The reason that there are so many carbon compounds is that the alkanes listed above can be varied very easily, with simple substitutions, in a nearly endless number of ways. Consider the fairly simple case of a propane molecule with two hydrogens replaced by chlorines to yield dichloropropane, as shown in Figure 3.3.
Figure 3.3. Isomers of dichloropropane
Note that all four of these compounds have three carbons in a chain (propane) and have two chlorines (dichloro). They are, however, not the same compound, as the locations of the chlorines are different in each case. A systematic way to distinguish between all four of these is needed, and there is also need to avoid redundancy. For instance, the first compound is the same as the one shown in Figure 3.4, except rotated 180 degrees. They are, in reality, the same compound, and it should be possible to know how to name them the same way.
Figure 3.4. 1,1-Dichloropropane. Despite appearances, this molecule is the same as the one in the top left of Figure 3.3