Question

Explain the longest chain rule and the smallest number for functional groups rule of nomenclature by the IUPAC system – with suitable examples.

Answer 1

The International Union of Pure and Applied Chemistry (IUPAC) has defined some universal rules for determining the name of any chemical compound in organic chemistry. According to this set of rules, the priority of the functional group and the length of the parent carbon chain is determined.

Complete step by step answer:

According to the rules of IUPAC nomenclature, the longest chain rule and the smallest number for functional groups can be determined. They are:
A. For the identification of the parent hydrocarbon chain, the chain must obey the following rules, in order of precedence:
(i) It should have the highest number of substituents or branches cited as prefixes.
(ii) It should have the maximum number of substituents of the suffix functional group. By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. If more than one functional group is present, the one with highest precedence should be used.
(iii) It should have the maximum length.
(iv) It should have the maximum number of multiple bonds.
(v) It should have the maximum number of single bonds.
For example, butane and 2-methylpropane have the same molecular formula but they have different structural formulas. The length of parent carbon chain in butane is 4 carbon atoms, whereas the length of parent carbon chain in 2-methyl propane is 3 carbon atoms.
B. For the identification of the functional group, if present in the parent chain, the highest order of precedence is followed.
The functional group should have the smallest number based on the preference in case there is more than one functional group present in the compound. For example, in the organic compound 2-oxo-propanoic acid, there are two functional groups present, oxo and carboxylic acid. But according to the preference order of the functional groups, carboxylic acid lies above the ketone group. Thus, the ketone group acts as a substituent while the carboxylic acid acts as the main functional group.

Note:
The numbering of the carbon chain in an organic compound is done in such a way that the combination of the numeric values comes out to be the lowest in case of multiple number of substituents present in the compound. For example, let us name the following organic compound:$C{H_3} - CH(Br) - CH(F) - CH(N{O_2}) - C{H_2} - C{H_3}$
The numbering of the compound can be done from left to right and from right to left. When the numbering is done from left to right, it comes out to be 234 and when it is done from right to left, the numbering comes out to be 345. The naming should be done in the order which has a lower numeric value. Thus, the name of the organic compound is: 2-bromo-3-fluoro-4-nitro hexane.