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Halogenated hydrocarbons: production, chemical properties, application

Hydrocarbons are a very large class of organic compounds. They include several main groups of substances, among which almost everyone finds wide application in industry, everyday life, nature. Of particular importance are halogenated hydrocarbons, which will be discussed in the article. They are not only of high industrial importance, but they are also an important raw material in many chemical syntheses, in the preparation of medicines and other important compounds. We pay special attention to the structure of their molecules, properties, and other features.

halogenated hydrocarbons

Halogenated hydrocarbons: general description

From the point of view of chemical science, this class of compounds includes all those hydrocarbons in which one or more hydrogen atoms are replaced by one or another halogen. This is a very extensive category of substances, since they are of great industrial importance. Within a fairly short time, people learned to synthesize almost all halogenated hydrocarbons, the use of which is necessary in medicine, the chemical industry, the food industry, and everyday life.

The main method for obtaining these compounds is the synthetic route in the laboratory and industry, since almost none of them are found in nature. Due to the presence of a halogen atom, they are highly reactive. This largely determines the scope of their application in chemical synthesis as intermediate products.

Since there are many representatives of halogenated hydrocarbons, it is customary to classify them according to various criteria. The basis is both the structure of the chain and the multiplicity of the bond, as well as the difference in the halogen atoms and their position.

Halogenated hydrocarbons: classification

The first separation option is based on generally accepted principles that apply to all organic compounds. The classification is based on the difference in the type of carbon chain, its cyclicity. On this basis distinguish:

  • saturated halogenated hydrocarbons;
  • unsaturated;
  • aromatic;
  • aliphatic;
  • acyclic.

The following separation is based on the form of the halogen atom and its quantitative content in the composition of the molecule. So, allocate:

  • mono-derivatives;
  • dipro derivatives;
  • three-;
  • tetra;
  • pentapro derivatives and so on.

If we talk about the form of halogen, then the name of the subgroup consists of two words. For example, monochloro derivative, triiodo derivative, tetrabromohalogenalkene and so on.

There is also another classification option, according to which mainly halogenated saturated hydrocarbons are separated. This is the number of the carbon atom to which the halogen is attached. So, allocate:

  • primary derivatives;
  • secondary
  • tertiary and so on.

Each specific representative can be ranked according to all characteristics and determine the full place in the system of organic compounds. So, for example, a compound with the composition of CH 3 —CH 2 —CH═CH — CCL 3 can be classified as follows. This is an unsaturated aliphatic trichloro derivative of pentene.

chemical properties of halogenated hydrocarbons

Molecule structure

The presence of halogen atoms cannot but affect both the physical and chemical properties, and the general structural features of the molecule. The general formula for this class of compounds has the form R-Hal, where R is a free hydrocarbon radical of any structure, and Hal is a halogen atom, one or more. The bond between carbon and halogen is highly polarized, as a result of which the molecule as a whole is prone to two effects:

  • negative inductive;
  • mesomeric positive.

In this case, the first of them is much more pronounced; therefore, the Hal atom always exhibits the properties of an electron-withdrawing substituent.

Otherwise, all structural features of the molecule are no different from those of ordinary hydrocarbons. The properties are explained by the structure of the chain and its branching, the number of carbon atoms, the strength of aromatic features.

Of particular note is the nomenclature of halogenated hydrocarbons. What is the proper name for these connections? To do this, you must follow a few rules.

  1. The chain numbering starts from the edge to which the halogen atom is closer. If there is any multiple connection, then the counting starts precisely from it, and not from the electron-withdrawing substituent.
  2. The name Hal is indicated in the prefix, the number of the carbon atom from which it departs should also be indicated.
  3. The last step is the name of the main chain of atoms (or ring).

An example of a similar name: CH 2 = CH-CHCL 2 - 3-dichloropropene-1.

Also, the name can be given according to rational nomenclature. In this case, the name of the radical is pronounced, and then - the halogen with the suffix -id. Example: CH 3 —CH 2 —CH 2 Br — propyl bromide.

Like other classes of organic compounds, halogenated hydrocarbons have a special structure. This allows many representatives to designate historical names. For example, fluorotan CF 3 CBrClH. The presence of three halogens at once in the composition of the molecule provides this substance with special properties. It is used in medicine, therefore it is more often used precisely by its historically established name.

halogenated aromatic hydrocarbons

Synthesis Methods

Methods for producing halogenated hydrocarbons are quite diverse. Five main methods for the synthesis of these compounds in the laboratory and industry can be distinguished.

