Isolation of 3-amino-4-nitrobenzyl acetate: evidence of an undisclosed impurity in 5-amino-2-nitrobenzoic acid

The crystal stucture of 3-amino-4-nitrobenzyl displays intramolecular resonance-assisted hydrogen bonding between the ortho amino and nitro groups in addition to an intermolecular network of hydrogen bonding and π-stacking.


Chemical Context
Often commercially available chemicals are sold with minor impurities in the range 1-5%; the user may choose to 'use as received' or further purify. The identities of the impurities are rarely disclosed in fine chemicals. Though these impurities may serve as benign spectators, in some cases they might hinder reactivity and/or produce undesirable by-products that are difficult to separate from the desired product. Therefore, it is important to identify these impurities to allow the users to decide if further purification is warranted. We recently purchased 5-amino-2-nitrobenzoic acid from Acros Organics # (5 g, 97%, AC33074-0050) for our ongoing studies of photoinduced decarboxylation of ortho-nitrobenzyl esters (Cabane et al., 2010;Pocker et al., 1978). The isolation of the title compound, 3-amino-4-nitrobenzyl acetate, after the reaction of crude (5-amino-2-nitrophenyl)methanol, prepared from the reduction of 5-amino-2-nitrobenzoic acid, with acetic anhydride suggests 3-amino-4-nitrobenzoic acid is an impurity in the commercially available starting material.

Supramolecular Features
The crystal structure of 3-amino-4-nitrobenzyl acetate has interesting supramolecular features. The molecules are arranged in layers held together by intermolecular N2-H2AÁ Á ÁO4 [3.005 (2) Å ] hydrogen bonding [graph set C1,1(9)] interactions between the carbonyl and amine groups forming a zigzag chain along the b-axis direction ( Fig. 2 and Table 1) lying in a plane parallel to (102). A view of a single layer along the ab plane, observed down the c axis ( Fig. 2) provides a representative illustration of the hydrogen-bonding interactions of 3-amino-4-nitrobenzyl acetate. Observing the unit cell along the b-axis (Fig. 3) shows four layers along the c axis separated at a distance of 3.3163 (10) Å with the arene groups stacked one above the other. The chains stack along the c axis byinteractions [centroid-centroid distances = 3.6240 (3) Å (symmetry code 1 À x, 1 À y, 1 À z) and 3.5855 (4) Å (symmetry code 1 À x, y, 3 2 À z)].

Database Survey
For a related benzyl acetate structure, see Kasuga et al. (2015). For alkyl-and aryl-3-amino-4-nitro-benzoates and benzoic acids displaying similar intramolecular hydrogen bonding between the amino and nitro groups, see: Narendra Babu et al.  A displacement ellipsoid plot of 3-amino-4-nitrobenzyl acetate (50% probability level). C-bound H atoms have been omitted for clarity.

Figure 3
A displacement ellipsoid plot of the unit cell of 3-amino-4-nitrobenzoic acid observed down the b axis. (Yoon et al. 1973). To a solution of 5-amino-2-nitrobenzoic acid (97%, 1.5 g, 8.2 mmol) dissolved in tetrahydrofuran (10 mL), borane-THF (27.6 mL, 1.0 M in THF, 27.6 mmol) was added dropwise by dropping funnel over 30 minutes. The reaction was stirred overnight at room temperature. The reaction was quenched with aqueous potassium hydroxide (2.45 M) until pH 11 was reached and continued to be stirred for 6 h, resulting in a greenish-brown solution. The solution was treated with a saturated solution of potassium carbonate followed by treatment with hydrochloric acid until pH 1 was reached. The reaction mixture was extracted with diethyl ether three times; organic portions were collected and dried with anhydrous sodium sulfate overnight.  (Aujard et al. 2006). Note: minor impurities were observed in the base line in the aromatic region.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms were refined freely.