Alkanes, or paraffin hydrocarbons, are the simplest of all classes of organic compounds. Their main characteristic is the presence in the molecule of only single, or saturated bonds, whence another name comes from - saturated hydrocarbons. In addition to the well-known oil and gas, alkanes are also found in many plant and animal tissues: for example, tsetse fly pheromones - alkanes containing 18, 39 and 40 carbon atoms in their chains; alkanes are also found in large quantities in the upper protective layer of plants (cuticle).
General information
Alkanes belong to the class of hydrocarbons. This means that only carbon (C) and hydrogen (H) will be present in the formula of any compound. What distinguishes them only is that all bonds in the molecule are single. The valency of carbon is 4; therefore, one atom in a compound will always be bound to four other atoms. Moreover, at least one bond will be of the carbon-carbon type, and the rest can be either carbon-carbon or carbon-hydrogen (the valency of hydrogen is 1, so it’s forbidden to think about hydrogen-hydrogen bonds). Accordingly, a carbon atom having only one CC bond will be called primary, two CC bonds - secondary, three - tertiary and four, by analogy, quaternary.
If we write down the molecular formulas of all alkanes in the figure, we get:
etc. It is easy to make a universal formula that describes any compound of this class:
This is the general formula for paraffin hydrocarbons. The totality of all possible formulas for them is a homological series. The difference between the two closest members of the series is (-CH 2 -).
Nomenclature of alkanes
The first and simplest in the series of saturated hydrocarbons is methane CH 4 . Next comes ethane C 2 H 6 having two carbon atoms, propane C 3 H 8 , butane C 4 H 10 , and from the fifth member of the homologous series, alkanes are called by the number of carbon atoms in the molecule: pentane, hexane, heptane, octane, nonane, dean, undecane, dodecan, tridecan and so on. However, several carbons can be called “at once” only if they are in the same linear chain. And this does not always happen.
This picture shows several structures whose molecular formulas coincide: C 8 H 18 . However, before us are three different compounds. Such a phenomenon, when several different structural formulas exist for one molecular formula, is called isomerism, and compounds are called isomers. Here the isomerism of the carbon skeleton is observed: this means that the isomers differ in the order of carbon-carbon bonds in the molecule.
All isomers that do not have a linear structure are called branched. In their nomenclature, the longest continuous chain of carbon atoms in the molecule is taken as the basis, and the “branches” are considered to be substituents of one of the hydrogen atoms in carbon from the “main” chain. Thus, 2-methylpropane (isobutane), 2,2-dimethylpropane (neopentane), 2,2,4-trimethylpentane are obtained. The number indicates the carbon number of the main chain, followed by the number of identical substituents, then the name of the substituent, then the name of the main chain.
The structure of alkanes
All four bonds at the carbon atom are covalent sigma bonds. To form each of them, carbon uses one of its four orbitals at the external energy level - 3s (one piece), 3p (three pieces). It is expected that since different types of orbitals participate in the binding, the resulting bonds in terms of their energy characteristics should be different. However, this is not observed - in the methane molecule, all four are the same.
To explain this phenomenon, the hybridization theory is used. It works as follows: it is assumed that a covalent bond is like two electrons (one from each atom in a pair) located exactly between the bound atoms. In methane, for example, four such bonds, so four pairs of electrons in the molecule will repel each other. To minimize this constant push, the central atom in methane has all four of its bonds so that they are as far apart as possible. At the same time, for even greater benefit, he mixes all his orbitals (3s - one and 3p - three), making four new identical sp 3 -hybrid orbitals from them. As a result, the “ends” of the covalent bonds on which the hydrogen atoms are located form a regular tetrahedron, in the middle of which there is carbon. This feint with the ears is called sp 3 hybridization.
All carbon atoms in alkanes are in sp 3 hybridization.
Physical properties
Alkanes with the number of carbon atoms from 1 to 4 are gases, from 5 to 17 are liquids with a pungent odor similar to the smell of gasoline, above 17 are solids. The boiling and melting points of alkanes increase along with the increase in their molar mass (and, accordingly, the number of carbon atoms in the molecule). It is worth saying that, with the same molar mass, branched alkanes have noticeably lower melting and boiling points than their unbranched isomers. This means that the intermolecular bonds in them are weaker, therefore, the general structure of the substance is less resistant to external influences (and when heated, these bonds are destroyed more quickly).
Despite these differences, on average, all alkanes are extremely non-polar: they practically do not dissolve in water (and water is a polar solvent). But the unsaturated hydrocarbons themselves from those that under normal conditions are liquids are actively used as non-polar solvents. So use n-hexane, n-heptane, n-octane and others.
Chemical properties
Alkanes are inactive: even in comparison with other organic substances, they react with an extremely limited list of reagents. Basically, these are reactions proceeding according to a radical mechanism: chlorination, bromination, nitration, sulfonation, etc. Methane chlorination is a classic example of chain reactions. Its essence is as follows.
A chemical chain reaction consists of several stages.
- first, chain nucleation occurs - the first free radicals appear (in this case, this occurs under the influence of photons);
- the next step is chain development. In the course of it, new substances are formed, which are the result of the interaction of some free radical and molecule; in this case, new free radicals are released, which, in turn, react with other molecules, and so on;
- when two free radicals collide and form a new substance, a chain break occurs - no new free radicals are formed, and the reaction decays in this branch.
Intermediate products of the reaction here are chloromethane CH 3 Cl, and dichloromethane CH 2 Cl 2 , and trichloromethane (chloroform) CHCl 3 , and carbon tetrachloride CCl 4 . This means that radicals can attack anyone: methane itself, and intermediate reaction products, more and more replacing hydrogen with halogen.
An extremely important reaction for industry is the isomerization of paraffin hydrocarbons. In the course of it, from branched alkanes, their branched isomers are obtained. This increases the so-called detonation resistance of the compound - one of the characteristics of automotive fuel. The reaction is carried out on an aluminum chloride catalyst AlCl 3 at temperatures of about 300 about C.
Alkane burning
Since elementary school, many people know that any organic compound burns with the formation of water and carbon dioxide. Alkanes are no exception; however, in this case the other is much more important. The property of paraffin hydrocarbons, especially gaseous ones, is the release of a large amount of heat during combustion. That is why almost all major fuels are produced from paraffins.
Hydrocarbon-based minerals
These are the remains of ancient living organisms that have come a long way in chemical changes without oxygen. Natural gas is on average 95% methane. The rest is ethane, propane, butane and minor impurities.
With oil, everything is much more interesting. It represents a whole bunch of the most diverse classes of hydrocarbons. But the main part is occupied by alkanes, cycloalkanes and aromatic compounds. Paraffin hydrocarbons of oils are divided into fractions (which include unsaturated neighbors) by the number of carbon atoms in the molecule:
- gasoline (5-7 C);
- gasoline (5-11 C);
- naphtha (8-14 C);
- kerosene (12-18 C);
- gas oil (16-25 C);
- oils - fuel oil, diesel fuel, lubricant and others (20-70 C).
In accordance with the fraction, the crude oil is used for different types of fuel. For this reason, the types of fuel (gasoline, naphtha - tractor fuel, kerosene - jet, diesel fuel) coincide with the fractional classification of paraffin hydrocarbons.