Detailed introduction of Halogenated Hydrocarbons

  • The compound in which the hydrogen atom in the hydrocarbon molecule is replaced by a halogen atom is called a halogenated hydrocarbon (Haloalkane), abbreviated as a halogenated hydrocarbon. The carbonyl group of the halogenated hydrocarbon is: (Ar)RX, X is a halogen atom, and may optionally be a functional group of a halogenated hydrocarbon, including fluorine, chlorine, bromine, and iodine.



    The way of halogenation is related to the chemical dosage and the characteristics of its chemical structure, the functional groups of organic compounds or halogenated halogen elements. Halogen is the general term for five elements: fluorine, chlorine, bromine, iodine, and astatine. Therefore, halogenation is also divided into fluorination, chlorination, bromination and iodination. Iodine is much more expensive than chlorine and bromine, so the most commonly used methods in chemical production are chlorination and bromination. Commonly used chlorinating agents are chlorine or hydrogen chloride. Because the fluorine gas is too oxidizing, the reactants are usually oxidized and decomposed directly, so the corresponding fluorinating agent is generally used for fluorination.

    Examples of halogenation are the chlorination of acetylene by hydrogen chloride to produce vinyl chloride, which becomes the raw material for the manufacture of plastic polyvinyl chloride; the chlorination of benzene to produce hexachlorobenzene and so on.

    Dehalogenation is the reverse reaction of halogenation, which removes halogen elements from the molecule. The most common is the dehydrohalogenation reaction.


    Organic compound

    There are many reaction pathways for halogenation of organic compounds, including radical halogenation, ketone halogenation, electrophilic halogenation and halogenation addition reactions.

    Free radicals

    Typically, saturated hydrocarbons are halogenated through free radicals without adding halogen elements, but instead hydrogen atoms are replaced by halogen elements. Halogenated species are often determined in regional chemistry by using relatively weak C-H bonds. This reaction tends to be the position of the second and third carbons because the more stable the structure will cause the free radicals to react with the transition energy levels. Free radical halogenation is used in the industrial production of methane chloride.

    CH4+ Cl2→ CH3Cl + HCl

    Enyne addition

    Unsaturated compounds, especially alkenes and alkynes, are addition halogenated.


    The addition halogenation of alkenes produces intermediate halide ions. In particular, this intermediate product can be separated out.

    Aromatic compounds

    The halogenation of aromatic compounds is an electrophilic halogenation reaction.

    RC6H5+ X2→ HX + RC6H4X

    This halogenation mechanism is affected by halogen elements. Fluorine and chlorine are relatively electrophilic and are relatively strong halogenating agents. Bromine is a weaker halogenating agent than fluorine and chlorine, but iodine is the weakest of them. . The mechanism of this dehydrohalogenation follows a reversal trend: iodine is the easiest to remove from organics and organic fluorides have the highest stability.


    Other methods

    The Hunsdiecker reaction is to produce short-finished halide from carboxylic acid. The first one will produce silver salt, which will then be oxidized with halogen elements.

    RCO2Ag + Br2→ RBr + CO2+ AgBr

    The Sandmeyer reaction is obtained from the aromatic halide diazonium salt, which is obtained from aniline.

    Hell–Volhard–Zelinsky halogenation is the halogenation of the alpha carbon in the condensation acid.


    According to halogen elements


    Fluorine reacts with saturated or unsaturated organic compounds, the reaction is fast and easy to explode. This reaction needs to be carried out in a professional environment. In practical applications, organic compounds are often halogenated electrochemically. Recently, it has been discovered that hydrogen fluoride can be used as a source of fluorine at the cathode. This method is called electrochemical fluorination (ECF). In addition to electrochemical production of fluorine, there are also various applications of fluorination reagents such as xenon difluoride and cobalt fluoride.


    Chlorination will produce an exothermic reaction. All saturated or unsaturated compounds react directly with chlorine. This composition often requires ultraviolet light to help crack the chlorine atom. Chlorination is commonly used in industry to make 1,2-dichloroethane (PVC) and ethane as a solvent. The other is oxychlorination by combining hydrogen chloride and oxygen. This effect must be directly chlorinated (with Cl2).


    Oxychlorination is a mixture of hydrogen chloride (HCl) and oxygen (O2) to chlorinate hydrocarbons. This method is very popular in industry because hydrogen chloride is cheaper than chlorine. The most commonly used reactant is alkene.

    CH2=CH2+ 2 HCl + ½ O2→ ClCH2CH2Cl + H2O

    The reaction starts with copper chloride (CuCl2, a catalyst produced from 1,2-dichloroethane). Sometimes CuCl2 is used as a co-catalyst in silica to help produce KCl, LaCl3, or AlCl3. In addition to silica, alumina, diatomaceous earth or pumice can also be used as auxiliary reactions. Because these reactions emit high energy (238kJ/mol), the catalytic temperature must be controlled to avoid pyrolysis. Catalysis plays an important role in the double bond chlorination of hydrocarbons, because CuCl2 is one of the compounds that provide the double bond of the chlorine atom.

    CH2=CH2+ 2 CuCl2→ 2 CuCl + ClH2C-CH2Cl

    Copper chloride is produced by the reaction of cuprous chloride with oxygen and then with hydrogen chloride.

    ClCH2CH2Cl → CH2=CHCl + HCl

    2 HCl + CH2=CH2+ ½ O2→ ClCH2CH2Cl + H2O

    HCl undergoes oxychlorination through the above-mentioned cyclic reaction, and the reactants are produced by themselves, which is also the reason why the industry prefers to use oxychlorination instead of chlorination.


    Bromination is more selective than chlorination because the reaction emits less energy. The general bromination is the addition of Br2 to alkenes. In nature, the bromination of saturated hydrocarbons and aromatic compounds is more common to form organic bromination. Compound. A commonly used catalyst is bromoperoxidase, which uses the combination of bromine and oxygen to form an oxidant. An example of bromination is the anesthetic halothane that can be organically synthesized in trichloroethylene.

    Organic bromine compounds are the most common halogenated compounds in nature, and their formation is catalyzed by bromoperoxidase. The ocean is estimated to release 1-2 million tons of bromoform and 56,000 tons of bromomethane each year.


    Iodine is the weakest halide and is the most difficult to react with most organic compounds. The addition of iodine and olefins is an analytical method called iodine, which is a method used to measure the degree of unsaturation of fat. The iodoform reaction kit will degrade methyl ketone.


    Inorganic chemistry

    Elements other than argon, neon, and helium will directly react with fluorine to form fluoride. Chlorine has strong selectivity, but it can still react with most metal elements and heavier non-metal elements. According to the trend, bromine has weaker activity, while iodine has the weakest activity. Possible reactions are like the chlorination of gold to produce gold chloride. Direct chlorination of metals is less important in industry because it is easier for metal oxides to react with hydrogen halides to carry out halogenation. The production of phosphorus trichloride and sulfur monochloride is an example of direct halogenation of inorganic compounds on a larger scale.


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