Halogenated hydrocarbons: production, chemical properties, application

2024 Author: Angel Austin | [email protected]. Last modified: 2023-12-17 05:14

Hydrocarbons are a very large class of organic compounds. They include several main groups of substances, among which almost every one is widely used in industry, everyday life, and nature. Of particular importance are halogenated hydrocarbons, which will be discussed in the article. They are not only of high industrial importance, but are also important raw materials for many chemical syntheses, obtaining medicines and other important compounds. Let's 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 broad category of substances, as they are of great industrial importance. For quite a short time peoplelearned how to synthesize almost all halogen derivatives of 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 occurs in nature. Due to the presence of a halogen atom, they are highly reactive. This largely determines the scope of their application in chemical syntheses as intermediates.

Since there are many representatives of halogenated hydrocarbons, it is customary to classify them according to different criteria. It is based on both the structure of the chain and the multiplicity of bonds, and the difference in the halogen atoms and their position.

Halogen derivatives of 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, they distinguish:

• limited halogenated hydrocarbons;
• unlimited;
• aromatic;
• aliphatic;
• acyclic.

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

• mono derivatives;
• diderivatives;
• three-;
• tetra-;
• penta derivatives and so on.

If we talk about the type of halogen, then the name of the subgroup consists of two words. For example, the monochloro derivative,triiodine derivative, tetrabromohaloalkene and so on.

There is also another classification option, according to which mainly halogen derivatives of 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 by all signs and determine the full place in the system of organic compounds. So, for example, a compound with the composition CH₃ - CH₂-CH=CH-CCL₃ can classify like this. It is an unsaturated aliphatic trichloro derivative of pentene.

The structure of the molecule

The presence of halogen atoms cannot but affect both the physical and chemical properties, and the general features 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 strongly polarized, as a result of which the molecule as a whole is prone to two effects:

• negative inductive;
• mesomeric positive.

The first of them is much more pronounced, so the Hal atom always exhibits the properties of an electron-withdrawing substituent.

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

The nomenclature of halogen derivatives of hydrocarbons deserves special attention. What is the correct name for these connections? To do this, you need to follow a few rules.

1. The numbering of the chain starts from the edge closest to the halogen atom. If there is any multiple bond, then the countdown starts 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 to name the main chain of atoms (or ring).

An example of a similar name: CH₂=CH-CHCL₂ - 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 name of the halogen with the suffix -id. Example: CH₃-CH₂-CH₂Br - propyl bromide.

Like other classes of organic compounds, halogenated hydrocarbons have a special structure. This allows many representatives to be designated by historical names. For example, halothane CF₃CBrClH. 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 the historical name that is most often used.

Synthesis Methods

Methods for obtaining halogen derivatives of hydrocarbons are sufficientvaried. There are five main methods for the synthesis of these compounds in the laboratory and industry.

1. Halogenation of conventional normal hydrocarbons. General reaction scheme: R-H + Hal₂ → R-Hal + HHal. The features of the process are as follows: with chlorine and bromine, ultraviolet irradiation is necessary, with iodine the reaction is almost impossible or very slow. The interaction with fluorine is too active, so this halogen cannot be used in its pure form. In addition, the halogenation of aromatic derivatives requires the use of special process catalysts - Lewis acids. For example, iron or aluminum chloride.
2. Obtaining halogen derivatives of hydrocarbons is also carried out by hydrohalogenation. However, for this, the starting compound must necessarily be an unsaturated hydrocarbon. Example: R=R-R + HHal → R-R-RHal. Most often, such an electrophilic addition is used to obtain chlorethylene or vinyl chloride, since this compound is an important raw material for industrial syntheses.
3. The effect of hydrohalogens on alcohols. General view of the reaction: R-OH + HHal→R-Hal + H₂O. A feature is the mandatory presence of a catalyst. Examples of process accelerators that can be used are phosphorus, sulfur, zinc or iron chlorides, sulfuric acid, a solution of zinc chloride in hydrochloric acid - Lucas reagent.
4. Decarboxylation of acid s alts with an oxidizing agent. Another name for the method is the Borodin-Hunsdicker reaction. The bottom line is the removal of a carbon dioxide moleculefrom silver derivatives of carboxylic acids when exposed to an oxidizing agent - halogen. As a result, halogen derivatives of hydrocarbons are formed. Reactions in general look like this: R-COOAg + Hal → R-Hal + CO₂ + AgHal.
5. Synthesis of haloforms. In other words, this is the production of trihalogen derivatives of methane. The easiest way to produce them is to treat acetone with an alkaline solution of halogens. As a result, the formation of haloform molecules occurs. Halogen derivatives of aromatic hydrocarbons are synthesized in the industry in the same way.

Special attention should be paid to the synthesis of unlimited representatives of the considered class. The main method is the treatment of alkynes with mercury and copper s alts 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 halogenation reactions of arenes or alkylarenes into a side chain. These are important industrial products as they are used as insecticides in agriculture.

Physical properties

The physical properties of halogen derivatives of hydrocarbons directly depend on the structure of the molecule. The boiling and melting points, 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 scores. In general, it is possible to characterize the physical parameters in several points.

