Regioselective dinitration of simple aromatics over zeolite Hβ/nitric acid/acid anhydride systems

Abstract

Various nitration systems comprising nitric acid, acid anhydride and zeolite Hβ in the absence of solvent are described. Direct double nitration of toluene with a nitric acid, propanoic anhydride and zeolite H system has been developed to give 2,4-dinitrotoluene in 98% yield, with a 2,4-:2,6-dinitrotoluene ratio of 123:1. This system also nitrates activated mono-substituted benzenes (anisole and phenetole) and moderately activated mono-substituted benzenes (ethylbenzene and propylbenzene) to give mainly 2,4-dinitro derivatives. The zeolite can be recovered, regenerated and reused to give almost the same yield as that given when fresh zeolite is used.

Regioselective nitration of 2- and 4-nitrotoluenes over systems comprising nitric acid, an acid anhydride and a zeolite

“…In order to gauge the influence of various zeolites, nitration of 2-NT (35 mmol) was carried out using various quantities of both nitric acid and acetic anhydride (Scheme 1) over 2 g of zeolite HY or Hβ at 50 °C for 4 h. These zeolites were chosen because both have large pores (2-NT might have difficulty entering the pores of small or medium pore zeolites) and have shown high regioselectivity in previous nitration studies. [13][14][15][16][17][18][19][20][21][22] The results obtained are recorded in Table. Table 1 shows that no reaction took place when HY zeolite (SiO 2 /Al 2 O 3 = 5.1) was used as a catalyst; the starting material was recovered quantitatively even when a large excess of nitrating reagent was used.…”

“…Since propionic anhydride had also proven to be more successful than acetic anhydride for double nitration of toluene, [20][21][22] it was of interest to study nitration of 2-NT and 4-NT using propionic anhydride to see what effect the anhydride would have on the reactivity of the mononitro compounds. Nitration of 2-NT (35 mmol) with nitric acid (70 mmol), propionic anhydride (75 mmol) and zeolite Hβ (2 g) for 4 h at 50 °C gave 2,4-DNT in 95.6% yield along with 4.2% of 2,6-DNT.…”

“…Despite the increased size of the substrate compared to toluene itself, it was clear that zeolite Hβ remained significantly better than zeolite HY in terms of both activity and selectivity for 2,4-DNT production. [13][14][15][16][17][18][19][20][21][22] Therefore, zeolite Hβ was selected as the catalyst for investigation of the nitration of 4-nitrotoluene (4-NT).…”

“…11 The 2,4-:2,6-DNT ratio was improved to 14 when the reaction was carried out with nitric acid over zeolite H. 12 We have shown that zeolites and other solid catalysts can play an important role in the enhancement of regionselectivity in nitration processes [13][14][15][16][17][18][19][20][21][22] and in many other electrophilic aromatic substitution reactions. [23][24][25][26][27][28][29][30][31] For example, 2,4-DNT was successfully produced in 96% yield with a 2,4-:2,6-DNT ratio of 70:1 by simple direct dinitration of toluene over a HNO 3 /acetic anhydride (Ac 2 O)/trifluoroacetic anhydride (TFAA)/Hβ system in two steps.…”

“…[20][21][22] Indeed, dinitration of toluene (1 molar equivalent) using nitric acid (3.4 molar equivalents) and propanoic anhydride (3.7 molar equivalents) over zeolite Hβ gave 2,4-DNT in 98% yield with a 2,4-:2,6-ratio of 123:1. [20][21][22] This is easily the most selective quantitative double nitration of toluene ever recorded.…”