Hydrocarbons

Hydrocarbons are a class of molecules that contain only carbon and hydrogen atoms. Some of them make very good fuels. Gasoline contains a mixture of hydrocarbons. Unsaturated hydrocarbons, which contain one or more double bonds, are useful chemicals for many reactions.

In organic chemistry, hydrocarbons may be classified into two categories, aromatic hydrocarbons (see below) that contain one or more benzene rings or similar rings and aliphatic hydrocarbons which includes all other types of hydrocarbons (see below).

Linear saturated hydrocarbons

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. Saturated hydrocarbon names generally end with the suffix "ane" which distinguishes them from unsaturated hydrocarbons, which end with the suffix "ene".

Linear saturated hydrocarbons are referred to as paraffins or alkanes. Their general formula is C_nH_n+2.

Some example alkanes

Name	Formula	CnH2n+2
Methane	CH4	CH4
Ethane	CH3–CH3	C2H6
Propane	CH3–CH2–CH3	C3H8
Butane	CH3–(CH2)2–CH3	C4H10
Pentane	CH3–(CH2)3–CH3	C5H12
Hexane	CH3–(CH2)4–CH3	C6H14
Heptane	CH3–(CH2)5–CH3	C7H16
Octane	CH3–(CH2)6–CH3	C8H18
Nonane	CH3–(CH2)7–CH3	C9H20
Decane	CH3–(CH2)8–CH3	C10H22

Linear unsaturated hydrocarbons

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 instead of the IUPAC designation. In addition, a numerical prefix is 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 C_nH_2n.

Some example alkenes

Name	Formula	CnH2n
Ethene	CH2=CH2	C2H4
Propene	CH2=CH–CH3	C3H6
1-Butene	CH2=CH–CH2–CH3	C4H8
2-Butene	CH3–CH=CH–CH3	C4H8
1-Pentene	CH2=CH–(CH2)2–CH3	C5H10
1-Hexene	CH2=CH–(CH2)3–CH3	C6H12

Linear unsaturated hydrocarbons containing two double bonds are referred to as dienes, diolefins or alkadienes. Their general formula is C_nH_2(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

(PD) Image: Milton Beychok
Example cycloalkanes.

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 C_nH_2n. 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 is C_nH_2(n+1-g) where g is the number of rings.

Cyclic unsaturated hydrocarbons

(PD) Image: Milton Beychok
Example cycloalkenes.

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 C_nH_2n-2.

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 C_nH_2n-4.

Aromatic hydrocarbons

(PD) Image: Milton Beychok
Example aromatics.

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 connnected 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 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.