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Halogenated hydrocarbons: production, chemical properties, application

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

Halogenated hydrocarbons: general characteristics

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. For quite a short time people have 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 of obtaining these compounds is a synthetic pathway in the laboratory and in industry, since practically none of them occur in nature. Due to the presence of a halogen atom, they have a high reactivity. This largely determines the field of their application in chemical synthesis as intermediate products.

Since representatives of halogenated hydrocarbons have a lot, it is customary to classify them according to different characteristics. The basis is the structure of the chain and the multiplicity of the bond, as well as the difference in the halogen atoms and the location of their position.

Halogenated hydrocarbons: classification

The first option of separation 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 the basis of this feature distinguish:

  • Limiting halogenated hydrocarbons;
  • Unsaturated;
  • Aromatic;
  • Aliphatic;
  • Acyclic.

The next division is based on the form of the halogen atom and its quantitative content in the molecule. So, they distinguish:

  • Mono derivatives;
  • Di-derivatives;
  • three-;
  • Tetra-;
  • Penta derivatives and so on.

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

There is also another classification option, according to which the halogenated derivatives of the ultimate hydrocarbons are separated. This is the number of the carbon atom to which the halogen is attached. So, they distinguish:

  • Primary derivatives;
  • Secondary;
  • Tertiary and so on.

Each specific representative can be ranked by all traits and determine the total place in the system of organic compounds. Thus, for example, a compound with the composition CH 3 -CH 2 -CH═CH-CCL 3 can be classified as follows. This is an unsaturated aliphatic trichloro derivative of pentene.

The structure of the molecule

The presence of halogen atoms can not but affect both the physical and chemical properties, and on the general outline of the structure of the molecule. The general formula for this class of compounds is 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, so that the molecule as a whole is prone to two effects:

  • Negative inductive;
  • Mesomeric positive.

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

Otherwise, all the features of the structure of the molecule do not differ from those of ordinary hydrocarbons. Properties are explained by the structure of the chain and its branching, the number of carbon atoms, the strength of aromatic features.

The nomenclature of halogenated hydrocarbons deserves special attention. How correctly should you call these connections? To do this, you must follow several rules.

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

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

The name can also be given on 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 historically formed names. For example, fluorotane CF 3 CBrClH. The presence of three halogens in the composition of the molecule provides this substance with special properties. It is used in medicine, so it is often used historically formed name.

Methods of synthesis

The methods for producing halogenated hydrocarbons are quite diverse. There are five main methods for synthesizing these compounds in the laboratory and industry.

  1. Halogenation of normal hydrocarbons of normal structure. The general reaction scheme is: RH + Hal2 → R-Hal + HHal. The peculiarities of the process are as follows: chlorine and bromine necessarily require ultraviolet irradiation, with iodine the reaction is practically impossible or very slow. With fluorine, the interaction is too active, so you can not use this halogen in its pure form. In addition, halogenating aromatic derivatives requires the use of special catalysts of the Lewis acid process. For example, iron or aluminum chloride.
  2. The production of halogenated hydrocarbons is also carried out by hydrohalogenation. However, for this, the starting compound must necessarily be an unsaturated hydrocarbon. Example: R = RR + HHal → RR-RHal. Most often, this 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 hydrohalogen on alcohols. General view of the reaction: R-OH + HHal → R-Hal + H 2 O. A special feature is the obligatory presence of a catalyst. Examples of process accelerators that can be used are: phosphorus, sulfur, zinc or iron chlorides, sulfuric acid, zinc chloride solution in hydrochloric acid - Lucas reagent.
  4. Decarboxylation of acid salts with an oxidizing agent. Another name for the method is the Borodin-Hunsdikker reaction. The essence lies in the splitting off of the carbon dioxide molecule from silver derivatives of carboxylic acids under the action of 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. Synthesis of haloforms. In other words, this is the production of trihalogenated methane derivatives. The simplest way to produce them is to apply acetal to the halogen solution. As a result, halogen molecules are formed. Halogen derivatives of aromatic hydrocarbons are synthesized in the same way in industry.

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

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

Physical properties

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

  1. Aggregate state: the first lower representatives - gases, subsequent to C 12 - liquids, higher - solids.
  2. They have a sharp, unpleasant, specific smell, almost all representatives.
  3. Very poorly soluble in water, but themselves - excellent solvents. In organic compounds dissolve very well.
  4. The boiling and melting temperatures increase with the number of carbon atoms in the main chain.
  5. All compounds except fluoro 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 a lot of similar similarities, because representatives vary greatly in composition and structure. Therefore, it is better to give values for each particular 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, since 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 form of the halogen atom is also important. The bond between carbon and Hal is strongly polarized, which provides an easy break with the release of free radicals. However, the easiest connection is precisely between iodine and carbon, which is explained by a natural 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, halogenoalkyls are most likely to react specifically to nucleophilic substitution. After all, a partially positive charge is concentrated on the carbon atom after disruption of the bond with the halogen. This allows the radical as a whole to become an acceptor of electron-negative particles. For example:

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

This explains the fact that halogenated hydrocarbons can be converted to virtually any class of organic compounds, it is only necessary to select an appropriate reagent that will provide the desired functional group.

