Hydrocarbons

Hydrocarbons – introduction

The compounds of carbon and hydrogen are called hydrocarbons. Their specific names come directly from the Greek names for the numbers of carbon atoms in their chains. The simplest of the hydrocarbons is methane. In its molecule, it has one carbon atom, which is connected to four hydrogen atoms. This structure results from the properties of organic compounds. In an organic compound molecule, the carbon is always tetravalent. It also means that there is a close relationship between the number of carbon atoms in a hydrocarbon molecule and the amount of hydrogen. If there are only single bonds between carbon atoms – then we are dealing with saturated hydrocarbons, otherwise (one or more double or triple bonds) they are unsaturated hydrocarbons.

The formation of homologous series, i.e. sets of compounds with similar structure and properties, is characteristic of hydrocarbons. They are formed as a result of attaching successive -CH₂– groups to the chain of the molecule. Moreover, for all homologues in a particular series, there are relationships between their boiling and melting points, density and other physicochemical properties. In addition to hydrocarbons, e.g. alcohols or carboxylic acids also form homologous series.

Among hydrocarbons, most of the compounds have the same empirical formula, but different structures, i.e. different structural formulae. They are called isomers, and depending on the occurring isomerism, the following are distinguished:

• Chain isomers refer to the carbon chain. This isomerism is related to the change in its length and the number of substituents.
• Multiple bond position isomerism refers to the possibility of unsaturated bonds between different carbon atoms in the chain.
• Cis-trans isomerism is characteristic of molecules in which there is no possibility of rotation around the bond. It is often found in alkenes and cyclic hydrocarbons.

Structure and properties of chain hydrocarbons

In aliphatic hydrocarbons, bonded carbon atoms form a characteristic chain consisting of consecutive -CH₂– groups. Therefore, they are called chain hydrocarbons. Depending on the structure, straight and branched chains can be distinguished. On the other hand, taking into account the occurrence of multiple bonds, we can distinguish alkanes, alkenes and alkynes.

Alkanes

Saturated chain hydrocarbons are called alkanes. They form a homologous series, where the general formula of the molecule is C_nH_2n+2 (where n is the number of carbon atoms in the molecule). All bonds between atoms in alkane molecules are single, atomic. The first three alkanes (methane, ethane, propane) have straight chain molecules. The next one – butane – has also a branched form (isobutane). Despite the identical empirical formula, these forms differ significantly in their properties, e.g. boiling point.

Most alkanes have similar physical and chemical properties. They show significant chemical passivity, react only with few substances, e.g. with chlorine. As the number of carbon atoms in a molecule increases, their boiling point increases. Thus, the first four alkanes in the homologous series are gases. Higher hydrocarbons (up to pentadecane) are liquids and the next ones are solids. All alkanes are flammable.

Alkenes

Alkenes contain one double bond in their molecules. Thus, they have two hydrogen atoms less compared to alkane molecules. The formula of alkenes in the homologous series is C_nH_2n. The first in the series is ethene. Ethene, and the next one: pentene, do not have isomers. However, from butene there is isomerism related to the position of the double bond in the molecule. There are also compounds that have two double bonds. They are called alkadienes.

The presence of a double bond has the greatest influence on the properties of alkenes. It causes that they readily undergo addition reactions with other molecules. They are also susceptible to polymerisation. As a result of polymerisation of ethene, polyethylene is obtained. It is a popular plastic. Alkenes, like alkanes, change their state of matter depending on the length of the chain. They are colourless and odourless. They do not dissolve in water.

Alkynes

Triple-bonded chain hydrocarbons are alkynes. In the homologous series of alkynes, the next molecules are characterised by the formula C_nH_2n-2. The first of the alkynes is ethyne, also called acetylene. Starting with butyne, there is isomerism in alkynes due to the location of the triple bond.

Alkynes have a higher degree of unsaturation than alkenes and are therefore more chemically active. They also show a tendency to polymerisation. The smallest molecules are gases. Their volatility decreases as the molecule size increases. In addition, they are colourless, odourless and do not dissolve in water.

Structure and properties of cyclic and aromatic hydrocarbons

Cyclic hydrocarbons are compounds of carbon and hydrogen that form closed systems, called rings. The simplest cycloalkane is a three-carbon ring – cyclopropane. The most important cycloalkane is cyclohexane, which, unlike cyclopropane or cyclobutane, does not have a ring strain, and its molecule can form the so-called conformations. In addition to cycloalkanes, there are also cycloalkenes and cycloalkynes.

Aromatic hydrocarbons are a group of unsaturated organic compounds. The simplest compound of this group is benzene with the empirical formula C₆H₆. There are no typical double bonds in the molecule of this compound, instead we are talking about de-localised π electrons.

Polycyclic aromatic hydrocarbons have slightly different properties. They have more than one ring in the molecule. The most important hydrocarbon of this group is naphthalene.

Obtaining hydrocarbons

Natural sources of hydrocarbons are crude oil and natural gas. Alkanes such as methane and ethane are part of natural gas, although it may contain small amounts of other alkanes as well. Crude oil contains a range of hydrocarbons with different chain lengths and branching. It is separated into individual fractions in the distillation process. They are an important source of organic compounds. Those with carbon chains that are too long are further processed in a cracking process. Aromatic hydrocarbons are largely obtained as a result of dry coal distillation (heating without access to air). Benzene is also obtained from crude oil in the process of catalytic processing of alkanes and cycloalkanes.