3.8 : Cycloalkanes

Organic molecules like sucrose and cholesterol are cyclic compounds composed of rings of atoms.

Cycloalkanes are saturated cyclic hydrocarbons containing rings of carbon atoms.

Simple cycloalkanes contain repeating units of CH₂ groups and have the general formula C_nH_2n. They contain two fewer hydrogen atoms than their corresponding acyclic form.

When naming cycloalkanes, the prefix "cyclo" — indicating the presence of a ring — is added to the name of the corresponding open-chain alkane, and together, they are called the parent.

Line-angle formulas simplify the structural representations of different cycloalkane rings. Simple polygons symbolize rings of different sizes, with each side of the polygon representing a carbon-carbon bond.

In a cycloalkane with a single substituent, the substituent's name precedes the parent name without the locant. The compound is named as an alkyl cycloalkane.

In the case of two non-identical substituents, the ring carbons are numbered beginning from the carbon with the substituent that comes earlier in the alphabetical order.

Numbering continues in the direction that gives the next lower number to the second substituent. Hence, the correct name is 1-ethyl-3-methylcyclohexane.

When multiple substituents — same or different — are present on the ring, the ring is numbered starting from the direction that gives the lowest set of locant values to all substituents.

This rule also applies if multiple substituents are attached to the same carbon atom in the ring.

If, in a substituted cycloalkane, the substituent has more carbon atoms than the ring, the name of the cyclic part appears before the name of the acyclic substituent. Meaning, the compound is named as cycloalkyl alkane instead of alkyl cycloalkane.

Bicycloalkanes contain two bridged rings that share two carbon atoms — known as the bridgehead carbons.

Due to the fused rings, bicycloalkanes contain two fewer hydrogen atoms than regular cycloalkanes, and therefore have the general formula C_nH_2n-2.