Animated Solution for Chemistry - Organic Chemistry: The correct decreasing order of priority for the functional groups of organic compounds in the IUPAC system of nomenclature is

Imagine you are the mayor of a bustling city called 'Organic Molecule'. In this city, there are many important officials—the functional groups. But when it comes to naming the city's main street (the parent chain), only the highest-ranking official gets the honor. This is exactly how the IUPAC nomenclature system works. When a molecule contains multiple functional groups, we need a strict hierarchy to decide who is the 'boss'—the principal functional group.

This principal group dictates the suffix of the molecule's name, while all other lower-ranking groups are demoted to mere substituents, taking up prefix positions.

So, who sits at the top of this chemical monarchy? The priority order is generally based on the oxidation state of the central carbon atom. The more oxidized the carbon, the higher its priority.

At the absolute pinnacle sits the Carboxylic Acid ($-\text{COOH}$). It is the undisputed king of organic functional groups.

Right below it is the Sulfonic Acid ($-{\text{SO}}_3\text{H}$).

Next in line are the derivatives of carboxylic acids. Their order is: Esters ($-\text{COOR}$) > Acid Halides ($-\text{COCl}$) > Amides ($-{\text{CONH}}_2$).

After the acid derivatives, we move to the Nitriles ($-\text{CN}$), followed by the carbonyl compounds: Aldehydes ($-\text{CHO}$) and then Ketones ($>\text{C=O}$).

Further down the ladder, we find Alcohols ($-\text{OH}$), Thiols ($-\text{SH}$), and Amines ($-{\text{NH}}_2$).

Let's bring our focus back to the question at hand. We are given four groups: $-\text{COOH}$, $-{\text{SO}}_3\text{H}$, $-{\text{CONH}}_2$, and $-\text{CHO}$.

Using our royal hierarchy, we know that the carboxylic acid ($-\text{COOH}$) is the highest.

Next comes the sulfonic acid ($-{\text{SO}}_3\text{H}$).

Then, we look for acid derivatives. We have an amide ($-{\text{CONH}}_2$), which takes the third spot.

Finally, the aldehyde ($-\text{CHO}$) sits at the bottom of this specific list.

Therefore, the correct decreasing order of priority is:

This perfectly matches option (a). Memorizing this sequence is a non-negotiable rite of passage for every chemistry student. It is the key that unlocks the language of organic chemistry!