Durene, nitration

We referred to the durene nitration phenomenon, although an electrophilic aromatic substitution is not directly related to the reactions of this section. There may, however, be an indirect correlation because we have the impression that... 

When the concentration of 2-phenylethanesulphonate anion was >0-5 mol 1, or when 2-mesitylethanesulphonate anion (v), " mesitylene-a-sulphonate anion, or iso-durene-a -sulphonate anion were nitrated, the initial part of the reaction deviated from a first-order dependence on the concentration of the aromatic towards a zeroth-order dependence. 

The rates of nitration of mesitylene-a-sulphonate anion (iii) and iso-durene-a -sulphonate anion (iv) in mixtures of aqueous nitric and perchloric acid followed a zeroth-order rate law. Although the rate of exchange of oxygen could not be measured because of the presence of perchloric acid, these results again show that, under conditions most amenable to its existence and involvement, the nitric acidium ion is ineffective in nitration. 

Work on the nitration of durene also gives evidence for the importance of the speed of mixing in nitration with nitronium salts. ... 

A. Dinitrodurene.—A solution of 13.4 g. (0.1 mole) (Notes 1,2) of durene (p. 32) in 100 cc. of chloroform is added to 75 cc. of concentrated sulfuric acid in an 800-cc. beaker provided with a thermometer and an efficient mechanical stirrer. The mixture is cooled to io°, and 16 g. (10.7 cc.) of fuming nitric acid (sp. gr. 1.5) (Note 3) is added drop by drop, with stirring, from a 125-cc. separatory funnel, the mixture being cooled in an ice-salt bath and the nitric acid added at such a rate that the temperature does not rise above 50° (about fifteen minutes is required for the addition). As soon as all the acid has been added the mixture is poured into a separatory funnel, the sulfuric acid layer is removed and the upper chloroform layer is immediately (Note 4) run into 500 cc. of 10 per cent sodium carbonate solution. The sulfuric acid layer is discarded because it contains very little dinitrodurene. Four portions are nitrated and the combined chloroform solutions are washed twice with 2.5 per cent sodium carbonate solution, dried over night with 30 g. of anhydrous calcium chloride, filtered, and the chloroform distilled off until crystals of dinitrodurene begin to appear. At this point four times the volume of hot 95 per cent ethyl alcohol is added (about... 

It is better to nitrate the durene in small batches, for a high yield and pure product depend upon not allowing the reaction mixture to remain in contact with the nitric acid very long. 

A similar (but somewhat less obvious) dichotomy results in the simultaneous ring and sidechain substitution of durene. Thus in this charge-transfer nitration, the addition of N02 to the cation radical DUR+- (72) occurs in competition with its deprotonation (73), in which the pyridine has been shown to act as a base (Masnovi et al., 1989) (Scheme 15). [Note that deprotonation of DUR+- also leads to aromatic dimers via the subsequent (oxidative) substitution of the benzylic radical formed in (73) (Bewick et al., 1975 Lau and Kochi, 1984).]... 

The reaction of durene with H NOj under the conditions just mentioned is another specific but principally important case. The reaction is accompanied with a strong enhanced absorption in N NMR spectrum with respect to a signal belonging to the product, that is, nitrodurene. Durene and naphthalene have very similar standard potentials (2.07 and 2.08 V, respectively, in AN see Ridd 1991). A significant difference between them is that, with durene, much of the nitration supposedly arises from ipso attack followed by rearrangement according to Scheme 4.41. 

According to Scheme 25, the NaphH + cation radical cannot be formed as a result of the interaction of NaphH with NO). In this sense, Ridd s work is relevant (Johnston et al. 1989,1991). The work shows that, to some extent, the nitration of naphthalene can indeed proceed through an outer-sphere electron transfer. Naphthalene, durene, and mesitylene were compared in their reactions with H15N03. The reactions were performed in a solvent containing trifluoroacetic acid (49%), nitromethane (50%), and water (1%) (weight-to-weight). The reaction mixture also contained some sodium azide and methanesulphonic... 

Fig. 1. Droplet model for nitration of durene + = nitronium ion O = durene M = mononitrodurene D = dinitrodurene...

Polymethylbenzenes undergo nitration more readily as illustrated by the conversion of sym-trimethylbenzene (mesitylene) to nitromesitylene (76%). Durene (sym-tetramethylbenzene) gives dinitrodurene (94%) no mononitrodurene is ever obtained. In the nitration of other polysub-stituted benzenes, certain anomalous reactions occur. Thus nitration of p-cymene (p-isopropyltoluene) and p-diisopropylbenzene gives in part p-nitrotoluene (8%) and p-nitroisopropylbenzene (50%), respectively, each resulting from the replacement of an isopropyl group by the nitro group. 

Aramides have a 40°C higher melting point with durene diamine than with p-phenylene diamine(5). Furthermore, durene diamine may be able to be produced at a price equivalent to or lower than that of p-phenylene diamine. This arises because the linear substitution is not generally simple in nitration reactions for durene, however, there is no other option. 

Analogous results were obtained for the nitrous acid catalyzed nitration of durene (26). Here, however, CIDNP signals were obtained also in the nitration of durene by N02 +, and their character suggested that durene and N02 were being formed not by the direct transfer of an electron from durene to N02 +, but by the homolysis of an ipso intermediate. Recombination of durene and N02 then led to nitrodurene. 

The reaction was found particularly useful as a relatively selective and mild nitration method, for example allowing mononitration of durene and other highly alkylated benzenes, which with mixed acid usually undergo dinitration. (Table XIl). Methyl nitrate-boron trifluoride can also be used to achieve dinitration of tet-ramethylbenzenes by using two and three molar excess of methyl nitrate, respectively. Relative yields of mono- and dinitro product compositions are shown in Table XIII. Other Friedel-Crafts type catalysts can also be used, but boron trifluoride was found to be the most suitable. Aluminum trichloride and titanium (IV) chloride in the nitration of pentamethylbenzene caused formation of significant amounts of chlorinated derivatives, whereas sulfuric acid led to nitrodemethylation products. 

E.g., CIDNP has been detected in the nitration of durene but not mesitylene ... 

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