Alkanes, or paraffinic 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, hence the other name - 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 are 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. The only difference is that all the bonds in the molecule are single. The valence of carbon is 4, therefore, one atom in a compound will always be bonded to four other atoms. Moreover, at least one bond will be of the carbon-carbon type, and the rest can be both carbon-carbon and carbon-hydrogen (hydrogen valency is 1, so think about hydrogen-hydrogen bondsforbidden). Accordingly, a carbon atom that has only one C-C bond will be called primary, two C-C bonds - secondary, three - tertiary and four, by analogy, quaternary.
If you write down the molecular formulas of all alkanes in the figure, you get:
- CH4,
- C2H6,
- C3H8.
and so on. It is easy to make a universal formula that describes any compound of this class:
C H2n+2.
This is the general formula for paraffinic hydrocarbons. The set of all possible formulas for them is a homologous series. The difference between the two closest members of the series is (-CH2-).
Alkanes nomenclature
The first and simplest in the series of saturated hydrocarbons is methane CH4. Next comes ethane C2H6, having two carbon atoms, propane C3H 8, butane C4H10, and from the fifth member of the homologous series, alkanes are named by the number of carbon atoms in the molecule: pentane, hexane, heptane, octane, nonane, decane, undecane, dodecane, tridecane and so on. However, several carbons can be called "at once" only if they are in the same linear chain. And this is not always the case.
This picture shows several structures whose molecular formulas are the same: C8H18. However, we have three different connections. Suchthe phenomenon when there are several different structural formulas for one molecular formula is called isomerism, and compounds are called isomers. Here there is an isomerism of the carbon skeleton: 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. Their nomenclature is based on the longest continuous chain of carbon atoms in the molecule, and "branches" are considered substituents of one of the hydrogen atoms at carbon from the "main" chain. So 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.
Alkanes structure
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 outer energy level - 3s (one piece), 3p (three pieces). It is expected that since different types of orbitals are involved in the bonding, then the resulting bonds should be different in terms of their energy characteristics. However, this is not observed - in the methane molecule, all four are the same.
The theory of hybridization is used to explain this phenomenon. It works as follows: it is assumed that a covalent bond is, as it were, two electrons (one from each atom in a pair) located exactly between the bound atoms. In methane, for example, there are four such bonds, so fourpairs of electrons in a molecule will repel each other. To minimize this constant pushing, the central atom in methane arranges all four of its bonds so that they are as far apart as possible. At the same time, for even greater benefit, he, as it were, mixes all his orbitals (3s - one and 3p - three), then making four new identical sp3-hybrid orbitals out of them. As a result, the "ends" of covalent bonds, on which hydrogen atoms are located, form a regular tetrahedron, in the middle of which there is carbon. This ear trick is called sp3-hybridization.
All carbon atoms in alkanes are in sp3-hybridization.
Physical properties
Alkanes with the number of carbon atoms from 1 to 4 - gases, from 5 to 17 - liquids with a pungent smell, similar to the smell of gasoline, above 17 - solids. The boiling and melting points of alkanes increase as their molar mass (and, accordingly, the number of carbon atoms in the molecule) increases. It is worth saying that at 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, so the overall structure of the substance is less resistant to external influences (and when heated, these bonds break down faster).
Despite such differences, on average, all alkanes are extremely non-polar: they practically do not dissolve in water (and water is a polar solvent). But themselvesunsaturated hydrocarbons from those that are liquids under normal conditions are actively used as non-polar solvents. This is how n-hexane, n-heptane, n-octane and others are used.
Chemical properties
Alkanes are inactive: even compared to other organic substances, they react with an extremely limited list of reagents. Basically, these are reactions that proceed according to the radical mechanism: chlorination, bromination, nitration, sulfonation, and so on. Methane chlorination is a classic example of chain reactions. Its essence is as follows.
A chemical chain reaction consists of several stages.
- first, the chain is nucleated - the first free radicals appear (in this case, this happens under the action 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 a molecule; this releases new free radicals, 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.
The intermediate reaction products here are both chloromethane CH3Cl and dichloromethane CH2Cl2, and trichloromethane (chloroform) CHCl3, and carbon tetrachloride CCl4. This means that radicals can attack anyone: both methane itself andintermediate products of the reaction, more and more replacing hydrogen with halogen.
The most important reaction for industry is the isomerization of paraffinic hydrocarbons. In the course of it, their branched isomers are obtained from unbranched alkanes. 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 AlCl3 at temperatures around 300oC.
Combustion of alkanes
Since elementary school, many have known that any organic compound burns to form water and carbon dioxide. Alkanes are no exception; however, in this case, something else is much more important. The property of paraffinic hydrocarbons, especially gaseous hydrocarbons, 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 gone through a long path of 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 is 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) according to the number of carbon atoms in the molecule:
- gasoline (5-7С);
- gasoline (5-11 C);
- naphtha (8-14 C);
- kerosene (12-18 C);
- gas oil (16-25 C);
- oils - fuel oil, solar oil, lubricants and others (20-70 C).
According to the faction, crude oil goes to different types of fuel. For this reason, the types of fuel (gasoline, ligroin - tractor fuel, kerosene - jet fuel, diesel fuel) coincide with the fractional classification of paraffinic hydrocarbons.
