Hydrocarbons are organic compounds that contain only carbon and hydrogen atoms. Widely used as fuels, their main source in nature is fossil.
The carbon atoms (C) are bonded together to form the molecule's skeleton, while the hydrogen atoms (H) protrude from this skeleton. Hydrocarbons are the only category of organic compounds without the functional group, in fact they are composed only of hydrogen and carbon atoms.
As the number of carbon atoms in the hydrocarbon chain increases, the number of possible hydrocarbon compounds increases.
The shortest chain possible is composed of a single carbon atom, which corresponds to methane, having the brute formula CH4.
There are three hydrocarbons having two carbon atoms:
ethane, of formula C2H6
ethene (or ethylene), C2H4
etino (or acetylene), C2H2. These three hydrocarbons differ in the number of bonds that hold the two contiguous carbon atoms together: one bond (so we speak of a "simple bond") in the case of ethane, two bonds ("double bond") in the case of ethylene and three bonds ("triple bond") in the case of acetylene.
In the structure of a generic hydrocarbon each carbon atom uses one to three valence electrons to form bonds with the adjacent carbon, while the remaining valence electrons of the carbon are used to form bonds with the hydrogen atoms. Inside the hydrocarbon molecule, however, there may be carbon atoms to which no hydrogen atoms are bonded, but only other carbon atoms (an example is the central carbon atom of the neopentane molecule).
Classification of hydrocarbons
A first distinction between the various hydrocarbons refers to their physical state in the conditions of environmental pressure and temperature, namely:
Solid or semi-solid hydrocarbons: constituents of asphalt, bitumen, paraffin waxes, etc.
Liquid hydrocarbons: constituents of (crude) petroleum, benzene, hexane, octane, etc.
Gaseous hydrocarbons: methane, ethane, propane, butane, etc. From the point of view of chemical properties, hydrocarbons are divided into two main classes:
aromatic hydrocarbons: endowed with "aromaticity", a chemical property imparted by a benzene ring, which makes them particularly stable;
aliphatic hydrocarbons: not endowed with aromaticity. The various types of hydrocarbons (alkanes, alkenes, etc.) described below are called "homologous series".
The classification described here is not intended to be exhaustive, in fact there are many types of hydrocarbons that are not well classifiable, such as for example the catenans, which are made up of several rings inserted into each other so that each can move independently of the other but without "melt" (in a similar way to the rings of a steel chain, hence the name).
Depending on the types of C-C bonds present in the molecule (single, double or triple) and their geometry (linear, branched, cyclic), aliphatic hydrocarbons are also divided into:
saturated hydrocarbons: with only single C-Calcane bonds (or paraffins): non-cyclic saturated hydrocarbons, ie having a chain that is not closed on itself; have general formula CnH2n + 2. In turn they can be linear or branched, depending on whether the carbon atoms follow each other continuously or with bifurcations
cycloalkanes: cyclic saturated hydrocarbons, having general formula CnH2n
bicycloalkanes: saturated hydrocarbons consisting of two contiguous cyclic chains
alkylcycloalkanes: saturated hydrocarbons consisting of a cyclic hydrocarbon chain linked to a non-cyclic chain
unsaturated hydrocarbons: contain at least one multiple C-C bond (i.e. double or triple). The bonds formed by two or more atoms present in the carbon chain that makes up the molecule's skeleton have trigonal geometry because they are ibr