Several classes of hydrocarbons are distinguished depending on the number of multiple bonds between carbon atoms. Let us dwell in more detail on diene compounds, the features of their structure, physical and chemical properties.
Structure
What are alkadienes? The physical properties of representatives of this class of organic compounds are similar to those of alkanes and alkenes. Dienes have the general formula SpN2p-2, complex bonds, therefore, belong to unsaturated hydrocarbons.
These bonds can be located in different positions, forming different variants of dienes:
- cumulated, in which multiple bonds are located on two sides of one carbon atom;
- conjugated (conjugated), in which one single is located between double bonds;
- isolated, in which several single species are located between double bonds.
In such substances, all the carbons in the double bond are in the sp2 hybrid state. What characteristics do alkadienes have? The physical properties of such compounds are determined precisely by their structural features.
Nomenclature
According to the systematic nomenclature, diene hydrocarbons are named according to the same principle by which they are called ethylene compounds. There are some distinguishing characteristics that can easily be explained by the presence of two double bonds in their molecules.
First, it is necessary to identify the longest carbon chain in the carbon skeleton, in which there are two double bonds. Based on the number of carbon atoms, the basis for the name is selected, then the suffix – diene is added to it. The numbers indicate the position of each link, starting with the smallest.
For example, according to the systematic nomenclature, the substance pentadiene-1, 3 has the following structure:
H 2 C = CH — CH = CH — CH 3.
In the systematic nomenclature there are some surviving names: allen, divinyl, isoprene.
Types of isomerism
Alkadienes, whose physical properties depend on the number of carbon atoms in the molecule, have several types of isomerism:
- positions of multiple bonds;
- carbon skeleton;
- interclass view.
Let us now dwell on issues related to determining the amount of isomers of diene hydrocarbons.
Isomer Tasks
"Determine the number of isomeric compounds and name the physical properties of alkadienes" - in grade 10 according to the school curriculum, in organic chemistry lessons, students are asked many questions of a similar nature. In addition, you can meet the tasks associated with unsaturated hydrocarbons in a single state exam in chemistry.
For example, you must specify all the isomers of the composition of C 4 H 6 , and also give them a name according to the systematic nomenclature. First of all, you can make all the alkadiene, the physical properties of which are similar to ethylene compounds:
H 2 C = CH — CH = CH 2.
This compound is a gaseous substance that is insoluble in water. According to a systematic nomenclature, it will be called butadiene -1.3.
When moving a multiple bond through the structure, one can obtain an isomer of the following form:
H 3 C-CH = CH = CH 2
It has the following name: butadiene -1.2
In addition to the isomers of the position of the multiple bond, for the C 4 H 6 composition, one can also consider interclass isomerism, namely, representatives of the class of alkynes.
Features of the preparation of diene compounds
How do alkadienes get? The physical and chemical properties of representatives of this class can only be fully studied if rational methods exist for their laboratory and industrial production.
Considering the fact that divinyl and isoprene are the most popular in modern production, we will consider options for obtaining these diene hydrocarbons.
In industry, these representatives of unsaturated compounds are obtained in the process of dehydrogenation of the corresponding alkanes or alkenes above the catalyst, which is chromium oxide (3).
Raw materials for this process are isolated during the processing of associated gas or from petroleum products.
Butadiene-1,3 was synthesized from ethanol in the process of dehydrogenation and dehydration by Academician Lebedev. This method, which involves the use of zinc or aluminum oxides as a catalyst and proceeding at a temperature of 450 degrees Celsius, was taken as the basis for the industrial synthesis of divinyl. The equation of this process has the following form:
2C 2 H5OH —————— H 2 C = CH — CH = CH 2 + 2H 2 O + H 2 .
In addition, it is possible to isolate isoprene and divil in small quantities by pyrolysis of oil.
Physical Features
In what aggregate state are alkadienes? Physical properties, the table of which contains information on the melting and boiling points, indicates that the lower representatives of this class are gaseous states with low boiling and melting temperatures.