1. Aggregate state: the first lowestrepresentatives - gases, subsequent to С₁₂ - liquids, above - solids.
2. Almost all representatives have a sharp unpleasant specific smell.
3. Very poorly soluble in water, but excellent solvents themselves. They dissolve very well in organic compounds.
4. Boiling and melting points increase with the 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 differ 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 that must be taken into account in the chemical industry and synthesis reactions is 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 (of the nucleophilic type) occur with secondary and tertiary haloalkyls.
2. The type of halogen atom is also important. The bond between carbon and Hal is strongly polarized, which makes it easy to break with the release of free radicals. However, the bond between iodine and carbon breaks most easily, which is explained by a regular change (decrease) in the bond energy in the series: F-Cl-Br-I.
3. The presence of aromaticradical or multiple bonds.
4. Structure and branching of the radical itself.

In general, halogenated alkyls react best with nucleophilic substitution. After all, a partially positive charge is concentrated on the carbon atom after breaking the bond with the halogen. This allows the radical as a whole to become an acceptor of electronegative particles. For example:

• OH^-;
• SO₄^2-;
• NO₂^-;
• CN^- and others.

This explains the fact that it is possible to go from halogen derivatives of hydrocarbons to almost any class of organic compounds, you just need to choose the appropriate reagent that will provide the desired functional group.

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

1. With various kinds of nucleophilic particles - substitution reactions. The result can be: alcohols, ethers and esters, nitro compounds, amines, nitriles, carboxylic acids.
2. Reactions of elimination or dehydrohalogenation. As a result of exposure to an alcoholic solution of alkali, a hydrogen halide molecule is cleaved. This is how an alkene is formed, low molecular weight by-products - s alt and water. Reaction example: CH₃-CH₂-CH₂-CH₂ Br + NaOH _(alcohol) →CH₃-CH₂-CH=CH ₂ + NaBr + H₂O. These processes are one of the main ways to synthesize important alkenes. The process is always accompanied by high temperatures.
3. Obtaining alkanes of a normal structure by the Wurtz synthesis method. The essence of the reaction is the effect on a halogen-substituted hydrocarbon (two molecules) with metallic sodium. As a strongly electropositive ion, sodium accepts halogen atoms from the compound. As a result, the liberated hydrocarbon radicals are closed by a bond, forming an alkane of a new structure. Example: CH₃-CH₂Cl + CH₃-CH₂ Cl + 2Na →CH₃-CH₂-CH₂-CH 3 _{+ 2NaCl.}
4. Synthesis of homologues of aromatic hydrocarbons by the Friedel-Crafts method. The essence of the process is the action of haloalkyl on benzene 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 also be oxidized in this way.
5. Getting the Greignard liquid. This reagent is a halogen-substituted hydrocarbon with a magnesium ion in the composition. Initially, metallic magnesium in ether acts on the haloalkyl derivative. As a result, a complex compound with the general formula RMgHal is formed, called the Greignard reagent.
6. Reduction reactions to alkane (alkene, arena). Carried out when exposed to hydrogen. As a result, a hydrocarbon and a by-product, hydrogen halide, are formed. General example: R-Hal + H₂ →R-H + HHal.

These are the main interactions in whichhalogen derivatives of hydrocarbons of various structures are able to easily enter. Of course, there are specific reactions that should be considered for each individual representative.

Isomerism of molecules

Isomerism of halogenated hydrocarbons is quite a 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 isomers to appear.

There are two main varieties of this phenomenon for this class of compounds.

1. Isomerism of the carbon skeleton of the radical and the main chain. This also includes the position of the multiple bond, if it exists in the molecule. As with simple hydrocarbons, starting from the third representative, formulas of compounds that have identical molecular but different structural formula expressions can be written. Moreover, for halogen-substituted 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 molecule. Its place in the name is indicated by a number, and even if it changes by only one, then the properties of such isomers will already be completely different.

Spatial isomerism is out of the question here, because halogen atoms make it impossible. Like all other organic compounds, haloalkyl isomers differ not only in structure, but also in physical and chemical properties.characteristics.

Derivatives of unsaturated hydrocarbons

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

1. Vinyl - when the Hal atom is located directly at the carbon atom of the multiple bond. Molecule example: CH₂=CCL_2.
2. With insulated position. The halogen atom and the multiple bond are located in opposite parts of the molecule. Example: CH₂=CH-CH₂-CH₂-Cl.
3. Allyl derivatives - the halogen atom is located to the double bond through one carbon atom, that is, it is in the alpha position. Example: CH₂=CH-CH₂-CL.

Of particular importance is vinyl chloride CH₂=CHCL. It is capable of polymerization reactions to form important products such as insulation materials, waterproof fabrics, and more.

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

Tetrafluoroethylene (or Teflon) is a polymer that has high-quality technical parameters. It is used for the manufacture of a valuable coating of technical parts, utensils, various appliances. Formula - CF₂=CF₂.

Aromatichydrocarbons and their derivatives

Aromatic compounds are those compounds that include a benzene ring. Among them there is also a whole group of halogen derivatives. Two main types can be distinguished by their structure.

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

Among the substances under consideration, there 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 for dressing seeds before sowing. It has an unpleasant odor, the liquid is quite caustic, transparent, and can cause lacrimation.
2. Benzyl bromide С₆Н₅CH₂Br. Used as an important reagent in the synthesis of organometallic compounds.
3. Chlorobenzene C₆H₅CL. Liquid colorless substance with a specific odor. It is used in the production of dyes, pesticides. It is one of the best organic solvents.

Industrial use

Halogen derivatives of hydrocarbons are used in industry and chemical synthesisvery wide. We have already spoken about unsaturated and aromatic representatives. Now let's denote in general the areas of use of all compounds of this series.

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