With the vast array of ways to arrange carbon atoms with respect to one another, sometimes the same chemical formula will give rise to an array of molecules that all have the same number and type of atoms, but with different structures, and therefore slightly or significantly different properties. These variations are called isomers.

Isomerization can be as simple as the rearrangement of substituents in a molecule. For an example, see the case of dichloropropane described earlier, Figure 3.3. There are four different ways to arrange two chlorine atoms on a propane backbone, leading to four different structural isomers of the compound.

A type of structural isomerization is observable for alkenes that can become important in plastics and polymers (see Section 3.5). Alkenes are different from alkanes in that rotation around a double bond is not possible, making the location of substituents on the two doubly bonded carbons distinct. If the substituents are on the same side of the double bond, the configuration is referred to as cis. If the substituents are on opposite sides, the configuration is trans. Figure 3.11 shows examples of cis and trans substitutions. These configurations can make a significant difference in chemical properties of materials made from the different isomers.

cis-Dichloroethene (left) and trans- dichloroethene (right)

Figure 3.11. cis-Dichloroethene (left) and trans- dichloroethene (right)

A more drastic example would be that of C2H6O. There are two ways, as shown in Figure 3.12, that these atoms can be arranged into a compound, but unlike dichloropropane, their properties are very different, as seen in Table 3.6.

Two structure of C2H6O. Ethanol on the left, dimethyl ether on the right

Figure 3.12. Two structure of C2H6O. Ethanol on the left, dimethyl ether on the right.

Table 3.6. Properties associated with structures in Figure 3.12



Dimethyl Ether

Melting point



Boiling point



Density (as liquid)

0.789 g/cm3

0.735 g/cm3

Solubility in water


71 g/L

Physiological effect


Weak depressant

These two molecules are ethanol and dimethyl ether. Their physical and chemical properties are quite distinct, even though they are structural isomers of each other.

A further grouping of compounds that are similar is the classification of congeners. This is a significantly broader and somewhat vaguer term than isomers. Congeners refer to molecules that have the same basic makeup, but vary in number or location of substituents. In organic chemistry, this usually refers to a molecule with a various number of same or similar substituents. As an example, chloroethane, dichloroethane, trichloroethane, and other enumerations of chloroethane would fall into a class of congeners.

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