IUPAC Nomenclature: A Comprehensive Guide for Chemistry Enthusiasts

• Arenes or Aromatic compounds: Closed structures made of carbon, which may or may not have other elements in the framework. For a compound to be aromatic, it must follow four rules: 

• Non-aromatic or Alicyclic compounds: Compounds without ring structure containing carbon along with elements. They don’t necessitate a continuous form of overlapping ring of p orbitals.

Priority Rules to Name Aromatic and Non-Aromatic Organic Compounds

In the following sections, we’ll learn the nomenclature of non-aromatic and aromatic compounds step by step:

Non-Aromatic/Alicyclic Compounds

Non-aromatic compounds are named by:

• Adding the prefix ‘cyclo’ to the alkane.
  
   

• In the presence of side chains, the alphabetic order of numbering is used.

  For example, a cyclic 6-carbon compound with methyl and propyl group will be named 1-methyl-3-propylcyclohexane. 

• In the presence of multiple bonds or functional groups, the corresponding suffixes are added to the word root. The number is done so as to give the minimum possible number to the multiple bond or functional group.

  

• In the presence of a cyclic ring attached to a carbon chain:

  ○ If the carbon chain has more number of carbon atoms than the cyclic ring, the name will be cycloalkylalkane (e.g. 2-cyclopropylhexane)

  ○ If the carbon chain has a lesser number of carbon atoms than the cyclic ring, the name will be a derivative of cycloalkane (e.g. 1-propyl cyclohexane)

Arenes or Aromatic Compounds

• The aromatic compounds with substituents are named either as ‘prefix + benzene’ or hold universal names. 

• The substituted benzene ring is numbered such that the substituent gets the lowest number. 

• In the presence of 2 substituents,

  ○ ortho (o) indicates compounds at 1 and 2 positions

  ○ meta (m) indicates compounds at 1 and 3 positions

  ○ para (p) indicates compounds at 1 and 4 positions

For example, the compound here will be named m-dichlorobenzene or 1, 3-dichlorobenzene.

• For three or more substituents, the numbering is done as per the lowest number of substituents, along with the lowest possible sum of all groups. The substitutes are listed alphabetically by name. 

For example, a benzene ring with chloro and 2 nitro groups will be named 1-chloro-2, 4-dinitrobenzene. 

What is Meant by the Homologous Series?

The series of compounds with common functional groups are called homologues and belong to the homologous series. The members of a specific series are represented by the general molecular formula. They differ only in the carbon chain length, i.e., two consecutive members differ by -CH₂ group in their molecular formulae. 

The presence of the following characteristics categorises homologous series: 

• The presence of the same functional group in all the members of the series.

• Presence of the same general formula for hydrocarbon series.

• Possession of the same chemical properties due to unvaried functional group.

• The physical properties show almost regular variation.

• The preparation method remains the same.

• The molecular mass differs by 14 amu from that of its neighbour.

Example: 

Homologues of alcohol series:

• Methyl alcohol
• Ethyl alcohol
• Propyl alcohol
• Butyl alcohol
• Pentyl alcohol
• Hexyl alcohol and so on.

Conclusion

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FAQ's

Q 1. What is IUPAC nomenclature?

Ans: IUPAC nomenclature is the systematic method of naming organic chemical compounds. It is based on the rules listed to name the compounds despite their structure and isomers logically. 

Q 2. How do I start naming a compound using IUPAC rules?

Ans: Begin by identifying the parent chain and assigning a root word to it. Number the chain for substituents. Name and alphabetically arrange substituents as prefixes. Next, identify and name functional groups, consider bond multiplicity, and include numbers to indicate substituent locations. If multiple functional groups are present, prioritise the one with higher naming priority. 

Q 3. What are functional groups, and how do they affect naming?

Ans: Functional groups are specific groups of atoms in the molecules that impact characteristic chemical reactivity to the molecules. The numbering in the presence of functional groups is done such that the carbon attached to these gets the lowest possible number in the chain. Thus, they are determining factors in the naming of the compounds. 

Q 4. How are branched compounds named?

Ans: Branched compounds are named by identifying such and numbering the longest carbon chain so that branches get the lowest numbers. The lowest number is preferred according to alphabetical order. 

Q 5. How do I name cyclic compounds?

Ans: Identify the parent chain (it should be larger than the substituent) and number the substituents (if present) such that they get the lowest number. Prefer numbering in alphabetical order in the presence of multiple substituents. Ensure adding the prefix ‘cyclo’ before the corresponding straight-chain alkane.