Select the principal functional group when the following groups are present in the molecule:
\[OH,\;COOH,\; - N{O_2}\]

Hint: Functional group refers to a group of bonds or atoms in a compound which is mainly responsible for the chemical reactions of that specific compound. The same type of functional group behaves in the same manner by undertaking the same reactions, irrespective of the compound.


Complete solution:
 The functional group in the molecule is identified by determining the appropriate suffix. Longest chain of the carbon atoms comprising the functional group can be numbered in a manner that the functional group is linked with the carbon atom having the lowest available number in the chain. While for polyfunctional compounds, among all functional groups one is selected as the fundamental or principal functional group. Thereafter, on this basis, compounds are named. The leftover functional groups are subordinate and are named employing appropriate prefixes and these are named substituents. The principal functional group is selected depending upon the predefined order of preference.
The order of priority for functional group in the decreasing order is given as follows:
carboxylic acid > sulphonic acid > esters > amide > nitrile > aldehydes > ketones > alcohol > amines > hydrocarbons > ethers > alkyl halides
Thus in the present case, carboxylic acid i.e. $ - COOH$ will be the principal functional group among all other groups present in the molecule.


Note: If multiple functional groups of similar type are present, then their number can be written as di (for two), tri (for three), etc before the suffix. In this scenario, the full name of the parent compound is written before the suffix.