Types of Hydrocarbons

What are Saturated Hydrocarbons?

It is said that Saturated hydrocarbons are hydrocarbons that contain only a single bond between carbon atoms. These are the simplest class of hydrocarbons. These are called saturated because each carbon atom is bonded to ‘n’ number of hydrogen atoms as possible. In simpler terms, the carbon atoms are saturated with hydrogen.

Let us see a saturated hydrocarbon example in the below figure. In this compound, each carbon atom, named ethane, is bonded to three hydrogen atoms. Each dash (-) represents a single covalent bond in the structural formula, where two atoms share one pair of valence electrons.

Here, we’ve taken an example of the general saturated hydrocarbon formula for Ethane (C2H6), where the structure is represented below.

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Types of Saturated Hydrocarbons

Saturated hydrocarbons are divided as a linear, branched, or a ring-shaped structure and can further be classified into one of the following types.

  • Alkanes

  • Cycloalkanes

It is essential to remember that even polycyclic alkanes (alkanes with several rings in their structures) are also categorized as cycloalkanes and, therefore, as a type of saturated hydrocarbons.

  • Alkanes

Alkanes may feature a linear or a branched carbon chain in their structures. All carbon atoms in these saturated hydrocarbons are sp3 hybridized. The melting point and boiling point of the alkane are related to the length of the carbon chain. The longer the carbon chain, the higher the melting or boiling point. This is because molecules with long carbon chains have high molecular weights.

At standard temperatures, alkanes containing up to four carbon atoms are gasses, and those containing 5 to 17 carbon atoms are liquid. Alkanes containing over 18 carbon atoms are solid at room temperature.

All the Alkanes containing more than three carbon atoms may exhibit chain isomerism. An illustration of possible chain isomers of butane (C₄H₁₀) is given below.

The possible number of chain isomers for an alkane depends upon the total number of carbon atoms in the alkane. For example, butane has only two chain isomers, but octane (C8H18) has a total of 18 possible chain isomers.

  • Cycloalkanes

Cycloalkanes are featured with a ring-shaped arrangement of sp3 hybrid carbon atoms. The ring in these saturated hydrocarbons can, however, be branched into side chains. 

The physical properties of cycloalkanes are a bit close to those of alkanes.

The boiling and melting points of cycloalkanes are generally higher than those of alkanes containing the same number of carbon atoms. 

Cycloalkanes are subjected to ring strain due to their composition. Even though cyclopropane has a carbon-carbon bond angle of 60°, it has the highest ring strain in all cycloalkanes.

What are Unsaturated Hydrocarbons?

It is stated that Unsaturated hydrocarbons are the organic compounds that are totally made up of carbon and hydrogen atoms and contain a double or a triple bond between two adjacent carbon atoms. Hydrocarbons that have at least one double bond between two adjacent carbon atoms are called alkenes. On the other hand, the hydrocarbons which contain a carbon-carbon triple bond are called alkynes. The distinguished chemical formula of saturated and unsaturated hydrocarbons is listed below.

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As per the IUPAC (International Union of Pure and Applied Chemistry) nomenclature of unsaturated hydrocarbons, the position of double or triple bond is described either by a number written before the name of the compound (as in 2,4 pentadiene) or by a number written before its suffix, ‘-ene’ or ‘-yne’ (in terms of pent-2-ene).

The physical properties of unsaturated and saturated hydrocarbons are quite similar. These types of hydrocarbons (excepting the aromatic hydrocarbons) are quite reactive and tend to undergo additional reactions with hydrogen halides, alcohols, elemental halogens, and many other compounds.

Types of Unsaturated Hydrocarbons

Depending on the types of bonds they contain, unsaturated hydrocarbons are further classified into Alkynes, Alkenes, and Hydrocarbons, which are explained below briefly.

  • Alkynes

The unsaturated hydrocarbons that consist of one or more triple bonds are known as alkynes. Names of specific alkynes always end with ‘-yne’ and have a prefix for the number of carbon atoms. The structural formula of the below diagram represents the smallest alkyne, named ethyne, with two carbon atoms and two hydrogen atoms, respectively (C2H2). Ethyne is also known as acetylene and is burned in acetylene torches, like the one represented in the Figure below. The flame of an acetylene torch is so hot that it can melt even a metal.

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  • Alkenes

Alkenes are referred to as unsaturated hydrocarbons that contain one or more double bonds. Always, the name of a specific alkene ends in ‘-ene’ and has a prefix indicating the number of carbon atoms. The structural formula in the below figure represents the smallest alkene, named ethene, with two carbon atoms and four hydrogen atoms (C2H4). Ethene is produced by most fruits and vegetables. It speeds up ripening.

The following representation shows the effect of Ethene on Bananas.

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Bananas on the left side were stored in a special bag which absorbs ethene, whereas the bananas on the right were stored without a bag.

Alkenes may have different shapes. They can also form straight chains, branched chains, or even rings. Alkenes with the same atoms but different shapes are known as isomers. Smaller alkenes have relatively high melting and boiling points. So they are gases at room temperature. Whereas the larger alkenes have lower melting and boiling points, and therefore they are liquids or waxy solids at room temperature.

Aromatic Hydrocarbons

Aromatic hydrocarbons are the cyclic hydrocarbons with double bonds. These compounds consist of six carbon atoms in a ring with an alternate single and double bonds. The smallest aromatic hydrocarbon is benzene, having just one ring. Its structural formula is given below. Larger aromatic hydrocarbons can consist of two or more rings, which are joined together between their carbon atoms by bonds. The name of aromatic hydrocarbons comes from their scent or strong aroma. Because of that, they are used in air fresheners and mothballs.

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FAQ (Frequently Asked Questions)

1. What are the Uses of Saturated Hydrocarbons?

  • Alkanes are commonly used as fuels, cooking oils, and solvents. A few more uses of saturated hydrocarbons are described below. 

  • Methane, the simplest alkane, is used as a fuel in different automobiles, water heaters, and ovens.

  • Liquid methane can also serve as rocket fuel in its highly refined form.

  • The propellant used in a number of aerosol sprays is a saturated hydrocarbon known as propane. This same compound is also used as a fuel for hot air balloons. 

  • Octane is an essential element of gasoline because it helps prevent engine damage.

  • Cycloalkanes are also used in motor fuels, diesel, petroleum gas, and other heavy oils.