Decreasing Priority Order of Functional Groups in IUPAC Nomenclature

To determine the correct order of decreasing priority of functional groups in IUPAC nomenclature, we recall the standard IUPAC priority table for principal functional groups in decreasing order:

$\text{Carboxylic acids } (-\text{COOH}) > \text{Esters } (-\text{COOR}) > \text{Acid Halides } (-\text{COX}) > \text{Amides } (-\text{CONH}_2) > \text{Nitriles } (-\text{CN}) > \text{Aldehydes } (-\text{CHO}) > \text{Ketones } (>\text{C}=\text{O}) > \text{Alcohols } (-\text{OH}) > \text{Amines } (-\text{NH}_2) > \text{Alkenes } (-\text{C}=\text{C}-) > \text{Alkynes } (-\text{C}\equiv\text{C}-)$

Evaluating the given options based on this priority order:

1. Option A: $-\text{CONH}_2, >\text{C}=\text{O}, -\text{CHO}, -\text{NH}_2, -\text{C}\equiv\text{C}-$

   • Incorrect, because Aldehydes ($-\text{CHO}$) have a higher priority than Ketones ($>\text{C}=\text{O}$).

2. Option B: $-\text{CONH}_2, -\text{COOCH}_3, -\text{CHO}, -\text{NH}_2, -\text{OH}$

   • Incorrect, because Esters ($-\text{COOCH}_3$) have a higher priority than Amides ($-\text{CONH}_2$), and Alcohols ($-\text{OH}$) have a higher priority than Amines ($-\text{NH}_2$).

3. Option C: $-\text{CONH}_2, -\text{CHO}, >\text{C}=\text{O}, -\text{NH}_2, -\text{C}\equiv\text{C}-$

   • Correct, because: $\text{Amide } (-\text{CONH}_2) > \text{Aldehyde } (-\text{CHO}) > \text{Ketone } (>\text{C}=\text{O}) > \text{Amine } (-\text{NH}_2) > \text{Alkyne } (-\text{C}\equiv\text{C}-)$

4. Option D: $-\text{CONH}_2, -\text{CHO}, -\text{CN}, -\text{NH}_2, -\text{C}\equiv\text{C}-$

   • Incorrect, because Nitriles ($-\text{CN}$) have a higher priority than Aldehydes ($-\text{CHO}$).

Hence, the correct order is given in Option C.