An infinite variety of compounds can be assembled from only carbon and hydrogen atoms. Such hydrocarbons are the simplest organic compounds, but they are also of prime economic importance because they include the constituents of petroleum and natural gas.
Propane, butane, and isobutane are all hydrocarbons with only single covalent bonds between carbon atoms. These hydrocarbons that lack double bonds, triple bonds, or ring structures make up the class called alkanes. See Table 1.
As the number of carbon atoms increases, so does the number of ways that they can be connected to form different isomers. You should realize that isomers are defined by the pattern of bonding between the carbons.
The two molecules in Figure 1 are not different isomers; they are both butane. Despite the crooked carbon chain of the molecule on the right, it still has the same condensed structural formula, as shown in Figure 2.
Figure 1. Both molecules are the same isomer of butane.
Figure 2. The condensed structural formula of butane.
An alkene is a hydrocarbon with at least one double bond between carbons. The simplest alkene is ethylene, C 2H 4. See Figure 3.
Figure 3. Ethylene—an alkene.
As is the case with the alkanes, each carbon atom in an alkene has precisely four bonds to fill its valence orbitals with eight electrons.
Another simple alkene is propene, C 3H 6. In Figure 4, propene demonstrates that alkenes can (and usually do) contain single bonds between some carbons. The existence of any double bond between carbons is the defining character.
Figure 4. Propene—one double and one single bond.
A hydrocarbon with a triple bond between carbons is an alkyne, and the simplest compound in this class is acetylene, C 2H 2, as shown in Figure 5.
Figure 5. Acetylene—an alkyne.
Once again, each carbon has exactly four bonds. Of course, the triple bond between carbons allows each carbon to bond to only one more atom. In acetylene, the single bond is to hydrogen, but in other alkynes, the single bond is to another carbon. Table 2 compares three hydrocarbons that contain the same number of carbon atoms.
IUPAC retains these common, nonsystematic names: *Ethylene is the nonsystematic name for ethene; **acetylene is the nonsystematic name for ethyne. These common names are generally accepted by IUPAC.
Look at the third column of the chart and appreciate the diminishing hydrogen content of the compounds as the number of carbon‐carbon bonds increases. Organic compounds with multiple carbon‐carbon bonds readily react with hydrogen gas.
The hydrogenation reaction is possible only for compounds with double or triple bonds, and such compounds are said to be unsaturated hydrocarbons. The addition of the hydrogen to the carbon atoms that were double‐ or triple‐bonded converts the unsaturated compound to a saturated hydrocarbon with only single bonds.
It is possible for long chains of carbons to loop around and form a closed ring structure. If you take the linear isomer of hexane in Figure 6 and delete the two hydrogens on the ends, the chain can form a hexagonal structure, as shown in Figure 7.
Figure 6. Hexane.
Figure 7. Cyclohexane.
Cyclohexane contains only single bonds and is representative of the simplest type of cyclic hydrocarbons.
A ring structure may possess double bonds, as in the following portrayal of the well‐known hydrocarbon benzene, which has the composition C 6H 6. See Figure 8.
Figure 8. Benzene.
The two representations of the benzene ring differ in the location of the three double bonds. The arrows between the structures represent hypothetical transitions between the two possible configurations. Only one variety of benzene exists with all six carbon‐carbon bonds having the same length and strength, so it seems best to regard the six extra electrons of the double bonds as being delocalized over the entire ring structure. Substances with benzene‐like rings are called aromatic compounds.
- Show the three isomers of pentane as condensed structural formulas.
- Write a balanced molecular reaction for the hydrogenation of acetylene to a saturated alkane. How many liters of hydrogen gas are needed to react completely with 100 liters of acetylene?