With an increase in relative molecular weight, there is a tendency to increase these indicators, a transition from a liquid state of aggregation.
It will help you to study in detail the physical properties of the alkadienes table. Photos showing the products obtained from these compounds are presented above.
Chemical properties
If we consider isolated (non-conjugated) double bonds, they have the same capabilities as typical ethylene hydrocarbons.
We have analyzed the physical properties of alkadienes; we will consider examples of their possible chemical interactions on butadiene -1.3.
Compounds having conjugated double bonds have a higher reactivity compared to other types of dienes.
Addition reactions
For all types of dienes, compound reactions are characteristic . Among them, we note halogenation. This reaction leads to the conversion of diene to the corresponding alkene. If hydrogen is taken in excess, a saturated hydrocarbon can be obtained. We represent the process in the form of an equation:
H 3 C — CH = CH = CH 2 + 2H 2 = H 3 C — CH 2 —CH 2 —CH 3.
Halogenation involves the interaction of a diene compound with a diatomic molecule of chlorine, iodine, and bromine.
The reaction of hydration (addition of water molecules) and hydrohalogenation (for diene compounds having a double bond in the first position) proceeds according to the Markovnikov rule. Its essence is that when the bond is broken, the hydrogen atoms will attach to those carbon atoms that have a smaller amount of hydrogen, and the atoms of the hydroxyl group or halogen will attach to those C atoms at which a smaller amount of hydrogen is located.
In diene synthesis, an ethylene compound or alkyno molecule is attached to a diene having conjugated double bonds.
These interactions are used in the production of various organic cyclic compounds.
The polymerization of representatives of diene compounds is of particular importance. The physical properties of alkadienes and their application are associated with this process. During their polymerization, rubbery, high molecular weight compounds are formed. For example, from butadiene-1,3, butadiene rubber having wide industrial application can be obtained.
Characterization of individual diene compounds
What are the physical properties of alkadienes? Briefly analyze the features of isoprene and divinyl.
Butadiene -1.3 is a gaseous gas with a specific pungent odor. It is this compound that is the starting monomers for the production of latexes, synthetic rubbers, plastics, as well as many organic compounds.
2-methylbutadiene-1,3 (isoprene) is a colorless liquid that is a structural component of natural rubber.
2-chlorobutadiene-1,3 (chloroprene) is a toxic liquid, which is the basis for the manufacture of vinylacetylene, the industrial production of synthetic chloroprene rubber.
Rubbers and rubbers
Rubber and rubber are elastomers. There is a division of all rubbers into synthetic and natural.
Natural rubber is a highly elastic mass, which is obtained from milk juice. Latex is a suspension of small particles of rubber in water that exists in tropical trees such as Brazilian Hevea, as well as in some plants.
This unsaturated polymer has a composition (C 5 H 8 ) p, in which the average molecular weight ranges from 15,000 to 500,000.
In the course of research, it was found that the structural unit of natural rubber has the form -CH2-C = CH-CH2-.
As its main distinguishing characteristics, one can note excellent elasticity, the ability to withstand significant mechanical deformations, maintain shape after stretching. Natural rubber is able to dissolve in some hydrocarbons, while forming viscous solutions.
Like diene compounds, it is capable of entering into addition reactions. Gutta-percha acts as a variety of isoprene polymer. This compound does not have increased elasticity, since it has differences in the structure of macromolecules.
Products made from rubber have certain disadvantages. For example, if the temperature rises, they become sticky, change their shape, and when the temperature drops, they become overly fragile.
In order to get rid of such shortcomings, in industry resort to vulcanization of rubber. The essence of this process is to give it heat resistance, elasticity when processing sulfur.
The process takes place at temperatures in the range of 140-180 ° C in special devices. As a result, rubber is formed, the sulfur content of which reaches 5%. She "sews" rubber macromolecules, forming a mesh structure. In addition to sulfur, rubber also contains additional fillers: dyes, plasticizers, antioxidants.
Due to the high demand in industry for rubber products, most of it is produced synthetically.