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S3.2

Functional groups: Classification of organic compounds

9 SL · +11 HL (SL 3.2.1-3.2.6) (HL 3.2.7-3.2.12)

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Parent topic: Classification of matter

Guiding question

Guiding questionHow does the classification of organic molecules help us to predict their properties?

The one big idea

Organic compounds are classified by functional group, which determines their physical and chemical properties. At HL, stereoisomerism, mass spectrometry, IR and ¹H NMR turn structural analysis into a problem you can solve from spectra alone.

What this rests on

Our editorial judgement.The dependencies in this section are this site’s own assessment of what a sub-topic rests on. They have notbeen verified by a chemistry teacher and are not part of the official IB guide. By contrast, section 8 (Linking questions) reproduces the IB’s own wording verbatim.

What rests on this

Core concepts that must be mastered

  • Representing organic compounds(3.2.1)Organic compounds can be represented by empirical, molecular, structural (full and condensed), stereochemical and skeletal formulas. You must identify different formula types and interconvert molecular, skeletal and structural formulas. A skeletal formula shows carbon atoms as line ends and corners, with hydrogen atoms on carbon implied; this is the standard notation for the rest of organic chemistry. Stereochemical formulas show 3-D arrangement explicitly (wedge–dash notation) and are needed at HL for stereoisomers. Carbon is unique: it forms four covalent bonds, chains and rings, and more compounds than all other elements combined — because it catenates (bonds to itself) stably and has an intermediate electronegativity that supports a wide range of bond polarities.
  • Functional groups and homologous series(3.2.2–3.2.4)A functional group gives characteristic physical and chemical properties; compounds are classified by functional group. You must identify by name and structure: halogeno (–X), hydroxyl (–OH), carbonyl (C=O, aldehyde or ketone), carboxyl (–COOH), alkoxy (–O–), amino (–NH₂), amido (–CONH–), ester (–COO–), phenyl (–C₆H₅). Include the terms "saturated" (single bonds only) and "unsaturated" (contains C=C or C≡C). A homologous series differs by a common structural unit, typically CH₂, and has a general formula: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH, carboxylic acids CₙH₂ₙ₊₁COOH. Successive members show trends in physical properties: melting and boiling points increase down the series because longer chains have more electrons and stronger London dispersion forces.
  • IUPAC nomenclature and structural isomers(3.2.5, 3.2.6)IUPAC nomenclature gives every compound a systematic name. You must apply IUPAC naming to saturated or mono-unsaturated compounds with up to six carbons in the parent chain containing one type of halogeno, hydroxyl, carbonyl or carboxyl group. The procedure: find the longest chain containing the principal functional group (this sets the parent name and suffix), number the chain to give the functional group the lowest possible locant, name substituents with prefixes, and assemble alphabetically. Numeric prefixes mono–hexa are used for multiple identical substituents. Both straight-chain and branched-chain isomers are covered. For example, CH₃CH(OH)CH₂CH₃ is butan-2-ol, not butan-3-ol (number from the end nearest the –OH). Structural isomers have the same molecular formula but different connectivity. You must recognise branched vs straight-chain isomers (butane vs 2-methylpropane), position isomers (propan-1-ol vs propan-2-ol), and functional group isomers (butanal vs butanone, both C₄H₈O). Structural isomers have different physical and often different chemical properties — branched alkanes have lower boiling points because branching reduces surface contact and thus London forces.
HL
