Picric acid, molecular structure

Picric acid, synthesis of, 628 Pinacol rearrangement, 646 Pineapple, esters in, 808 Piperidine, molecular model of, 939 structure of. 918 P1TC, see Phenylisothiocyanate, 1031-1032... 

In the dihydrochloride (602), the second molecule of acid is not firmly bound because of protonation of the second basic site by internal hydrogen bond formation. This would suggest that in the dipicrate, one mole of picric acid participates in salt formation, and that the second mole of picric acid is added to the hydrogen-bonded cation (LXI) forming a molecular complex. The existence of a monopicrate might thus serve to point out structures with steric inhibition of hydrogen bonding. 

These dyes are now of only minor commercial importance, but are of interest for their small molecular structures. The early nitro dyes were acid dyes used for dyeing natural animal fibers such as wool and silk. They are nitro derivatives of phenols, e.g., picric acid (4) or naphthols, e.g., C.I. Acid Yellow 1, 10316 [846-70-8] (5). 

Fig. 1.3 Molecular structures of picric acid (PA), tetryl, trinitrotoluene (TNT), Nitroguanidine (NQ), pentaerythritol tetranitrate (PETN), hexogen (RDX), octogen (HMX), hexanitrostilbene (HNS) and triaminotrinitrobenzene (TATB).

The crystal and molecular structure was determined of the picrates (174) of aniline (la), its A-Me derivatives, 1,2- (lj), 1,3- (lk) and 1,4-phenylenediamine (11). Except for lj, combining with two picric acid molecules, the rest show 1 1 stoichiometry. The main structural feature is H-bonding of various strengths, with O-N distances from 267 pm for A-methylaniline to 288 pm for lj. The acid proton in these compounds sits on the N atom forming ammonium salts, as shown by the Cl-O bond distances near 125 pm for a picrate anion, rather than 130 to 134 pm for picric acid or its tt-complexes with benzene and PAH. The picrates of N-methylaniline and lk also show jr-stacking to some extent, especially in the latter case266. 

Ammonium picrate, CjH2(N02)30NH4, is a high explosive when dry and flammable when wet. It is composed of yellow crystals that are slightly soluble in water. The four-digit UN identification number is 1310 for ammonium picrate wetted with not less than 10% water. It is used in pyrotechnics and other explosive compounds. The structure and molecular formula for ammonium picrate are shown in Figure 3.18. Notice the similarity to picric acid and TNT. You may notice that the structure and formula of ammonium picrate do not follow the usual rules of bonding. 

Trinitrophenol, also known as picric acid, is composed of yellow crystals and is a nitro hydrocarbon derivative. It is shipped with not less than 10% water as a wetted explosive. There is a severe explosion risk when shocked or heated to 572°F, and it reacts with metals or metallic salts. In addition to being flammable and explosive, it is toxic by skin absorption. Picric acid has caused disposal problems in school and other chemistry laboratories where the moisture has evaporated from the container as the material ages. When the picric acid dries out, it becomes a high explosive closely related to TNT. Picric acid has been found in various amounts in school labs across the country. In a dry condition, picric acid is dangerous and should be handled by the bomb squad. The structure and molecular formula for picric acid are shown in Figure 6.3. 

TNT and picric acid have been replaced gradually by tetryl, RDX, and nitroglycerine (Section 9-11). On the research front, chemists are continuing to explore novel structures. A case in point is octanitrocubane, synthesized in 2000, in which ring strain adds to the brisance of the compound. Its molecular formula, CgNgOig, indicates the potential to generate 8 CO2 -f 4 N2 molecules, with an associated 1150-fold volume expansion. A recent example from 2012 is the exotic salt TKX-50, in which two lone carbon atoms are surrounded by 10 nitrogens and the equivalent of 4 H2O moieties. 

The crystal structures of 13 co-crystals formed by -alkylcarboxylic acids and pyrazine in 2 1 stoichiometry have been reported [85,86]. The acids include formic acid, which is liquid at room temperature, up to the tridecandioic acid which is solid. Pyrazine is also a solid. Clearly, the early members of this series cannot be called solvates only because the co-former is liquid, while from decanoic to tridecandioic the acid/pyrazine systems are co-crystals. Another example is the two-component crystals formed by picric acid forms and benzene, naphthalene and anthracene (amongst others), which contain stacks of alternating molecular components that are clearly comparable in all three structures [87-89]. All of these crystals are stable at room temperature. 

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