3-Nitrobenzoic Acid: Relative Molecular Mass and Mechanisms

Basic Information

English name: 3-Nitrobenzoic acid

CAS No.: 121-92-6

Molecular formula: C₇H₅NO₄

Molecular weight: 167.12

EINECS No.: 204-508-5

m-Nitrobenzoic acid is an important organic synthesis intermediate widely used in pharmaceutical manufacturing, dye industry, photosensitive materials, and chemical analysis. This article discusses the relative molecular mass of m-nitrobenzoic acid and its calculation method, along with its physicochemical properties, synthesis methods, and applications, providing fundamental data support for related chemical research and industrial production.

1. Molecular Structure and Formula

The molecular formula of m-nitrobenzoic acid is C₇H₅NO₄. The structure is based on a benzene ring, with a carboxyl group (-COOH) attached to the carbon atom at position 1 and a nitro group (-NO₂) attached to the carbon atom at position 3 (i.e., the meta position). This meta-substituted structural feature reflects the presence of two strong electron-withdrawing groups – the nitro and carboxyl groups – which are both meta-directing groups in electrophilic aromatic substitution reactions, directing incoming groups preferentially to the meta position.

This meta-disubstituted molecular structure endows 3-Nitrobenzoic acid with unique physicochemical properties. The presence of the nitro group enhances the acidity of the carboxyl group on the benzene ring, making its acid dissociation constant (Ka, approximately 3.48×10⁻⁴ at 25°C) significantly higher than that of benzoic acid. Meanwhile, the synergistic electron-withdrawing effect of the nitro and carboxyl groups gives the compound good reactivity in reduction reactions, esterification reactions, and others, laying the structural foundation for its wide application in organic synthesis.

2. Relative Molecular Mass Calculation

Relative molecular mass, also known as molecular weight, is the sum of the atomic masses of all atoms in a molecule. For m-nitrobenzoic acid, the relative molecular mass can be calculated as follows:

Carbon atoms (C): There are 7 carbon atoms in the molecule, each with a relative atomic mass of 12.01. The total mass of carbon atoms is: 7 × 12.01 = 84.07.

Hydrogen atoms (H): There are 5 hydrogen atoms in the molecule, each with a relative atomic mass of 1.008. The total mass of hydrogen atoms is: 5 × 1.008 = 5.04.

Nitrogen atom (N): There is 1 nitrogen atom in the molecule, with a relative atomic mass of 14.01. The total mass of the nitrogen atom is: 1 × 14.01 = 14.01.

Oxygen atoms (O): There are 4 oxygen atoms in the molecule, each with a relative atomic mass of 16.00. The total mass of oxygen atoms is: 4 × 16.00 = 64.00.

Summing these masses gives the relative molecular mass of m-nitrobenzoic acid:

84.07 + 5.04 + 14.01 + 64.00 = 167.12

Through calculation, we find that the relative molecular mass of 3-Nitrobenzoic acid is approximately 167.12. This data is crucial for accurate chemical stoichiometry and reaction feed ratio calculations, helping researchers make precise quantitative analyses in experimental design and synthesis process development.

3. Physicochemical Properties

m-Nitrobenzoic acid typically appears as white or slightly yellow monoclinic flaky crystals with an almond-like odor and bitter taste. At 20°C, its relative density is 1.494. Its melting point is approximately 142°C, and its boiling point at 760 mmHg is about 340.7°C. The flash point of this compound is 190°C, and its vapor pressure at 25°C is about 3.26×10⁻⁵ mmHg, classifying it as a low-volatility solid substance.

In terms of solubility, m-nitrobenzoic acid is slightly soluble in cold water at room temperature (1 g dissolves in about 320 mL of water), more soluble in hot water, and soluble in common organic solvents such as ethanol, diethyl ether, chloroform, methanol, and acetone. 1 g of product dissolves in approximately 3 mL of ethanol, 4 mL of diethyl ether, 18 mL of chloroform, about 2 mL of methanol, and 2.5 mL of acetone. It is practically insoluble in benzene, carbon disulfide, and petroleum ether.

The compound is chemically stable under normal temperature and pressure, but the nitro group in its molecule is highly reactive and can undergo various chemical transformations under appropriate conditions, making it an important synthetic intermediate.

4. Chemical Reaction Mechanisms

The chemical reactions of m-nitrobenzoic acid mainly revolve around its two core functional groups – the nitro group and the carboxyl group. In-depth study of these reaction mechanisms is of great significance for understanding and optimizing its synthesis and transformation processes.

4.1 Reduction Reaction

The nitro group in the m-nitrobenzoic acid molecule can be reduced to an amino group under appropriate conditions, yielding m-aminobenzoic acid. This reduction reaction can be carried out under various conditions: in acidic or neutral media, chemical reduction is commonly performed using metal reducing agents such as iron powder, zinc powder, or stannous chloride; under catalytic hydrogenation conditions, catalysts such as palladium on carbon, platinum on carbon, or Raney nickel can be used to efficiently reduce the nitro group to an amino group under a hydrogen atmosphere.

In terms of electrochemical reduction, studies have shown that 3-Nitrobenzoic acid can undergo cathodic electroreduction on a copper electrode in a sulfuric acid system. The electrode process is diffusion-controlled, and the electroreduction reaction is accompanied by an irreversible subsequent chemical reaction. The addition of the inorganic salt stannous chloride exhibits a significant co-catalytic effect on this electrolytic reduction process. The reduction peak potential shows a linear relationship with the solution acidity, while the reduction peak current first increases and then decreases with increasing acidity.