  • HL: Stereoisomerism(3.2.7)Stereoisomers have the same connectivity but different spatial arrangement. Cis–trans isomerism occurs in non-cyclic alkenes (where the C=C bond prevents rotation) and in C3/C4 cycloalkanes: the cis isomer has the identical or similar groups on the same side, the trans isomer on opposite sides. The E–Z system is not assessed. A chiral carbon has four different groups attached; it gives rise to enantiomers — non-superimposable mirror images. You must draw stereochemical formulas using wedge–dash notation: wedges come out of the plane, dashes go behind. Enantiomers have different optical properties: they rotate plane-polarised light in opposite directions. A racemic mixture contains equal amounts of both enantiomers and is optically inactive. Enantiomers behave identically except in chiral environments (biological systems, chiral catalysts) — this is why a drug may work as one enantiomer but not the other. A carbon with four different-looking groups is not necessarily chiral — the four groups must be genuinely different as whole substituents, not just different at the first atom.
HL
  • HL: Mass spectrometry, IR, ¹H NMR and combined structural analysis(3.2.8–3.2.12)Mass spectrometry of organic compounds causes fragmentation. The molecular ion M⁺ is the highest m/z peak (ignoring isotope peaks) and gives the molecular mass — it is not necessarily the tallest peak (the tallest is the base peak, the most stable fragment). Fragmentation patterns reveal structural features: loss of 15 (CH₃⁺), 29 (C₂H₅⁺ or CHO⁺), 31 (CH₂OH⁺), 43 (C₃H₇⁺ or CH₃CO⁺), 45 (COOH⁺ or OC₂H₅⁺). IR spectra identify bond types: you interpret the functional group region (above ~1500 cm⁻¹) using a table of characteristic wavenumbers (data booklet). Key absorptions: O–H (broad, ~3200–3600 cm⁻¹), N–H (~3300–3500 cm⁻¹), C–H (~2850–3100 cm⁻¹), C≡N (~2200–2260 cm⁻¹), C=O (strong, ~1650–1750 cm⁻¹), C=C (~1620–1680 cm⁻¹), C–O (~1000–1300 cm⁻¹). A molecule is IR active if its vibration changes the dipole moment — this is why O₂ and N₂ are not greenhouse gases but CO₂, H₂O and CH₄ are. ¹H NMR gives information on the different chemical environments of hydrogen atoms. You interpret spectra from three features: the number of signals (equals the number of distinct H environments), the chemical shift δ (in ppm, identifies the environment — data booklet gives characteristic shifts), and the relative area (integration trace, gives the ratio of hydrogens in each environment). Signals split into clusters of peaks: the n+1 rule — a signal is split into n+1 peaks by n equivalent hydrogens on the adjacent carbon(s). So singlet (no neighbours), doublet (1 neighbour), triplet (2 neighbours), quartet (3 neighbours). In combined structural analysis, you interpret data from multiple techniques — molecular formula (from combustion analysis or MS), functional groups (from IR), H environments (from ¹H NMR), and fragmentation (from MS) — to determine a complete molecular structure. The workflow: use the molecular ion to get the molecular mass and formula; use IR to identify functional groups; use the number of NMR signals to count H environments; use integration to find the H ratio; use splitting to identify connectivity; use chemical shift to confirm the identity of each environment. Cross-check everything against the molecular formula.
  • How this sub-topic connectsStructure 3.2 is the heaviest HL block in the entire course (+11 HL hours, more than SL hours). The functional groups learned here are the reactants in every organic mechanism (R3.4: nucleophilic substitution, electrophilic addition, electrophilic substitution). Intermolecular forces (S2.2) explain the trends in boiling points within homologous series. At HL, the spectroscopic techniques connect back to the nuclear atom (S1.2: mass spectrometry), emission spectra (S1.3: energy transitions), and bonding (S2.2: IR activity requires a changing dipole). Stereoisomerism connects to reaction mechanisms (R3.4: SN2 is stereospecific, producing a single enantiomer; SN1 produces a racemic mixture). Condensation polymers (S2.4 HL) are built from the amide and ester functional groups introduced here.

