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 CnH2n. 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 CnH2n-2.
3.8:
Cycloalkanes
Cycloalkanes are saturated cyclic hydrocarbons with carbon atoms arranged in the form of rings. They have two fewer hydrogen atoms than the corresponding acyclic alkane; therefore, their general formula is CnH2n. The structural formulas of cycloalkanes are simplified using the line-angle representation. The regular polygons are used to represent the cycloalkane rings, with each side representing a carbon-carbon bond.
The IUPAC nomenclature of cycloalkanes follows similar rules that apply to open-chain alkanes with few minor variations. The prefix “cyclo” is added to the corresponding open-chain name to indicate the presence of a ring. A monosubstituted cycloalkane is named by adding the substituent name before the parent name without specifying the locant. For cycloalkanes with multiple identical or non-identical substituents, the locant values are assigned in such a way that the substituents receive the lowest set of values. If the substituent has more carbon atoms than the ring, the name of the cyclic part is written before the acyclic part. However, as per the updated IUPAC rules,1 irrespective of the carbon atoms, the ring always receives preference, and hence, it is considered the parent.
Bicycloalkanes are formed by fusing two rings that share two carbon atoms known as bridgeheads. They have two fewer hydrogens than the cycloalkanes and hence have the general formula CnH2n-2.
Suggested Reading
- K.-H. Hellwich, R. M. Hartshorn, A. Yerin, T. Damhus, A. T. Hutton, Brief Guide to the Nomenclature of Organic Chemistry(IUPAC Technical Report), Pure Appl. Chem., 2020; 92(3): 527–539