4.2 Hydrolysis Reaction of Nitrobenzoate Esters and Single Electron Transfer Mechanism

The hydrolysis of m-nitrobenzoate esters (such as methyl m-nitrobenzoate, ethyl m-nitrobenzoate, and phenyl m-nitrobenzoate) under alkaline conditions is not a simple nucleophilic substitution process but involves a single electron transfer (SET) mechanism. Electron spin resonance (ESR) studies have confirmed that when m-nitrobenzoate esters react with potassium hydroxide in dimethyl sulfoxide, ESR signals of radical anions are detected in the reaction solution. Spin trapping techniques have confirmed the formation of hydroxyl radicals (·OH) during the reaction. The addition of radical trapping agents such as nitroso-tert-butane, phenyl tert-butyl nitrone, and oxygen all leads to a decrease in the yield of the product m-nitrobenzoic acid. These experimental pieces of evidence collectively indicate that an electron transfer step exists in the alkaline hydrolysis of nitrobenzoate esters, and the reaction proceeds through radical intermediates.

Furthermore, the nucleophilic substitution reaction of m-nitrobenzoate esters with sodium thiophenolate also involves a radical IPSO substitution mechanism. Spin trapping techniques have detected the presence of phenylthiyl radicals, and diphenyl disulfide has been isolated from the product mixture, further confirming the important role of radical mechanisms in this type of reaction.

4.3 Carboxyl-Related Reactions

The carboxyl group in the m-nitrobenzoic acid molecule is a reactive functional group and can undergo a series of typical carboxylic acid reactions. In esterification reactions, it reacts with alcohols under acid catalysis to form the corresponding m-nitrobenzoate esters; in acyl chloride formation reactions, it reacts with thionyl chloride or phosphorus pentachloride to form m-nitrobenzoyl chloride – the latter is an important intermediate for the further synthesis of amides, esters, and other derivatives. The strong electron-withdrawing effect of the nitro group enhances the acidity of the carboxyl group, making it more reactive than benzoic acid in reactions such as acyl chloride formation.

5. Synthesis and Preparation Methods

The industrial synthesis of m-nitrobenzoic acid mainly follows the following process routes. The most commonly used method employs benzoic acid as the starting material, which is nitrated using sodium nitrate in the presence of concentrated sulfuric acid, achieving a yield of about 60%. In the specific operation, dried and finely ground sodium nitrate is mixed with benzoic acid, and the mixture is added in portions to concentrated sulfuric acid. The temperature is raised to 90°C and the reaction is stirred for about 1.5 hours. After cooling, the mixture is poured into ice water to precipitate the product.

To improve the regioselectivity and yield of the nitration reaction, a two-step method involving prior esterification followed by nitration can be used: first, benzoic acid is esterified with methanol under sulfuric acid catalysis to form methyl benzoate, which is then nitrated using mixed acid (a mixture of concentrated nitric acid and concentrated sulfuric acid), and finally hydrolyzed to obtain m-nitrobenzoic acid. This method is slightly more complex to operate but can achieve a yield close to 70%. Additionally, direct nitration of benzoic acid using mixed acid can also yield the target product, with a yield of about 60%.

In the nitration reaction, the nitro group preferentially enters the meta position of the benzene ring because the carboxyl group in the benzoic acid molecule is a meta-directing group. Its electron-withdrawing effect reduces the electron cloud density at the ortho and para positions of the benzene ring, while the electron cloud density at the meta position is relatively higher, thus directing the nitronium ion to attack the meta position and generate the target product.

6. Main Application Areas

As an important fine chemical intermediate, 3-Nitrobenzoic acid has wide applications in multiple fields. In the pharmaceutical industry, it is a key intermediate in the synthesis of the vascular contrast agent iodylamic acid and the diagnostic agent acetrizoic acid. It can also be used to synthesize drug molecules with antiviral, antibacterial, antitumor, and antiepileptic activities.

In the fields of photosensitive materials and dye industry, m-nitrobenzoic acid is an important raw material for photosensitive dyes, used in the preparation of photosensitive materials and functional colorants, playing a significant role in photosensitive systems such as printing plates and photoresists. In organic synthesis, this compound can be used to further prepare various chemical products including m-dimethylaminobenzoic acid, m-nitrobenzoyl chloride, and 3-nitropropiophenone.

In the field of chemical analysis, m-nitrobenzoic acid can serve as a metal ion precipitant for the determination of alkaloids and the metal thorium. It is also used as a standard substance for the determination of carbon, hydrogen, and nitrogen content in organic microanalysis. Furthermore, this compound is used as an intermediate in the synthesis of anesthetics and as a raw material for the electrochemical synthesis of 5-aminosalicylic acid.

m-Nitrobenzoic acid is an important organic intermediate with the molecular formula C₇H₅NO₄ and a molecular weight of 167.12. At room temperature, it appears as white or slightly yellow crystals. The meta-disubstituted benzene ring structure, featuring two strong electron-withdrawing groups (nitro and carboxyl), endows it with strong acidity and good reactivity. As a key intermediate in the pharmaceutical, dye, and photosensitive material industries, 3-Nitrobenzoic acid is widely used in the synthesis of drugs such as iodylamic acid and acetrizoic acid. It also plays an important role in chemical analysis as a metal ion precipitant and as a standard substance for organic microanalysis. A thorough understanding of its relative molecular mass, physicochemical properties, and reaction mechanisms holds significant theoretical and practical value for optimizing synthesis processes and expanding application fields.