Halogenation

As halogenation is in the transfer of a chemical element or compound into a halide, or a salt-like compound having a halogen covalently built designated. This is possible for both inorganic and organic compounds. Depending on the halogen distinction is made between fluorination, chlorination, bromination or iodination.

• 2.1 halogenation of aromatics
• 2.2 halogenation of alkanes
• 2.3 halogenation in the allyl position or benzyl position
• 2.4 halogenation of ketones and aldehydes 2.4.1 mechanism of the halogenation of aldehydes and ketones

Addition reactions

Halogen, hydrogen halide or hypohalous acid react with unsaturated compounds.

Addition of halogens to alkenes or alkynes

Alkenes formed by the reaction with halogen molecules vicinal dihaloalkanes. Alkynes add halogen gradually. There arise tetra haloalkanes. Here the example of the reaction of Br2 shown with ethyne:

The first step is the formation of vicinal Dihalogenalkens ( here 1,2- dibromoethene ).

In the second step creates the tetra alkyl halide ( here 1,1,2,2- tetrabromoethane ).

Addition of hydrogen halides to alkenes or alkynes

Addition of hydrogen halide to an alkene provides a mono haloalkane. Alkynes of reacting with hydrogen halide to a Monohalogenalken, wherein the halogen atom is added to one of two sp2 -hybridized carbon atom orbitals of carbon-carbon double bond. An example for the addition of hydrogen halides to olefins is the chlorination of 2-butene:

Addition hypohalous acid to alkenes

The addition of hypohalous acid to alkenes provides halohydrins, ie both sp2 -hybridized carbon atoms in the starting material are transformed into sp3 -hybridized carbon atoms in the product in which at one of these carbon atoms, a hydroxy group, and the other is a halogen atom is bound.

Addition of Halogens to free radicals

Because of the ease of homolytic cleavage of halogen molecules react spontaneously with this free organic radicals.

Substitution reactions

Halogenation of aromatics

Halogenate a variant to aromatics is electrophilic aromatic substitution. Here, grab activated halides the aromatic system to electrophilic. An example is the chlorination of benzene:

If you leave chlorine in the presence of a Lewis acid to react with benzene, chlorobenzene arise and hydrogen chloride. Iron (III ) chloride or aluminum trichloride usually acts as a Lewis acid and is used for activation of chlorine, that would otherwise not react with benzene. The reaction with bromine would proceed analogously.

Halogenation of alkanes

The halogenation of alkanes proceeds radically and leads to haloalkanes.

Halogenation in the allyl position or benzyl position

The halogenation of alkenes in the allyl position or alkylaromatic in the benzyl -position proceeds under free radical and results in substitution of halogenated alkenes and halogenated in the side chain alkyl aromatics.

Halogenation of ketones and aldehydes

Halogenation of ketones is a formal electrophilic substitution reaction with a proton as a leaving group. It does not proceeding according to such a mechanism, but via the enol of the ketone in question and there are α - halogenated ketones. The halogenation of aldehydes is similar.

Mechanism of the halogenation of aldehydes and ketones

The halogenation of carbonyl groups, such as are present in aldehydes and ketones, can run either acid or base catalysis. In the following, the mechanism is demonstrated exemplarily of the acid catalysed bromination of acetone:

The acetone is used (1), with its enol form of the keto -enol tautomerism in equilibrium. If we now add bromine, then rearranges this as described in the enol form, and it forms the oxonium ion 2, which has a more mesomeric structure. In addition, a bromine ion, which deprotonates the oxonium ion 2 in the following step forms. This simply gives brominated acetone 3 Repeating. These steps twice for this brominated acetone 3, so you can get a triple brominated acetone 4 This is called an α, α, α - trihalogenated acetone.

The rate-determining step in this reaction is the formation of the enol. The reaction rate decreases with each molecule added to the halogen, since the halogens are electron withdrawing. Thus they hinder the rearrangement of electrons in the molecule to the enol form. It is also said that the compound has been reduced Enolisierbarkeit.

Halogenation of carboxylic acids

Carboxylic acids can be converted by reaction with thionyl chloride to carboxylic acid chlorides. Other chlorinating agent such as phosphorus trichloride or phosphorus pentachloride can also be employed.

Halogenation of ethers

Ether α' - Dichlorethern are at low temperatures to α -chloro- α and, chlorierbar.

Substitution of hydroxyl groups

Hydroxyl groups in alcohols are substituted by chlorine or hydrogen chloride by means of hydrogen bromide with bromine. In particular for primary and secondary alcohols are often added thionyl chloride ( SOCl2 ) for chlorination and phosphorus tribromide ( PBr3 ) was used for bromination. In order to replace hydroxy groups with iodine in addition to hydrogen iodide in the laboratory and phosphorus triiodide, which is prepared in situ from phosphorus and iodine can be used.

Substitution of the oxygen of the carbonyl groups

Upon heating of aldehydes or ketones with phosphorus trichloride or phosphorus tribromide geminal dichlorides or dibromides arise.

Substitution of nitrogen-containing groups by halogen

Reaction of diazonium salts with the formation of organic halogen compounds. In addition to the Sandmeyer reaction here is the Balz- Schiemann reaction of importance.

Halogenation at the nitrogen atom of carboxylic acid amides

Carboxamides, which carry at least one hydrogen atom on the amide nitrogen atom react with hypohalites. It created N- Halogencarbonsäureamide. An example is N-bromosuccinimide ( NBS), which is obtained in an aqueous solution of one equivalent of succinimide and each of a base and bromine.

Halogenation in Inorganic Chemistry

In Inorganic Chemistry, halogenation is involved, inter alia, for the formation of salts, hydrogen halides, and halogen oxygen acids.