Nomenclature of Alicyclic Compounds
Compounds containing one or more rings but do not show aromatic characters are called alicyclic compounds.
The following rules are generally used to name alicyclic compounds.
Cyclo : Prefix 'cyclo' is added to the word root of the alicyclic compounds.
Lowest sum rule : When two or more substituents are present in the alicyclic ring, the numbering is done in such a way that the sum of the positional numbers (locants) of the substituents is the lowest.
Number alphabetically : If two or more substituents are present, then the numbering is done from the substituent which comes first in the alphabetical order provided it satisfies the lowest sum rule.
Example : 1-Ethyl-2-methylcyclohexane
Priority : According to IUPAC 2013 recommendation, if both alicyclic ring and side chain are present in the compound, the compound is named as a derivative of ring regardless of the length. Prior to 2013 rules, priority was given to the part having greater number of carbon atoms, but not anymore.
Example : Since the priority of ring is higher than that of side chain, the following compound is a derivative of ring.
Example : Although the side chain has more carbon atoms, the following compound is a derivative of alicyclic ring. Notice that earlier it was named cyclopropylbutane as length of the carbon chain used to be given priority.
Butylcyclopropane (according to iupac 2013 rules)
Cyclopropylbutane (according to earlier rules)
Example : Look at the way the following compound is named.
Notice that the part inside the brackets is named according to the rules we learned in complex substituents nomenclature.
Presence of a Functional Group : Functional group is given the highest priority while naming compounds. Study the following examples :
Example : In the following example, the compound is a derivative of the side chain because of the presence of a functional group in the side chain.
Example : Look at the way a compound is named when the substituent part contains a multiple bond.
Principal functional group : If the alicyclic ring as well as the side chain contains the functional group, the compound is named as a derivative of the one which contains the principal functional group.
Example : In the following compound, the principal functional group is attached to the side chain; consequently, the compound is a derivative of the side chain.
Example : The following compound is a derivative of the alicyclic ring due to the presence of the principal functional group in the ring.
Numbering : The following order is followed while numbering an alicyclic ring :
Priority : Functional group > Multiple bond > Substituents
Example : In the following compound, the double bond is given preference because there is no functional group (Cl is considered as substituent).
Example : Preference is given to the functional group followed by the double bond and the substituents in the following example :
Presence of an aromatic ring : If a compound contains an alicyclic ring and a benzene ring, then the compound is named as a derivative of alicyclic ring if it contains more carbon atoms than benzene else the compound is named as a derivative of benzene.
Example : The following compound is named as a derivative of benzene because both the rings have equal number of carbon atoms.
Example : Cyclohexylbenzene
Example : The following compound is named as a derivative of alicyclic ring because it contains more number of carbon atoms. Notice that the prefix phenyl is used for benzene as substituent.
Example : Phenylcyclooctane
Two alicyclic rings are attached : When two alicyclic rings are attached, then the compound is named as a derivative of the one that contains greater number of carbon atoms.
Example : Cyclopropylcyclobutane
Presence of carbon containing functional group If the functional group that is attached to the alicyclic ring contains a carbon, and the carbon is not the part of the ring, the carbons are not included in the parent name of the alicyclic systems. Furthermore, different suffixes are used in these situations. These types of functional groups with their prefixes and suffixes are listed in the table below :
|-COX (X=F,Cl,Br,I)||Halocarbonyl||Carbonyl halide|
|-COOR||Alkoxycarbonyl or Carbalkoxy||Alkyl carboxylate|
Example 1: Cyclohexanecarbonitrile
Example 2: Cyclohexanecarbaldehyde
Example 3: 4-(2-Hydroxyeth-1-yl)cyclohexane-1-carboxylic acid