Functional Group Priorities in Organic Nomenclature

Group		As a substituent:
from highest to lowest priority:			examples:
onium ion
carboxylic acid		carboxy
sulfonic acid		sulfo
anhydride
ester		alkoxycarbonyl
acid chloride		chloroformyl
amide		amido or carbamoyl
nitrile		cyano	3-cyanopentanoic acid

Group		As a substituent:
aldehyde		oxo or formyl	4-oxobutanoyl chloride 4-formylcyclohexanecarbonitrile
ketone		oxo	methyl 3-oxopentanoate
alcohol		hydroxy	3-hydroxycyclopentanone
phenol
thiol		mercapto
amine		amino	2-aminophenol
peroxide		peroxo
organometallic

Group		As a substituent:
alkene		alken or alkenyl	3-butenal
alkyne		alkynyl
ether		alkoxy	6-methoxy-3-heptene
halogen		halo	4-bromo-2-pentyne
nitro		nitro	2-nitrotoluene