Organic functional groups — the 24 you have to recognize

A functional group is the reactive fragment of an organic molecule. Two compounds with the same carbon skeleton but different groups behave completely differently: ethanol (CH₃CH₂OH) is a solvent you can drink in trace amounts, ethanamine (CH₃CH₂NH₂) is a base that reeks of fish, ethanal (CH₃CHO) polymerises on standing. The chemistry lives in the group, not the alkyl chain.

Learning to spot groups on sight lets you predict three things without further thought: reactivity (what this molecule attacks or is attacked by), physical properties (boiling point, water solubility, IR/NMR signatures), and the correct IUPAC name. In substitutive nomenclature the highest-priority group becomes the parent-chain suffix and every other group becomes a prefix; in the older functional-class nomenclature the group is named as a separate word (e.g. “ethyl methyl ether”, “acetyl chloride”). Modern IUPAC prefers substitutive, but retained names (phenol, aniline, acetone, acetic acid) are still allowed and are what most chemists actually say.

This card is the 24-group short list every general and organic chem course expects on sight.

The core table

Names and prefixes are drawn from the IUPAC 2013 Recommendations (Blue Book), Ch. P-6 and Table 4.1 in P-14.5.2. Common-name equivalents (acetone, acetic acid, acetaldehyde, aniline) are IUPAC-retained. Cross-checked against Clayden 2e Ch. 2 and Vollhardt & Schore 8e Ch. 2.

IUPAC principal-group priority (which one becomes the suffix)

When a molecule has more than one functional group, exactly one is chosen as the principal characteristic group and named as the parent-chain suffix. Every other group is named as a substituent prefix. The seniority ladder below is the standard organic-course cut of IUPAC 2013 Blue Book P-14.5.2 (Table 4.1) / P-41 — higher rows win.

Worked read: 4-hydroxybutanoic acid has both -OH and -COOH. Acid outranks alcohol, so -oic acid is the suffix and the alcohol drops to hydroxy- prefix — never “4-carboxybutan-1-ol”. Similarly 3-oxobutanoic acid (acetoacetic acid) keeps the acid as suffix and the ketone becomes oxo-.

Two ranking gotchas worth memorising:

• Halogens, nitro, and ether/alkoxy groups are always prefixes — they never claim the suffix, regardless of what else is present.
• Double and triple bonds are ranked separately: they enter the parent name as -ene / -yne but do not compete with characteristic groups. Butenoic acid keeps -oic acid as suffix and -ene as the unsaturation infix (but-2-enoic acid).

Ether vs ester vs amide vs anhydride — telling the -CO-O / -CO-N cluster apart

Half of student naming errors are in this small family. Read left-to-right through the carbonyl:

• Ether R-O-R’ — no carbonyl. Unreactive under most acid/base conditions; the C-O-C is the only feature.
• Ester R-CO-O-R’ — carbonyl on the left, single-bonded O to another carbon on the right. Named “alkyl (from the O-R’ side, i.e. from the alcohol) alkanoate (from the acyl side, i.e. from the acid)”. Ethyl ethanoate = ethyl (from ethanol) + ethanoate (from ethanoic acid).
• Amide R-CO-N< — carbonyl bonded straight to N. The C=O withdraws density from N, so amide N is a very weak base (protonation happens at O, not N).
• Anhydride R-CO-O-CO-R’ — two acyl groups sharing one bridging oxygen. Hydrolyses to two acid molecules.
• Acyl chloride R-CO-Cl — the most reactive of this cluster; hydrolyses on contact with water.

Common student trip-ups

1. The nitrile carbon is part of the parent chain. Acetonitrile CH₃CN is ethanenitrile, C1 = the CN carbon, so it’s a two-carbon parent, not one. This trips people because “cyano-” (the prefix form) does NOT count in the chain — CN as a suffix does, as a prefix does not.
2. The aldehyde and carboxylic-acid carbon is always C1. These groups can only sit at the end of a chain, and IUPAC numbers from that end. You never write “2-al” or “3-oic acid”.
3. Ester word order. “Ethyl ethanoate” = ethyl (the R’ from the alcohol side, R’-O-) + ethanoate (the acyl side, R-CO-). Beginners flip these because both halves are “ethyl-something”.
4. Ether vs ester — one letter, worlds apart. R-O-R’ (ether) is nearly inert; R-CO-O-R’ (ester) hydrolyses to acid + alcohol under acid or base catalysis. Do not skim.
5. Amide (-CO-NH₂) vs amine (-NH₂). The C=O next to N in an amide delocalises the N lone pair into the carbonyl. Amines have pKₐ(BH⁺) ≈ 9–11 (weak base). Amides have pKₐ(BH⁺) ≈ 0 for O-protonation; the N is essentially non-basic (Clayden 2e Ch. 8).
6. Phenol vs alcohol. Phenol (Ar-OH) has pKₐ ≈ 10; ethanol has pKₐ ≈ 16. The aromatic ring stabilises the phenoxide by delocalising the negative charge into the ring — a millionfold difference in acidity from a functional-group swap that looks identical on paper.

See also

• Common Ka and Kb values — the pKa numbers behind phenol vs alcohol and amine vs amide.
• Polyatomic ions — the inorganic counterpart of this card.
• Molar mass calculator — parses organic formulas from Hill notation.
• pH calculator — weak-acid / weak-base ICE-table treatment for the groups on this card.

Sources

• IUPAC, Nomenclature of Organic Chemistry: Recommendations and Preferred Names 2013 (Blue Book) — Ch. P-6 (characteristic groups and their suffixes/prefixes) and P-14.5.2 / P-41 (Table 4.1, seniority of the principal characteristic groups).
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed. (Oxford, 2012) — Ch. 2, pp. 27-33 (functional-group table and reactivity preview).
• Vollhardt & Schore, Organic Chemistry: Structure and Function, 8th ed. (Freeman, 2018) — Ch. 2 §2-4 and the inside front cover (“Common functional groups”).
• Bruice, Organic Chemistry, 8th ed. (Pearson, 2016) — Ch. 3 (an introduction to organic compounds; functional-class summary).
• Smith, March’s Advanced Organic Chemistry, 8th ed. (Wiley, 2020) — Appendix A, cross-checked for edge cases (enamine, imine, epoxide nomenclature).