The ability of carbon to form bonds with itself allows for the possibility of the formation of cyclic compounds. In nature, cyclic compounds with ring sizes from 3 to 30 carbons are known; five- and six-member rings are especially common. For a simple cycloalkane the general molecular formula is C_nH_2n, where n is the total number of carbons. You will note that this differs from the general formula for an alkane (C_nH_2n+2) by the lack of the two additional hydrogens (the "+ 2 term"). As a general rule, every ring which is constructed from an alkane reduces the number of hydrogens in the molecular formula for the parent hydrocarbon by 2. Thus one ring gives C_nH_2n, two rings within the molecule would give a molecular formula of the type C_nH_2n-2, three rings, C_nH_2n-4, etc. Thus by simply examining the molecular formula of an alkane or cycloalkane, you can immediately calculate the number of rings within the molecule. This notion can be expanded to also include double bonds (which also reduce the number of hydrogens in an alkane by two) to give the concept of degree of unsaturation, which is covered in a later section. Using this simple calculation, the total number of rings and multiple bonds in a molecule can be calculated, based simply on the observed molecular formula. Molecular models of cycloalkanes with n = 3 to 7 are shown below. You should note that, in the smaller ring sizes (3, 4 and 5), the bond angles are significantly less than the optimal 109.5^o. This results in a significant amount of ring strain in these compounds which makes many small rings susceptible to ring-opening reactions. The bond angles in a six-membered ring match well with the tetrahedral geometry of carbon and there is virtually no ring strain in these compounds. Rings which are seven-membered and larger are highly distorted, and again display significant ring strain.

The nomenclature for a simple cycloalkane is based on the parent hydrocarbon, with the simple addition of the prefix cyclo. A three-membered ring is therefore cyclopropane, four-membered, cyclobutane, five-membered, cyclopentane, six-membered, cyclohexane, etc.

As a convenient shortcut, cyclic structures are usually drawn using line (structural or line-angle) drawings, as shown above. Again, it is important to understand that every vertex in these drawings represents a -CH₂- group, every truncated line a -CH₃ group and intersections of three or four lines represent 3^o or 4^o carbons, respectively.

Substituents on cycloalkanes are named using the conventions described for alkanes, with the exception that, on rings bearing only one substituent, no number is needed; otherwise numbering proceeds to produce the lowest number at the first point of difference.

Polycyclic carbons, such as those shown below, are common in organic chemistry. Carbons in these compounds which are shared between attached rings are termed bridgehead carbons, and, in the special case where only one carbon is shared between rings, the bridging carbon is referred to as a spiro carbon. Polycyclic compounds are named and numbered using a complex system to indicate ring sizes and attachments, and will be covered later.