  1. Halogenation of normal normal hydrocarbons. General reaction scheme: RH + Hal 2 → R-Hal + HHal. The features of the process are as follows: with chlorine and bromine, ultraviolet radiation is absolutely necessary, with iodine the reaction is almost impossible or very slow. Interaction with fluorine is too active, therefore it is impossible to use this halogen in its pure form. In addition, when halogenating aromatic derivatives, special process catalysts, Lewis acids, must be used. For example, iron or aluminum chloride.
  2. Obtaining halogenated hydrocarbons is also carried out by hydrohalogenation. However, for this, the starting compound must necessarily be unsaturated hydrocarbon. Example: R = RR + HHal → RR-RHal. Most often, such an electrophilic addition is used to produce chloroethylene or vinyl chloride, since this compound is an important raw material for industrial syntheses.
  3. The effect of hydrohalogens on alcohols. General reaction type: R-OH + HHal → R-Hal + H 2 O. A specific feature is the obligatory presence of a catalyst. Examples of process accelerators that can be used: phosphorus, sulfur, zinc or iron chlorides, sulfuric acid, a solution of zinc chloride in hydrochloric acid - Lucas reagent.
  4. Decarboxylation of acid salts with an oxidizing agent. Another name for the method is the Borodin-Hunsdicker reaction. The bottom line is the removal of a carbon dioxide molecule from silver derivatives of carboxylic acids when exposed to an oxidizing agent - halogen. As a result, halogenated hydrocarbons are formed. The reactions in general form look like this: R-COOAg + Hal → R-Hal + CO 2 + AgHal.
  5. The synthesis of haloforms. In other words, this is the preparation of trihalogenated methane. The easiest way to produce them is to expose acetone to an alkaline halogen solution. As a result, haloform molecules are formed. In the same way, halogenated aromatic hydrocarbons are synthesized in industry.

Particular attention should be paid to the synthesis of unsaturated representatives of the class in question. The main method is exposure to alkynes with mercury and copper salts in the presence of halogens, which leads to the formation of a product with a double bond in the chain.

Halogenated aromatic hydrocarbons are obtained by halogenation of arenes or alkylarenes in the side chain. These are important industrial products, as they are used as insecticides in agriculture.

halogenated hydrocarbons application

Physical properties

The physical properties of halogenated hydrocarbons directly depend on the structure of the molecule. The boiling and melting temperatures, the state of aggregation are affected by the number of carbon atoms in the chain and possible branches to the side. The more of them, the higher the rates. In general, physical parameters can be characterized in several points.

  1. Aggregate state: the first lower representatives are gases, the following up to C 12 are liquids, and above are solids.
  2. Almost all representatives have a sharp unpleasant specific smell.
  3. Very poorly soluble in water, but they themselves are excellent solvents. They dissolve very well in organic compounds.
  4. The boiling and melting points increase with increasing number of carbon atoms in the main chain.
  5. All compounds except fluorine derivatives are heavier than water.
  6. The more branches in the main chain, the lower the boiling point of the substance.

It is difficult to identify many similarities in common, because the representatives vary greatly in composition and structure. Therefore, it is better to give values ​​for each specific compound from a given series of hydrocarbons.

Chemical properties

One of the most important parameters, which is necessarily taken into account in the chemical industry and synthesis reactions, are the chemical properties of halogenated hydrocarbons. They are not the same for all representatives, as there are a number of reasons for the difference.

  1. The structure of the carbon chain. The simplest substitution reactions (nucleophilic type) occur in secondary and tertiary haloalkyls.
  2. The type of halogen atom is also important. The bond between carbon and Hal is strongly polarized, which ensures its easy rupture with the release of free radicals. However, the bond between iodine and carbon is most easily broken, which is explained by a regular change (decrease) in the binding energy in the series: F-Cl-Br-I.
  3. The presence of an aromatic radical or multiple bonds.
  4. The structure and branching of the radical itself.

In general, haloalkyls enter the reaction of nucleophilic substitution best. Indeed, after a bond with a halogen is broken, a partially positive charge is concentrated on the carbon atom. This allows the radical as a whole to become an acceptor of electron-negative particles. For instance:

  • OH - ;
  • SO 4 2- ;
  • NO 2 - ;
  • CN - and others.

This explains the fact that one can go from almost any class of organic compounds from halogenated hydrocarbons, you just need to select the appropriate reagent that will provide the desired functional group.

In general, we can say that the chemical properties of halogenated hydrocarbons are the ability to enter into the following interactions.

