3.2 Functional groups: Organic compounds
- Syllabus
- First assessment 2025
- Topic
- 3.2
- Level
- SL
Learning objectives
3.2.1Organic formulas• Types: empirical, molecular, structural (full/condensed), stereochemical, skeletal• 3D models• Interconvert molecular, structural, and skeletal formulas3.2.2Functional groups• Give characteristic properties• Groups: halogeno, hydroxyl, carbonyl, carboxyl, alkoxy, amino, amido, ester, phenyl• Identify functional groups by name and structure3.2.3Homologous series• Successive members differ by CH₂• Series: alkanes, alkenes, alkynes, halogenoalkanes, alcohols, aldehydes, ketones, carboxylic acids, ethers, amines, amides, esters• Recognize general formulas and functional group patterns3.2.4Physical property trends• Melting and boiling points in homologous series• Explain trends using chain length, branching, polarity, and IMF3.2.5IUPAC nomenclature• Systematic naming for organic/inorganic compounds• Up to 6 carbons, one functional group type• Apply nomenclature to saturated or mono-unsaturated compounds with one functional group3.2.6Structural isomers• Same molecular formula, different connectivities• Types: branched, straight-chain, position, functional group isomers• Include primary, secondary, and tertiary alcohols, halogenoalkanes, and amines
Organic Formula Representations: From Ratio to Structure
| Representation | What it preserves or shows |
|---|---|
| Empirical | Simplest whole-number atom ratio |
| Molecular | Actual atom counts |
| Structural/condensed | Connectivity in compact form |
| Skeletal | Carbon framework and implied hydrogen |
| Stereochemical/3D | Spatial arrangement |
Translate representations without changing atom connectivity. For a skeletal formula, count every vertex and line end as carbon, add enough hydrogens to give carbon four bonds, and write heteroatoms explicitly. Then verify both the molecular formula and the connectivity.
An empirical formula is a ratio, not necessarily the complete molecule: hydrogen peroxide has molecular formula H₂O₂ but empirical formula HO. Reduce all subscripts by their greatest common factor; if no common factor exists, the molecular and empirical formula are identical.
Matching atom totals alone cannot prove two drawings are the same compound: connectivity and, where relevant, stereochemistry must also agree. Do not reduce a molecular formula when the subscripts already have no common factor.
Organic Functional Groups
| Family | Complete recognition pattern | Bounded characteristic cue |
|---|---|---|
| Halogenoalkane | C–F/Cl/Br/I | polar C–X bond |
| Alcohol / hydroxyl | C–OH, not the –OH inside –COOH | can donate and accept hydrogen bonds |
| Aldehyde | terminal –CHO carbonyl | polar C=O; terminal carbonyl |
| Ketone | –CO– between carbons | polar C=O; internal carbonyl |
| Carboxylic acid | –COOH | acidic proton and hydrogen bonding |
| Ether / alkoxy | C–O–C | oxygen accepts hydrogen bonds but has no O–H donor |
| Amine / amino | C–N without adjacent carbonyl | basic lone-pair chemistry; N–H species may donate H bonds |
| Amide / amido | –CONH₂/–CONHR/–CONR₂ | nitrogen directly attached to carbonyl |
| Ester | –COO– between carbon groups | carbonyl and single-bond O in one group |
| Phenyl | C₆H₅– attached as a substituent | aromatic ring pattern |
Identify the whole local bonding pattern before naming the group; these cues support classification, not a complete reaction mechanism.
Identify the characteristic atoms and bonding pattern first, then give the functional-group name and relevant property context.
Identify the complete bonding pattern: an aldehyde has a terminal –CHO carbonyl, a ketone has C=O between carbons, and an ester contains –C(=O)–O–. Do not label every O–H as an alcohol or every C–N as an amine without checking the neighbouring carbonyl.
Saturation describes carbon-carbon bonding: a saturated compound has only C-C single bonds, while an unsaturated compound contains at least one C=C or C≡C bond. A carbonyl C=O does not by itself make the carbon skeleton unsaturated. Identify saturation separately from identifying hydroxyl, carbonyl, carboxyl or other functional groups.
IUPAC Nomenclature
| Step | Naming decision |
|---|---|
| 1 | Choose the longest parent chain |
| 2 | Number to give the relevant feature the lowest locant |
| 3 | Identify unsaturation and functional group |
| 4 | Assemble prefixes, locants, and suffix |
Apply the systematic sequence to saturated or mono-unsaturated compounds with up to six carbons and one functional-group type.
For CH₃CH(OH)CH(CH₃)CH₃, choose the four-carbon chain containing –OH, number from the end that gives –OH the lower locant, and name 3-methylbutan-2-ol. The principal suffix controls numbering before a substituent does; check locants, punctuation and retained unsaturation at the end.
Structural Isomers
Structural isomers have the same molecular formula but different atom connectivities. Types include straight-chain/branched, position, and functional-group isomers.
Classify primary, secondary, and tertiary alcohols, halogenoalkanes, and amines by the carbon or nitrogen environment attached to the functional group.
| Family | Primary / secondary / tertiary test |
|---|---|
| Alcohol | count carbon groups attached to the carbon bearing -OH: 1 / 2 / 3 |
| Halogenoalkane | count carbon groups attached to the carbon bearing X: 1 / 2 / 3 |
| Amine | count carbon groups attached directly to N: 1 / 2 / 3 |
C₄H₁₀O can represent different carbon skeletons, different –OH positions, or an ether instead of an alcohol. Draw each connectivity once, then compare molecular formulae. Rotating or redrawing one connectivity does not create a new structural isomer.
For alcohols and halogenoalkanes, classify the carbon carrying the functional group; for amines, classify the nitrogen by how many carbon groups are bonded to it. Do not use the position number alone: butan-2-ol is secondary because its OH-bearing carbon is attached to two other carbons.