Halogenation reaction, also known as halogenation reaction, refers to the reaction in which hydrogen or other groups in organic compounds are replaced by halogen to form halogen-containing organic compounds.

Halogenation reaction plays an important role in organic synthesis. Through halogenation reaction, a variety of halogen-containing organic compounds can be prepared.

The reaction in which halogen atoms are introduced into organic compound molecules to form carbon-halogen bonds to obtain halogen-containing compounds is called halogenation reaction. Depending on the introduction of halogen atoms, halogenation reactions can be divided into chlorination, bromination, iodination and fluorination. Among them, chlorination and bromination are more commonly used, and the chlorination reaction is particularly widely used. Halogenation has been widely used in industries such as medicine, pesticides, dyes, spices, plasticizers, flame retardants and their intermediates to prepare various important raw materials, fine chemical intermediates and industrial solvents. It is an important part of organic synthesis. one of the important positions.

Introduction to halogenation reactions

By introducing halogens into organic compound molecules, there are two main purposes:

① Give organic compounds some new properties, such as reactive dyes containing fluorochloropyrimidine reactive groups, which have excellent dyeing properties.
② After making the halogen derivatives, a series of intermediates containing other groups can be prepared through further conversion of the halogen group. For example, the dye intermediate p-nitroaniline can be obtained by reacting p-nitrochlorobenzene with ammonia water. 2,4-Dinitrochlorobenzene can be hydrolyzed to produce the intermediate 2,4-nitrophenol, etc.
Due to the different chemical properties of halogenated aliphatic hydrocarbons, aromatic hydrocarbons and their derivatives, the halogenation requirements are different and the types of halogenation reactions are also different. Halogenation methods are divided into:
①Substituted halogenation, such as halogenation of alkanes, aromatic hydrocarbons and their derivatives.
②Addition halogenation, such as halogenation of unsaturated hydrocarbons and their derivatives.
③Replacement halogenation, if the existing functional groups on the organic compound are converted into halogen groups.

Common halogenation reactions include halogenation of alkanes, aromatic ring halogenation and side chain halogenation of aromatic hydrocarbons, alcohol hydroxyl and carboxylic acid hydroxyl groups are replaced by halogens, α-active hydrogens of carbonyl compounds such as aldehydes and ketones are replaced by halogens, and halogens in halogenated hydrocarbons Exchange etc. In addition to direct halogenation with halogens such as chlorine and bromine, commonly used halogenating reagents include hydrohalic acid, thionyl chloride, phosphorus pentachloride, and phosphorus trihalide.

Halogenation reaction principle

Substituted halogenation mainly includes substituted halogenation on the aromatic ring, substituted halogenation on the aromatic ring side chain and aliphatic hydrocarbons. Substituted halogenation is most common with substituted chlorination and substituted bromination.

Substituted halogenation on the aromatic ring is an electrophilic substitution reaction, and its general reaction formula is

Ph-H +X2→Ph-X+HX

This is an important type of reaction in fine organic synthesis and can produce a series of important aromatic halogenated derivatives. For example:

In this type of reaction, Lewis acids such as aluminum trichloride, ferric trichloride, ferric bromide, tin tetrachloride, zinc chloride, etc. are commonly used as catalysts, and their function is to promote the polarization dissociation of halogen molecules.

Substituted halogenation on aromatic rings is generally an ionic electrophilic substitution reaction. First, the polarized halogen molecules or halogen ions make an electrophilic attack on the aromatic ring to form a σ-complex, and then quickly lose a proton to obtain halogenated aromatic hydrocarbons.

The purpose of halogenation

(1) It can improve the dyeing performance and improve the dye fastness of dyes. For example, the fastness of tetrabromoindigo is better than that of indigo, with brighter color and good fastness.
(2) Introduce other groups through hydrolysis, alcoholysis and amination of halo groups (mainly -Cl, -Br), mainly -OH, -OR and -NH2.
(3) Through the halogen group, a ring-forming condensation reaction is performed to further synthesize the dye.