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Hydrocarbons

Hydrocarbons* are a class of molecules that contain only carbon and hydrogen atoms. The majority of hydrocarbons found naturally occur in petroleum crude oil and in raw natural gas.

Hydrocarbon categories

The two major hydrocarbon categories are the aliphatics and the aromatics. The aliphatics may be further sub-categorized as alkanes, alkenes, cycloalkanes, and cycloalkenes. The aromatics are those hydrocarbons that contain one or more benzene rings or similar rings. Each of these hydrocarbon types are discussed below.[1]

Linear saturated hydrocarbons (alkanes)

The simplest hydrocarbons are linear molecules in which each carbon atoms is bonded to two other carbons atoms, in a linear fashion, except for the carbon atoms at the ends, which are only bonded to one other carbon atom. Linear saturated hydrocarbons are referred to as paraffins or alkanes and their
general formula is CnHn+2.

HC_Alkanes.png

Linear unsaturated hydrocarbons (alkenes)

HC_Alkenes.png

Unsaturated hydrocarbons are useful precursor molecules for many reactions. Because they contain one or more double bonds, a large variety of chemical transformations are possible. Unsaturated hydrocarbons generally end with the "ene" suffix, although common names are sometimes used. In addition, a numerical prefix is often used to indicate the position of the double bond(s).

Linear unsaturated hydrocarbons containing a single double bond are referred to as olefins or alkenes. Their general formula is CnH2n.

Linear unsaturated hydrocarbons containing two double bonds are referred to as dienes, diolefins or alkadienes. Their general formula is CnH2(n-1) and some example dienes are:

  • 1,2-Butadiene: CH2=C=CH–CH3 or C4H6
  • 1,2-Pentadiene: CH2=C=CH–CH2–CH3 or C5H8

Linear unsaturated hydrocarbons containing a triple bond are referred to as alkynes. The simplest example is ethyne (acetylene): H–C≡C–H or C2H2

Cyclic saturated hydrocarbons

HC_Naphthenes.png

Cyclic saturated hydrocarbons have a closed ring of carbon atoms in a polygon configuration having the same number of vertices as the number of carbon atoms, with each carbon atom being bonded to two hydrogen atoms and to two other carbon atoms. They are referred to as cycloalkanes, cycloparaffins or naphthenes.

Cycloalkanes with a single ring of carbon atoms have a general formula of CnH2n. The adjacent image depicts two such single ring cycloalkanes, namely cyclopentane (C5H10) and cyclohexane (C6H12).

Cycloalkanes may also have two fused (i.e., conjoined) rings. For example, two cyclohexane rings may be fused, so that two of the carbon atoms are shared by each of the two rings, to form decalin (C10H18) which is referred to as a bicycloalkane.

The general formula for cycloalkanes (single rings or fused multiple rings) is CnH2(n+1-g) where g is the number of rings.

Cyclic unsaturated hydrocarbons

HC_Cycloalkenes.png

Cyclic unsaturated hydrocarbons have a closed ring of carbon atoms in a polygon configuration having the same number of vertices as the number of carbon atoms. If the hydrocarbons have a ring containing a single double bond between two of the carbons, the hydrocarbons are referred to as cycloalkenes or cycloolefins. The adjacent image depicts two such cycloalkenes with single double bonds, namely cyclopentene (C5H8) and cyclohexene (C6H10).

The general formula for single-ring cycloalkenes with a single double bond is CnH2(n-1).

If single-ring alkenes have two double bonds, they are referred to as cyclodienes or cyclodiolefins. An example is cyclopentadiene (C5H6), and the general formula for single-ring cycloalkenes with two double bonds is CnH2(n-2).

Aromatic hydrocarbons

HC_Aromatics.png

Aromatic hydrocarbons have a closed ring of 6 carbon atoms in the shape of a hexagon with 3 of the carbon atoms having a double bond with one connected carbon atom and a single bond with another connected carbon. Such a ring is referred to as a benzene ring and all hydrocarbons containing one or more such rings are referred to as aromatics or aryl compounds.

The adjacent image depicts three of the most common aromatics, namely benzene (C6H6), toluene (C7H8) and o-xylene (C8H10). There are three possible xylenes and they are referred to as ortho-xylene (o-xylene), meta-xylene (m-xylene) and para-xylene (p-xylene).

Aromatics may have two or more rings, either in fused or other configurations, and may have many different side groups or side chains (such as the methyl side group in toluene). There are quite literally hundreds (if not thousands) of various aromatic hydrocarbons.

References

  1. ^ Editor: Robert C. Weast (1975). Handbook of Chemistry and Physics, 56th Edition, CRC Press, ISBN 0-87819-455-X.

Contributor

  • Milton Beychok