Aromatic amines refer to amines with an aromatic substituent, i.e. -NH2, -NH or nitrogen-containing group attached to an aromatic hydrocarbon. The structure of aromatic hydrocarbons usually contains one or more benzene rings, that is, there is a direct chemical bond between the nitrogen atom and the carbon atom of the benzene ring. Aniline is the simplest example of this class of compounds. Aromatic amine molecules have higher reactivity. Aromatic amines are generally high-boiling liquids or low-melting solids with special odor and high toxicity.
The preparation of aromatic amines generally cannot be obtained directly by introducing amino groups into aromatic rings, but by indirect methods. Since aromatic nitro compounds are easily obtained by nitration, the reduction of aromatic nitro compounds is a common method for preparing aromatic primary amines. Nitro compounds can be reduced to aromatic primary amines with chemical reducing agents under acidic or alkaline conditions. Commonly used reducing agents are composed of metals and acids. Metals can be iron, zinc, tin, and acids can be hydrochloric acid, acetic acid and sulfuric acid. Among them, iron powder and hydrochloric acid are the cheapest and suitable for mass production in industry. In addition to the reduction of iron powder and hydrochloric acid, tin and hydrochloric acid are also commonly used in the preparation of aromatic amines in the laboratory.
Aromatic amines are high-boiling liquids or low-melting solids with special odor and high toxicity. For example, aniline can be poisoned by inhalation, ingestion, or absorption through the skin. If 0.25mL is ingested, it will cause serious poisoning. Beta-naphthylamine and benzidine are substances that cause malignant tumors.
Aromatic amines have the basicity of amines, which are generally weaker and can react with acids to form salts. In addition, aromatic amines also have some characteristics.
Aromatic amines with hydrogen on N are very easy to be oxidized, and the oxidation products are different with the different types of oxidants and reaction conditions.
2. Oxidation of aromatic amines
In the electrophilic substitution reaction of aromatic amines, H2N-, RNH-, R2NH-, ArNH-, etc. are all ortho-para-positioning groups, while the corresponding ammonium salts, such as H3N+, are meta-positioning groups.
(1) Halogenation: When aniline is directly halogenated, it is difficult for the reaction to stay in the first generation stage, except for the reaction with iodine to generate iodoaniline.
(2) Acylation: There are hydrogens on the nitrogen of the primary and secondary aromatic amines. Since acylation can occur on the nitrogen of the amino group and on the carbon of the benzene ring, the yield is relatively low. However, after the amino group is protected with an acyl group, the acylation reaction on the aromatic ring carbon can proceed smoothly.
(3) Sulfonation: sulfuric acid first forms a salt with aniline during sulfonation. If oleum is used as the sulfonating reagent, the reaction is carried out at room temperature, and the meta-substituted product is mainly obtained. If the sulfonation is carried out with concentrated sulfuric acid and the reaction is carried out under the condition of heating for a long time, the main product is the para-substituted product.
(4) Nitration: During direct nitration, the amino group is easily oxidized by nitric acid, and the conversion rate is low. In order to prevent the oxidation of the amino group, the acetylation reaction of amine is often used to protect the amino group, and the acetyl group is removed after the reaction.