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.

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

Basic introduction Chinese name: Halogenation reaction Foreign name: halogenating reaction Alias: Halogenation reaction Halogenation reagents: chlorine, bromine, active chlorine, etc. Purpose: can improve dyeing performance and introduce other functional groups Application: Introduction to halogenation reactions in the synthesis of organic compounds, principles of halogenation reactions

, Purpose of halogenation, influencing factors, types, halogenation reagents and examples, Introduction to halogenation reaction The reaction in which halogen atoms are introduced into organic compound molecules to form carbon-halogen bonds to obtain halogen-containing compounds is called a 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.

By introducing halogen into organic compound molecules, there are two main purposes: ① To give organic compounds some new properties, such as reactive dyes containing fluorochloropyrimidine reactive groups, which have excellent dyeing properties.

② After making 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.

The hydrolysis of 2,4-dinitrochlorobenzene can 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.

Principle of halogenation reaction Substituted halogenation mainly includes substituted halogenation on the aromatic ring, substituted halogenation of 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. The general reaction formula is Ph-H +X 2 →Ph-X+HX. This is an important type of reaction in fine organic synthesis and can produce a series of important aromatic hydrocarbons.

Halogenated derivatives.

For example: Lewis acids such as aluminum trichloride, ferric trichloride, ferric bromide, tin tetrachloride, zinc chloride, etc. are commonly used as catalysts in this type of reaction. Their function is to promote the polarization and 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 -NH 2 .

(3) Through the halogen group, a ring-forming condensation reaction is performed to further synthesize the dye.

Influencing factors The main factors affecting the reaction are as follows: ① Aromatic hydrocarbon substituents.

The electronic effect of the substituents on the aromatic ring has a great influence on the ease of halogenation of substitutions on the aromatic ring and the position of the halogenation.

There is an electron-donating group attached to the aromatic ring, so the halogenation reaction proceeds easily, and multiple halogenation phenomena often occur. It is necessary to appropriately select and control the reaction conditions, or use protection, removal and other means to make the reaction stay in the single or double halogenation stage.

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If there are electron-withdrawing groups on the aromatic ring, the reaction will be more difficult. Lewis acid catalyst must be used to perform halogenation at a higher temperature, or a more active halogenating reagent may be used to make the reaction proceed smoothly.

If there are electron-donating groups in addition to electron-withdrawing groups on the aromatic ring, the halogenation reaction will be much smoother.

The halogenation of naphthalene is easier than that of benzene and can be carried out in solvent or molten state.

The chlorination of naphthalene is a parallel series of reactions. The monochlorination products include two isomers, clonaphthalene and eclonaphthalene, while the dichlorination products can have up to 10 isomers.

②Halogenating reagent.

Directly using fluorine and aromatic hydrocarbons to produce fluorinated aromatic hydrocarbons, the reaction is very violent and needs to be carried out under argon or nitrogen dilution at -78°C, so it has no practical significance.

Halogenating reagents used in the synthesis of other halogenated aromatic hydrocarbons include halogen (SOCl 2 ), etc.

If iodine is used for iodination reaction, due to the hydrocarbons produced, it is necessary to add an oxidizing agent at the same time, or to add alkali, or to add and remove it, in order for the iodination reaction to proceed smoothly.

If strong iodine, N-bromo(chloro)succinimide (NBS), hypochlorous acid, and sulfuryl chloride are used, hydrogen iodide has reducing properties and can reduce the iodoaromatic hydrocarbons to the raw material aromatic hydrogen iodide to form insoluble hydrogen iodide.

Good results can be obtained by iodizing aromatic hydrocarbons with metal oxides of water iodide and its oxidizing agent ICl.