain a first-ever estimate of the minimum value of kpE for the poly(styryl)triflate ester of ca. 750 TmoH-s"1 (from kpE = RE/m-[E], assuming that [E] = [HA]J, which is ca. 100 times as great as the kpu for poly(styryl)perchlorate ester. 

One obvious test of the F-Cat theory was to try to synthesise the perchlorate ester of styrene and to find out whether it will act as a polymerisation initiator however, the ester is stable only in the presence of an excess of styrene. This proved to be the first instance of the stabilisation of a hyperactive ester by an electron donor to give a species which is sufficiently long-lived to be an effective propagator (C). 

Inevitably, most of the papers dealing with the F-Cat polymerisations contain some reference to the alleged solvation of the polystyryl perchlorate ester by four styrene molecules. However this feature must now be ignored, but its omission does not affect any of the arguments. A full explanation of this error has been submitted to the Journal of Polymer Science. 

Detailed studies led Gandini and Plesch to formulate the concept of pseudocationic polymerisations. These are reactions which show many of the characteristics of cationic polymerisations, but do not involve ions. Since they could see no other alternative compatible with general chemical knowledge, they formulated the reactive species as an ester, and they were able to support this view by direct experiments (formation of the ester in the styrene solution by metathesis). The evidence indicates that in the system styrene, perchloric acid, methylene dichloride, the poly(styryl perchlorate) ester requires four molecules of styrene for its stabilisation. When these are no longer available, the ester ionises, and the residual styrene is consumed by a very fast, truly cationic polymerisation ionisation of the ester is a complicated reaction which has been only partly elucidated. The initiation and propagation of the pseudocationic polymerisation can be represented thus ... 

The following discussion is based on our earlier conclusion that the propagating species in these polymerisations is oligostyryl perchlorate ester, which is stabilised in solution by excess styrene. One question arising concerns the number of molecules of styrene per ester molecule required for this stabilisation another concerns the kinetics and mechanism of the ionogenic reactions which ensue once the styrene concentration has been reduced by polymerisation to such a low level that the quantity of styrene no longer suffices to stabilise the ester. 

We conclude that the polymerisation of styrene by perchloric acid in methylene dichloride at 0 °C involves at least two types of independent propagating species. Rate studies and conductivity measurements, in conjunction with an independent study of molecular weight distributions [26, 27], indicate that these species include the perchlorate ester and the free polystyryl carbenium ion and that the term pseudocationic is a very appropriate description of what is a distinctive form of organic reaction. 

Methyl, ethyl and propyl perchlorates, readily formed from the alcohol and anhydrous perchloric acid, are highly explosive oils, sensitive to shock, heat and friction [1], Many of the explosions which have occurred on contact of hydrox-ylic compounds with cone, perchloric acid or anhydrous metal perchlorates are attributable to the formation and decomposition of perchlorate esters [2,3,4], Safe procedures for preparation of solutions of 14 sec-alkyl perchlorates are described. Heated evaporation of solvent caused explosions in all cases [5], l-Chloro-2-propyl, iram-2-chlorocyclohexyl, l-chloro-2-propyl, 1,6-hexanediyl, hexyl, and 2-propyl perchlorates, prepared by a new method, are all explosive oils [6],... 

Existing knowledge on perchloric acid and its salts was reviewed extensively in 1960 in a monograph including the chapters Perchloric Acid Alkali Metal, Ammonium and Alkaline Earth Perchlorates Miscellaneous Perchlorates Manufacture of Perchloric Acid and Perchlorates Analytical Chemistry of Perchlorates Perchlorates in Explosives and Propellants Miscellaneous Uses of Perchlorates Safety Considerations in Handling Perchlorates [1], There is a shorter earlier review, with a detailed treatment of the potentially catastrophic acetic anhydride-acetic acid-perchloric acid system. The violently explosive properties of methyl, ethyl and lower alkyl perchlorate esters, and the likelihood of their formation in alcohol-perchloric acid systems, are stressed. The instability of diazonium perchlorates, some when damp, is discussed [2],... 

A mixture of an inorganic perchlorate salt, a glycol and a polymer exploded violently after heating at 265-270°C. It was thought that the glycol may have been oxidised, but formation of the glycol perchlorate ester(s) seems a likely alternative cause. 

One interesting aspect of the polymerization of styrene is its possible pseudoca-tionic propagation. It was observed, for example, that in the presence of HC104 styrene forms only oligomers and no real cationic species could be found.160 161 This led to the suggestion that the initiating species is a perchlorate ester stabilized by monomer molecules ... 

Bottles of HC104 should not be stored on wooden shelves, because acid spilled on wood can form explosive cellulose perchlorate esters. Perchloric acid also should not be stored near organic reagents or reducing agents. A reviewer of this book once wrote, I have seen someone substitute perchloric acid for sulfuric acid in a Jones reductor experiment with spectacular results—no explosion but the tube melted ... 

A related pseudo-cationic propagation via a covalent perchlorate ester has bee proposed for the polymerisation of styrene by perchloric acid (135), though the situation in this system is extremely complex (136). 

Group 14 (IVA) Perchlorates. Perchlorates containing organic carbon have been reported, as have diazomum perchlorates, oxonium perchlorates, and the perchlorate esters. Extreme caution must be used in working with organic perchlorates many decompose violently when healed, contacted with other reagents, or subjected to mechanical shock. 

Contact of the hot concentrated acid or the cold anhydrous acid with cellulose (as paper, wood fibre or sawdust, etc.) is very dangerous and may cause a violent explosion. Many fires have been caused by long-term contact of diluted acid with wood (especially in fume cupboards) with subsequent evaporation and ignition [1,2]. Contact of cellulose acetate with 1200 1 of uncooled anhydrous acid in acetic anhydride led to a particularly disastrous explosion [1], and interaction of benzyl cellulose with boiling 72% acid was also explosive [3], Perchlorate esters of cellulose may have been involved in all these incidents. 

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