  1. With nucleophilic particles of various kinds - substitution reactions. The result can be: alcohols, ethers and esters, nitro compounds, amines, nitriles, carboxylic acids.
  2. Elimination or dehydrohalogenation reactions. As a result of exposure to an alcohol solution of alkali, the hydrogen halide molecule is cleaved. This forms alkene, low molecular weight by-products - salt and water. Reaction example: CH 3 —CH 2 —CH 2 —CH 2 Br + NaOH (alcohol) → CH 3 —CH 2 —CH = CH 2 + NaBr + H 2 O. These processes are one of the main methods for the synthesis of important alkenes. The process is always accompanied by high temperatures.
  3. Obtaining alkanes of normal structure by the method of Wurz synthesis. The essence of the reaction is the impact on the halogen-substituted hydrocarbon (two molecules) with sodium metal. As a strongly electropositive ion, sodium accepts halogen atoms from the compound. As a result, the released hydrocarbon radicals are closed together by a bond, forming a new alkane. Example: CH 3 —CH 2 Cl + CH 3 —CH 2 Cl + 2Na → CH 3 —CH 2 —CH 2 —CH 3 + 2NaCl.
  4. The synthesis of homologues of aromatic hydrocarbons according to the Friedel-Crafts method. The essence of the process is the exposure of benzene to halogenated in the presence of aluminum chloride. As a result of the substitution reaction, the formation of toluene and hydrogen chloride occurs. In this case, the presence of a catalyst is necessary. In addition to benzene itself, its homologues can be oxidized in this way.
  5. Getting Greniard fluid. This reagent is a halogen-substituted hydrocarbon with a magnesium ion in its composition. Initially, the effect of metallic magnesium in ether on the halogenated alkyl is carried out. As a result, a complex compound is formed with the general formula RMgHal, called the Greniard reagent.
  6. Reduction reactions to alkane (alkene, arena). Carried out by exposure to hydrogen. As a result, a hydrocarbon is formed and a by-product - hydrogen halide. General example: R-Hal + H 2 → RH + HHal.

These are the main interactions into which halogenated hydrocarbons of various structures are able to easily enter. Of course, there are specific reactions that should be considered for each specific representative.

halogenated hydrocarbons structure

Isomerism of molecules

The isomerism of halogenated hydrocarbons is a completely natural phenomenon. After all, it is known that the more carbon atoms in the chain, the higher the number of isomeric forms. In addition, unsaturated representatives have multiple bonds, which also causes the appearance of isomers.

Two main varieties of this phenomenon can be distinguished for this class of compounds.

  1. Isomerism of the carbon skeleton of the radical and the main chain. The position of the multiple bond, if it exists in the molecule, can also be attributed to this. As with simple hydrocarbons, starting from the third representative, one can write the formulas of compounds having identical molecular but different structural formula expressions. Moreover, for halogenated hydrocarbons, the number of isomeric forms is an order of magnitude higher than for their corresponding alkanes (alkenes, alkynes, arenes, and so on).
  2. The position of the halogen in the composition of the molecule. Its place in the name is indicated by a number, and even if it changes by only one, the properties of such isomers will already be completely different.

We are not talking about spatial isomerism, since halogen atoms make this impossible. As with all other organic compounds, isomers of haloalkyls differ not only in structure, but also in physical and chemical characteristics.

halogenated unsaturated hydrocarbons

Unsaturated hydrocarbon derivatives

Of course, there are many similar compounds. However, it is halogen derivatives of unsaturated hydrocarbons that interest us. They can also be divided into three main groups.

  1. Vinyl - when the Hal atom is located directly at the carbon atom of a multiple bond. An example of a molecule: CH 2 = CCL 2.
  2. With an isolated position. A halogen atom and a multiple bond are located in opposite parts of the molecule. Example: CH 2 = CH — CH 2 —CH 2 —Cl.
  3. Allyl derivatives - a halogen atom is located to a double bond through one carbon atom, that is, it is in the alpha position. Example: CH 2 = CH — CH 2 —CL.

Of particular importance is a compound such as vinyl chloride CH 2 = CHCL. It is capable of polymerization reactions with the formation of important products, such as insulating materials, waterproof fabrics and so on.

Another representative of unsaturated halogen derivatives is chloroprene. Its formula is CH₂ = CCL-CH = CH₂. This compound is the starting material for the synthesis of valuable types of rubber, which are fire resistant, have a long service life and poor gas permeability.

Tetrafluoroethylene (or Teflon) is a polymer that has quality technical parameters. Used for the manufacture of valuable coatings for technical parts, utensils, various appliances. The formula is CF 2 = CF 2 .

Aromatic hydrocarbons and their derivatives

Aromatic compounds are those compounds that include a benzene ring. Among them there is also a whole group of halogen derivatives. There are two main types of them in structure.

  1. If the Hal atom is bonded directly to the nucleus, that is, the aromatic ring, then it is customary to call the compounds haloarenes.
  2. The halogen atom is not bound to the ring, but to the side chain of atoms, that is, a radical extending into the side branch. Such compounds are called arylalkyl halides.

Among the substances under consideration, several representatives of the greatest practical importance can be named.

  1. Hexachlorobenzene - C 6 Cl 6 . Since the beginning of the 20th century, it has been used as a strong fungicide, as well as an insecticide. It has a good disinfecting effect, so it was used for seed treatment before sowing. It has an unpleasant odor, the liquid is quite caustic, transparent, can cause lacrimation.
  2. Benzyl bromide C 6 H 5 CH 2 Br. .
  3. 6 5 CL. , . , . .

Industrial use

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Source: https://habr.com/ru/post/G14712/

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