Nomenclature and Isomerism

The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature.the name of the compound is written out with the substituents in alphabetical order followed by the base name (derived from the number of carbons in the parent chain). Commas are used between numbers and dashes are used between letters and numbers. There are no spaces in the name

Nomenclature

The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature.

In general, an IUPAC name will have three essential features:

• A root or base indicating a major chain or ring of carbon atoms found in the molecular structure.
  • A suffix or other element(s) designating functional groups that may be present in the compound.
  • Names of substituent groups, other than hydrogen, that complete the molecular structure.

 

Here is a simple list of rules to follow. Some examples are given at the end of the list.

1. Firstly, the longest chain is to be identified which is known as parent chain
2. Identification of all substituents groups is to be done
3. Number the carbons of the parent chain from the end that gives the substituents the lowest numbers. When comparing a series of numbers, the series that is the "lowest" is the one which contains the lowest number at the occasion of the first difference. If two or more side chains are in equivalent positions, assign the lowest number to the one which will come first in the name.
4. If the same appending group occurs more than once, the location of each point on which the appendix group is given. Repetition of an appending group makes prefix occur di, tri, tetra etc.
5. If there are two or more different appending groups then, alphabetical order is used to enlist them. The prefix used for the appending group in alphabetical order is iso. The prefixes secondary and tertiary are used when compared.
6. If chain of equal length is to be made parent chain, then, following points aka priority:

The greatest number of side chains.
Substituents having the lowest- numbers.
Having the greatest number of carbon atoms in the smaller side chain.
Having least branched side chains

7. The prefix cyclo is directly used for cyclic hydrocarbons.

In summary, the name of the compound is written out with the substituents in alphabetical order followed by the base name (derived from the number of carbons in the parent chain). Commas are used between numbers and dashes are used between letters and numbers. There are no spaces in the name.

CH₄ → methane

C₂H₆ → Ethane

CH₃ —CH₃ → Ethane

C₃H₈ → Propane

CH₃ —CH₂ —CH₃ → n-propane

CH₃ —CH₂ — CH₂ —CH₂— CH₃ → n-pentane

→ Isohexane OR 2 – methyl pentane (IUPAC)

→ Isobutane OR 2 − methyl propane

IUPAC Nomenclature

Prefix + word root + primary suffix + secondary suffix

 

Prefix → 3-methyl

Word root → Hex

Primary suffix → ane

Therefore the IUPAC name is 3-methyl hexane

 E.g. Pentane (C₅H₁₂₎

E.g. Hexane (C₆H₁₄)