Organic Functional Groups — Names, Structures, and Properties
| Functional Group | General Formula | IUPAC Suffix / Prefix | Example Compound | Example Formula | Key Properties | Naming Priority |
|---|---|---|---|---|---|---|
| Carboxylic acid | R–COOH | -oic acid | Acetic acid | CH₃COOH | Acidic, H-bonding, forms dimers | 1 |
| Ester | R–COO–R' | -oate | Ethyl acetate | CH₃COOC₂H₅ | Fruity odor, hydrolyzable | 2 |
| Amide | R–CONH₂ | -amide | Acetamide | CH₃CONH₂ | High bp, peptide bonds | 3 |
| Aldehyde | R–CHO | -al | Formaldehyde | HCHO | Easily oxidized, reducing agent | 4 |
| Ketone | R–CO–R' | -one | Acetone | CH₃COCH₃ | Polar, good solvent | 5 |
| Alcohol | R–OH | -ol | Ethanol | C₂H₅OH | H-bonding, high bp | 6 |
| Phenol | Ar–OH | -ol (phenol) | Phenol | C₆H₅OH | Weakly acidic (pKa ~10) | 6 |
| Amine (primary) | R–NH₂ | -amine | Methylamine | CH₃NH₂ | Basic, fishy odor | 7 |
| Amine (secondary) | R₂NH | -amine | Dimethylamine | (CH₃)₂NH | Basic, more nucleophilic | 7 |
| Amine (tertiary) | R₃N | -amine | Trimethylamine | (CH₃)₃N | Basic, no N–H bond | 7 |
| Thiol | R–SH | -thiol | Ethanethiol | C₂H₅SH | Strong odor, forms disulfides | 8 |
| Ether | R–O–R' | -ether / oxy- | Diethyl ether | C₂H₅OC₂H₅ | Low reactivity, good solvent | 9 |
| Alkene | C=C | -ene | Ethene (ethylene) | C₂H₄ | Addition reactions, pi bond | 10 |
| Alkyne | C≡C | -yne | Ethyne (acetylene) | C₂H₂ | Terminal H is acidic, linear | 11 |
| Alkane | C–C, C–H | -ane | Methane | CH₄ | Unreactive, combustion | 12 |
| Alkyl halide | R–X (X=F,Cl,Br,I) | halo- | Chloromethane | CH₃Cl | SN1/SN2/E1/E2 reactions | — |
| Acyl halide | R–COX | -oyl halide | Acetyl chloride | CH₃COCl | Very reactive, hydrolyzes easily | — |
| Acid anhydride | R–CO–O–CO–R' | -anhydride | Acetic anhydride | (CH₃CO)₂O | Reactive acylating agent | — |
| Nitrile | R–C≡N | -nitrile / -carbonitrile | Acetonitrile | CH₃CN | Polar aprotic solvent, hydrolyzable | — |
| Nitro | R–NO₂ | nitro- | Nitromethane | CH₃NO₂ | Electron-withdrawing, explosive | — |
| Sulfide (thioether) | R–S–R' | -sulfide | Dimethyl sulfide | (CH₃)₂S | Nucleophilic at S | — |
Naming priority follows IUPAC 2013 recommendations: lower priority number = higher seniority and claims the suffix. R denotes an alkyl or aryl substituent; Ar denotes an aromatic ring. Groups with priority null (alkyl halide, nitro, ether, etc.) are always cited as prefixes regardless of context. The 'key property' column lists the most diagnostic feature for spotting the group in spectra or reactions (carboxylic acid dimers, amide N–H IR around 3300 cm⁻¹, ketone carbonyl at 1715 cm⁻¹, nitrile around 2250 cm⁻¹). Source: IUPAC Recommendations on Organic Nomenclature (2013), Clayden's Organic Chemistry.
Frequently Asked Questions
How do you determine which functional group gets the suffix in IUPAC naming?
The senior group — carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene > alkyne > alkane — claims the suffix and anchors the parent chain numbering. Lower-priority groups appear as prefixes: -OH becomes hydroxy- when a ketone is present, C=O becomes oxo- when a carboxylic acid is present, -NH₂ becomes amino-. Number the chain so the senior group gets the lowest locant. For example, 4-oxopentanoic acid: the acid is senior, ketone demoted to oxo-.
What is the difference between an aldehyde and a ketone?
Both share the C=O carbonyl, but aldehydes carry it at chain-terminal position with at least one hydrogen on the carbonyl carbon (R–CHO), while ketones place it between two carbon substituents (R–CO–R'). Aldehydes oxidize easily to carboxylic acids (Tollens, Fehling, KMnO₄ all positive); ketones resist oxidation under those conditions. Aldehydes are also more electrophilic — less steric hindrance and less alkyl donation into the carbonyl — so they undergo nucleophilic addition (hydration, hemiacetal formation) faster than ketones.
Why do carboxylic acids have such high boiling points?
Each –COOH carries both a hydrogen-bond donor (O–H) and acceptor (C=O), so two molecules pair into a cyclic dimer locked by two hydrogen bonds. The dimer is so stable it persists even in the gas phase, effectively doubling the molecular weight that has to vaporize. Acetic acid boils at 118 °C while acetaldehyde (similar molar mass, only an acceptor) boils at 20 °C. The same dimerization explains why short carboxylic acids show abnormally high vapor-phase molecular weights by mass spec.