Nitrobenzoic acid

The nitrobenzoic acids constitute a group of substances derived from both the benzoic acid as well as from nitrobenzene in chemistry. The structure consists of a benzene ring with an added carboxy ( -COOH) and nitro ( - NO2) as a substituent. Due to their different arrangement of three structural isomers arise with the empirical formula C7H5NO4. 4-nitrobenzoic acid is mainly used in the analysis of organic substances.

Representation

3- nitrobenzoic acid obtained from methyl benzoate by reaction with fuming nitric acid in the presence of concentrated sulfuric acid. After the nitration of the resulting 3- nitrobenzoate is saponified to the acid.

However, when the benzoic acid is nitrated directly, mostly produced 3.5 dinitrobenzoic.

In general, the nitrobenzoic acids from the nitrotoluene by oxidation of the methyl group are accessible. Thus, 4-nitrotoluene is oxidized with sodium dichromate in the presence of concentrated sulfuric acid to 4-nitrobenzoic acid.

Properties

The nitrobenzoic acids are colorless to yellowish crystalline solids. The melting points differ significantly. 4- nitrobenzoic acid, which has the highest symmetry, has the highest melting point. The nitrobenzoic acids have, because of the -M effect of the nitro group to a higher acidity compared to benzoic acid. The pKa values ​​are therefore correspondingly lower ( benzoic acid: 4,20 ).

Use

Analysis

4-nitrobenzoic acid is mainly used in the analysis of organic substances by derivatization. Liquid substances or those with a low melting point can be converted to a slightly crystallized derivatives: alcohols can be identified, for example by measuring the melting points of its esters of 4-nitrobenzoic acid, or 3,5- dinitrobenzoic. For this purpose the substance to be analyzed is reacted with 4-nitrobenzoic acid in the presence of small amounts of sulfuric acid.

The melting points of these derivatives are usually sharp. This reaction is also suitable for many amines.

The derivatives of 3,5- dinitrobenzoic generally have higher melting points than that of 4-nitrobenzoic acid. You are then preferably selected when the melting point of 4-nitrobenzoic acid is too low and no more accurate determination is possible.

If the substance in question is more sensitive, and instead they direct reaction with the corresponding acid chloride, 4- nitrobenzoyl chloride. In this way, for example, derivatives of amino acids are available.

Synthesis

3 -aminobenzoic acid may be obtained by the reduction of 3- nitrobenzoic acid. Suitable reducing agents are, for example elemental zinc in hydrochloric acid or hydrazine.