Picryl bromide

Tetrabromomethane shows two types of n-bonding with aromatic donors that are contrasted in Fig. 8a and b, showing over-the-rim coordination to the aromatic C-C bond and over-the-center coordination to the benzene ring. The over-the-rim coordination is generally similar to that observed in the dibromine complexes but the C - Br distance in the former is longer, in agreement with weaker acceptor abilities of tetrabromomethane. Note that picryl bromide shows similar bromine coordination to the outer pyrene C-C bond with Br - C distances of 3.35 and 3.39 A [83]. A second type of coordination was reported earlier in ihc p-xylcnc complex [80] and recently in the associate with dimethylnaphtalene [53]. 

Picryl bromide can be coupled at or below room temperatures by copper powder used in only 10 % excess, to give coupling or reduction products depending on the solvent and stoichiometric conditions (Eq. 3.17) [105]. Sonications were performed with a pulsed wave, a technique not frequently used. 

Orlova and co-workers "i reacted 1,2,3,5-tetranitrobenzene with hydrochloric and hydro-bromic acids to form picryl chloride and picryl bromide respectively. The same chemists treated 2,3,4,6-tetranitroaniline and 2,3,4,6-tetranitrophenol (121) with aqueous solutions of hydrogen halides to form 3-halo-2,4,6-trinitroanilines and 3-halo-2,4,6-trinitrophenols (122 and 123) respectively. 

Nelson, K. A. Adolph, H. G. Ullmann reaction of picryl bromide in the presence of ultrasound. Synth. Commun. 1991, 21, 293-305. 

The decrease of the metal particle size is insufficient to explain this effect, and the ultrasonic desorption of the products from the surface was also invoked, a case of after-reaction cleaning. Picryl bromide, despite the steric hindrance, undergoes coupling in 15 min at room temperature in nitrobenzene with copper powder in only 10% excess (Eq. 45). 

C22H11NO6S, Phenothiazine pyromellitic dianhydride complex, 46B, 609 C22H12BrN30g, Fluoranthene - picryl bromide (polymorph I), 41B, 687 C22H12N4, 7,7,8,8-Tetracyanoquinodimethane - naphthalene (1 1), 42B, 493... 

The coupling of aryl halides with copper is called the Ullmann reaction.m The reaction is of broad scope and has been used to prepare many symmetrical and unsymmetrical biaryls.187 When a mixture of two different aryl halides is used, there are three possible products, but often only one is obtained. For example, picryl chloride and iodobenzene gave only 2,4,6-trinitrobiphenyl.188 The best leaving group is iodo, and the reaction is most often done on aryl iodides, but bromides, chlorides, and even thiocyanates have been used. 

In the presence of equivalent K hydroxide, TNT reacts as follows with various halides, such as 4-nitrobenzyl bromide, picryl chloride, or 2,4-dinitrofluorobenzene (Ref 61) ... 

N[C6H2(N02)3].C0 mw 660.42, N 16,97% citron-yel ndls (from phenol+acet ac), mp >200°(dec) sol in hot phenol boiling ethylene bromide v si sol in most other solvs was prep>d by reaction of pyridine N-picryl-anthranilic chloride at 100° (Refs 2 3)... 

Dinitro-chlorobenzene 2,4,6-Trinitro-chlorobenzene (picryl chloride) Vinyl chloride Vinyl bromide... 

Thiazolo[3,2-6]isoquinolines.—[CaNS-CsN-Ce]. In continuation of their investigation of cyclizations involving the loss of nitrous acid, Krbhnke et alf have described a synthesis of thiazolo[3,2-Z>]isoquinolines. 2-Methyl-3-phenacylbenzothiazolium bromide (315 R = Ph) is found to react with picryl chloride to form (316). In common with picryl-substituted ylides, this is cyclizable, involving its nitroaryl and A -methylene groups, to yield... 

The order of reactivity of picryl halides is the reverse of that observed in the reactions of the corresponding alkyl halides.1 Alkyl fluorides react only slowly with nucleophiles whereas picryl fluoride reacts extremely rapidly with hydroxide ions to produce picric acid. Addition of sodium hydroxide to a picryl halide in aqueous dioxan produces a transient red intermediate. The present study has been concerned with the measurement of the rates of formation and decay of the intermediates derived from picryl fluoride, chloride and bromide using stopped-flow spectrophotometry. The spectra of the intermediates are similar to those attributed to Meisenheimer complexes. Picryl fluoride reacts to give this intermediate in a few milliseconds. The rate of appearance of the intermediate was therefore only measurable at very low base concentrations (less than 6 x 10 M hydroxide). Under these conditions the rate was found to... 

Unfortunately neither picryl chloride nor bromide gave first order plots for the disappearance of the intermediate. Analogue computer analysis showed that this was not simply due to competition between rates of formation and disappearance. Good agreement between observed and analogue computer simulated rates was obtained if a second intermediate was introduced into the model. The most likely species corresponding to this intermediate is the Meisenheimer complex produced by addition of hydroxide at the 3-position. This model produces the scheme shown below ... 

The presence of two intermediates would be difficult to detect as the spectra of II and III are similar. The equilibrium reaction between I and III effectively reduces the standing concentration of I. Corrected values for the rate of disappearance of I as a function of hydroxide concentration show that this reaction is first order in both picryl halide and hydroxide (figure 2). This base dependence shows that the formation of the 1-adduct (II) is slow compared with chloride or bromide loss from II. It is therefore not surprising that the reactivities of the picryl halides have little in common with their alkyl analogues. The rate constants for all three halides are shown in table 1. 

Examination of the equilibrium constants for the formation of III shows that picryl chloride and bromide are quickly converted to the inactive adduct III and that this is the coloured species observed (very little II is present). As the rates of formation of I from III (k 3) and of II from I (k] [0H J) are comparable in these cases the rate determining step is a combination of these two reactions. Apart from making meaningless any discussion of the relative values of the overall rates of hydrolysis it underlines the dangers involved in complex equilibrium reactions. The values of the rates and equilibrium constants for the formation of III might be taken to indicate a fast pre-equilibrium reaction followed by a slow reaction to give products. This is clearly not the case as shown by the marked curvature found in a pseudo first order plot. 

The simple model originally derived for picryl fluoride must also be reconsidered in the light of the more complicated model for the bromide and chloride. Inspection of the ratio of the rate constants for the formation of the 1- and 3- Meisenheimer complexes (k] /k3) for the bromide (0.03) and chloride (0.19) leads to the conclusion that this ratio may be close to unity for the fluoride. The colour observed in the reactions of picryl... 

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