In general, it can be said 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. As a result, alcohols, ethers and esters, nitro compounds, amines, nitriles, carboxylic acids can be obtained.
  2. Elimination or dehydrohalogenation reactions. As a result of the action of an alkali alcohol solution, the hydrogen halide molecule is cleaved off. So alkene is formed, low-molecular 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 ways of synthesis of important alkenes. The process is always accompanied by high temperatures.
  3. Preparation of alkanes of a normal structure by the Würz synthesis method. The essence of the reaction is the effect on the halogen-substituted hydrocarbon (two molecules) by metallic sodium. As a highly electropositive ion, sodium accepts halogen atoms from the compound. As a result, the liberated hydrocarbon radicals are closed together by a bond, forming an alkane of a new structure. Example: CH 3 -CH 2 Cl + CH 3 -CH 2 Cl + 2Na → CH 3 -CH 2 -CH 2 -CH 3 + 2NaCl.
  4. Synthesis of homologues of aromatic hydrocarbons by the Friedel-Crafts method. The essence of the process is in the action of benzene with haloalkyl 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, it is also possible to oxidize its homologues in this way.
  5. Preparation of Greniard liquid. This reagent is a halogenated hydrocarbon with a magnesium ion in the composition. Initially, metal magnesium in the ether is exposed to the haloalkyl derivative. As a result, a complex compound is formed with the general formula RMgHal, referred to as the Grenyar reagent.
  6. Reaction of reduction to alkane (alkene, arene). Carried out under the action of hydrogen. As a result, a hydrocarbon is formed and a by-product, hydrogen halide. An example in the general form: R-Hal + H 2 → RH + HHal.

These are the main interactions in which halogen derivatives of hydrocarbons of different structures can readily enter. Of course, there are specific reactions that should be considered for each particular representative.

Isomerism of molecules

The isomerism of halogenated hydrocarbons is quite natural. 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.

We can distinguish two main varieties of this phenomenon for this class of compounds.

  1. Isomerism of the carbon skeleton of a radical and a backbone. This also includes the position of the multiple bond, if it exists in the molecule. As with simple hydrocarbons, starting from the third representative, it is possible to write down the formulas of compounds having identical molecular but different structural formula expressions. Moreover, for halogenated hydrocarbons, the amount of isomeric forms is an order of magnitude higher than for the corresponding alkanes (alkenes, alkynes, arenes, etc.).
  2. The position of the halogen in the molecule. Its place in the title is indicated by a digit, and even if it changes only by one, the properties of such isomers will be completely different.

It is not a question of spatial isomerism, since halogen atoms make this impossible. Like all other organic compounds, the halogenoalkyl isomers differ not only in structure, but also in their physical and chemical characteristics.

Derivatives of unsaturated hydrocarbons

Of course, there are many such connections. However, we are interested in the halogen derivatives of unsaturated hydrocarbons. They can also be divided into three main groups.

  1. Vinyl - when the atom Hal is located directly at the carbon atom of a multiple bond. Example of the molecule: CH 2 = CCL 2.
  2. With isolated position. The halogen atom and the multiple bond are located in opposite parts of the molecule. Example: CH 2 = CH-CH 2 -CH 2 -Cl.
  3. Allylic derivatives - a halogen atom is located to double bond through one carbon atom, that is, 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 the like.

Another representative of unsaturated halogen derivatives is chloroprene. Its formula is CH₂ = CCL-CH = CH. This compound is the starting material for the synthesis of valuable types of rubber, which differ in fire resistance, long service life, poor permeability for gases.

Tetrafluoroethylene (or Teflon) is a polymer that has qualitative technical parameters. It is used for the manufacture of valuable coverage of technical parts, dishes, various appliances. The formula is CF 2 = CF 2 .

Aromatic hydrocarbons and their derivatives

Aromatic are those compounds, which include a benzene ring. Among them there is also a whole group of halogen derivatives. We can distinguish two main types of them in structure.

  1. If the Hal atom is directly connected to the nucleus, that is, the aromatic ring, then the compounds are commonly called haloarenes.
  2. The halogen atom is bonded not to the ring, but to the side chain of the atoms, that is, the radical that extends into the lateral branch. Such compounds are called arylalkyl halides.

Among the substances under consideration are several representatives of the greatest practical importance.

  1. Hexachlorobenzene - C6Cl6. 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 to dress seeds before sowing. Has an unpleasant smell, the liquid is quite caustic, clear, can cause lachrymation.
  2. Benzyl bromide C 6 H 5 CH 2 Br. Used as an important reagent in the synthesis of organometallic compounds.
  3. Chlorobenzene C 6 H 5 CL. Liquid colorless substance with a specific odor. Used in the manufacture of dyes, pesticides. It is one of the best organic solvents.

Use in industry

Halogenated hydrocarbons are very widely used in industry and chemical synthesis. We have already spoken about the unsaturated and aromatic representatives. Now let's designate in general the use areas of all compounds of this series.

  1. In construction.
  2. As solvents.
  3. In the production of textiles, rubber, rubbers, dyes, polymeric materials.
  4. For the synthesis of many organic compounds.
  5. Fluoride derivatives (freons) are refrigerants in refrigeration plants.
  6. Used as pesticides, insecticides, fungicides, oils, drying oils, resins, lubricants.
  7. Go to the manufacture of insulation materials, etc.

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