From the point of view of chemistry, propane is a saturated hydrocarbon with typical properties of alkanes. However, in some areas of production, propane is understood as a mixture of two substances - propane and butane. Next, we will try to figure out what propane is and why it is mixed with butane.
The structure of the molecule
Each propane molecule consists of three carbon atoms linked to each other by simple single bonds, and eight hydrogen atoms. It has the molecular formula C3H8. The C-C bonds in propane are covalent non-polar, but in the C-H pair, carbon is slightly more electronegative and slightly pulls the common electron pair towards itself, which means that the bond is covalent polar. The molecule has a zigzag structure due to the fact that carbon atoms are in a state of sp3-hybridization. But, as a rule, the molecule is said to be linear.
There are four carbon atoms in the butane molecule С4Н10, and it has two isomers: n-butane (has a linear structure) and isobutane (hasbranched structure). Often, they do not separate upon receipt, but exist as a mixture.
Physical properties
Propane is a colorless and odorless gas. It dissolves very poorly in water, but it dissolves well in chloroform and diethyl ether. It melts at tpl=-188 °С, and boils at tkip=-42 °С. It becomes explosive when its concentration in the air exceeds 2%.
The physical properties of propane and butane are very close. Both butanes also have a gaseous state under normal conditions and are odorless. Practically insoluble in water, but interact well with organic solvents.
The following characteristics of these hydrocarbons are also important in the industry:
- Density (the ratio of mass to volume of a body). The density of liquid propane-butane mixtures is largely determined by the composition of hydrocarbons and temperature. As the temperature rises, volumetric expansion occurs, and the density of the liquid decreases. With increasing pressure, the volume of liquid propane and butane is compressed.
- Viscosity (the ability of substances in a gaseous or liquid state to resist shear forces). It is determined by the forces of adhesion of molecules in substances. The viscosity of a liquid mixture of propane with butane depends on temperature (with its increase, the viscosity decreases), but a change in pressure has little effect on this characteristic. Gases, on the other hand, increase their viscosity with increasing temperature.
Finding in nature and obtaining methods
The main natural sources of propane are oil andgas fields. It is contained in natural gas (from 0.1 to 11.0%) and in associated petroleum gases. Quite a lot of butane is obtained in the process of distillation of oil - separating it into fractions, based on the boiling points of its components. Of the chemical methods of oil refining, catalytic cracking is of the greatest importance, during which the chain of high-molecular-weight alkanes is broken. In this case, propane forms about 16-20% of all gaseous products of this process:
СΗ3-СΗ2-СΗ2-СΗ 2-СΗ2-СΗ2-СΗ2-СΗ 3 ―> СΗ3-СΗ2-СΗ3 + СН 2=CΗ-CΗ2-CΗ2-CΗ3
Large amounts of propane are formed during the hydrogenation of various types of coal and coal tar, they reach 80% of the volume of all gases produced.
It is also widespread to obtain propane by the Fischer-Tropsch method, which is based on the interaction of CO and H2 in the presence of various catalysts at elevated temperature and pressure:
nCO + (2n + 1)Η2 ―> C Η2n+2 + nΗ2O
3CO + 7Η2 ―> C3Η8 + 3Η 2O
Industrial volumes of butane are also isolated during oil and gas processing by physical and chemical methods.
Chemical properties
From structural features of moleculesdepend on the physical and chemical properties of propane and butane. Since they are saturated compounds, addition reactions are not characteristic of them.
1. substitution reactions. Under the action of ultraviolet light, hydrogen is easily replaced by chlorine atoms:
CH3-CH2-CH3 + Cl 2 ―> CH3-CH(Cl)-CH3 + HCl
When heated with a solution of nitric acid, the H atom is replaced by the NO group2:
СΗ3-СΗ2-СΗ3 + ΗNO 3 ―> СΗ3-СΗ (NO2)-СΗ3 + H2O
2. Cleavage reactions. When heated in the presence of nickel or palladium, two hydrogen atoms are split off with the formation of a multiple bond in the molecule:
CΗ3-CΗ2-CΗ3 ―> CΗ 3-СΗ=СΗ2 + Η2
3. decomposition reactions. When a substance is heated to a temperature of about 1000 ° C, the pyrolysis process occurs, which is accompanied by the breaking of all chemical bonds present in the molecule:
C3H8 ―> 3C + 4H2
4. combustion reactions. These hydrocarbons burn with a non-smoky flame, releasing a large amount of heat. What propane is known to many housewives who use gas stoves. The reaction produces carbon dioxide and water vapor:
C3N8 + 5O2―> 3CO 2 + 4H2O
Combustion of propane in conditions of lack of oxygen leads to the appearance of soot and the formation of carbon monoxide molecules:
2C3H8 + 7O2―> 6SO + 8H 2O
C3H8 + 2O2―> 3C + 4H 2O
Application
Propane is actively used as a fuel, since 2202 kJ / mol of heat is released during its combustion, this is a very high figure. In the process of oxidation, many substances necessary for chemical synthesis are obtained from propane, for example, alcohols, acetone, carboxylic acids. It is necessary to obtain nitropropanes used as solvents.
As a propellant used in the food industry, has the code E944. Blended with isobutane, it is used as a modern, environmentally friendly refrigerant.
Propane-butane mixture
It has many advantages over other fuels, including natural gas:
- high efficiency;
- easy return to gaseous state;
- good evaporation and combustion at ambient temperature.
Propane fully meets these qualities, but butanes evaporate somewhat worse when the temperature drops to -40°C. Additives help to correct this deficiency, the best of which is propane.
Propane-butane mixture is used for heating and cooking, for gas welding of metals and cutting them, as a fuel for vehicles and for chemicalsynthesis.
