Alkenes are hydrocarbons containing two fewer hydrogen atoms than the corresponding alkane. The general formula for alkenes is C_nH_Zn. These molecules will have a double bond as illustrated in Figure 12.

Because there are fewer hydrogen atoms than the maximum possible, alkenes are unsaturated hydrocarbons. The chief source for alkenes is the cracking of alkanes.

Figure 12 Alkene

Alkynes

The third of the aliphatic hydrocarbons are the alkynes. These compounds are unsaturated like the alkenes. They contain two fewer hydrogens than the corresponding alkane, C.1-12.-2. The alkyne hydrocarbons contain a triple bond between at least one set of carbon atoms as illustrated in Figure 13.

Figure 13 Alkyne

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Aromatics

The other broad class of hydrocarbons is the aromatic hydrocarbon. Rather than being arranged in straight chains, as the aliphatics are, these are cyclic formations such as in benzene. The derivatives of cyclic hydrocarbons have pleasant (sometimes toxic) odors. The benzene in rubber cement is a familiar odor. The cyclic compounds have alternating single - double bonds as illustrated in Figure 14.

Aromatic hydrocarbons are very stable chemically, and act very much like alkanes. They will undergo substitution reactions rather than additions.

Figure 14 Aromatic