Quantitative non-negotiables

  • Interconvert molecular, skeletal and structural formulas.
  • Apply IUPAC nomenclature to saturated or mono-unsaturated compounds with up to six carbons in the parent chain, containing one type of halogeno / hydroxyl / carbonyl / carboxyl group.
  • Identify structural isomers: branched, straight-chain, position and functional group isomers.
  • At HL: draw cis–trans isomers and chiral centres (wedge–dash), and identify enantiomers.HL
  • At HL: interpret mass spectra (molecular ion and fragmentation), IR spectra (functional group region, characteristic wavenumbers), and ¹H NMR spectra (number of signals, chemical shift, integration, splitting patterns).HL
  • At HL: combine data from multiple techniques to determine a molecular structure.HL

Common failure modes

  • M-71 — Any carbon with four different-looking groups is chiral.

    Confidence: verified

    Why it’s wrong: The four groups must be genuinely different as whole substituents, not just different at the first atom.

    Correction: Check the entire branch.

  • M-72 — Structural isomers have the same properties.

    Confidence: verified

    Why it’s wrong: Different connectivity gives different physical and often different chemical properties.

    Correction: E.g. branched alkanes have lower boiling points because branching reduces surface contact and thus London forces.

  • M-76 — The molecular ion peak in a mass spectrum is the tallest peak.

    Confidence: verified

    Why it’s wrong: The tallest is the base peak (most stable fragment).

    Correction: The molecular ion is the highest-m/z peak (ignoring isotope peaks).

  • M-77 — In ¹H NMR, the number of peaks equals the number of hydrogens.

    Confidence: verified

    Why it’s wrong: The number of signals equals the number of distinct chemical environments.

    Correction: The integration (relative area) gives the ratio of hydrogens in each environment; the splitting (n+1) gives the number of hydrogens on adjacent carbons.

What "HL standard" actually looks like

HLHL adds stereoisomerism (cis–trans, chirality, enantiomers, optical activity, racemic mixtures), mass spectrometry fragmentation, IR spectroscopy (functional group region and greenhouse gases), ¹H NMR (chemical shift, integration, splitting n+1), and combined spectroscopic structure determination. This is the heaviest HL block in the entire course (+11 h).

See the command terms reference →

Linking questions

Official IB.The questions in this section are the IB’s own linking questions, reproduced verbatim from the guide. They are not this site’s editorial judgement — see section 3 for that distinction.

  1. Official IB

    How does the fragmentation pattern of a compound in the mass spectrometer help in the determination of its structure?

    Structure 1.2.3 → S3.2 Functional groups: Classification of organic compounds

  2. Official IB

    What is the importance of approximation in the determination of an empirical formula?

    Structure 1.4.4 → S3.2 Functional groups: Classification of organic compounds

  3. Official IB

    What features of a molecule make it "infrared (IR) active"?

    Structure 2.2.6 → S3.2 Functional groups: Classification of organic compounds

  4. Official IB

    To what extent does a functional group determine the nature of the intermolecular forces?

    Structure 2.2.9 → S3.2 Functional groups: Classification of organic compounds

  5. Official IB

    What are the structural features of some plastics that make them biodegradable?

    Structure 2.4.4 → S3.2 Functional groups: Classification of organic compounds

  6. Official IB

    What functional groups in molecules can enable them to act as monomers for addition reactions?

    Structure 2.4.5 → S3.2 Functional groups: Classification of organic compounds

  7. Official IB

    What functional groups in molecules can enable them to act as monomers for condensation reactions?

    Structure 2.4.6 → S3.2 Functional groups: Classification of organic compounds

  8. Official IB

    What is unique about carbon that enables it to form more compounds than the sum of all the other elements' compounds?

    Structure 3.2.1 → S2.2 The covalent model

  9. Official IB

    What are the advantages and disadvantages of different depictions of an organic compound (e.g. structural formula, stereochemical formula, skeletal formula, 3D models)?

    Structure 3.2.1 → S2.2 The covalent model

  10. Official IB

    What is the nature of the reaction that occurs when two amino acids form a dipeptide?

    Structure 3.2.2 → S2.4 From models to materials

  11. Official IB

    How can functional group reactivity be used to determine a reaction pathway between compounds, e.g. converting ethene into ethanoic acid?

    Structure 3.2.2 → R3.2 Electron transfer reactions

  12. Official IB

    What is the influence of the carbon chain length, branching and the nature of the functional groups on intermolecular forces?

    Structure 3.2.4 → S2.2 The covalent model

  13. Official IB

    How does the fact that there are only 3 isomers of dibromobenzene support the current model of benzene's structure?

    Structure 3.2.6 → S2.2 The covalent model

  14. Official IB

    What features of a molecule determine whether it is IR active or not?

    Structure 3.2.9 → S2.2 The covalent model

  15. Official IB

    What properties of a greenhouse gas determine its "global warming potential"?

    Structure 3.2.9 → R1.3 Energy from fuels

  16. Official IB

    Why do larger hydrocarbons have a greater tendency to undergo incomplete combustion?

    Reactivity 1.3.3 → S3.2 Functional groups: Classification of organic compounds

  17. Official IB

    Why is carbon dioxide described as a greenhouse gas?

    Reactivity 1.3.3 → S3.2 Functional groups: Classification of organic compounds

  18. Official IB

    How does the nature of the functional group in a molecule affect its physical properties, such as boiling point?

    Reactivity 3.2.9 → S3.2 Functional groups: Classification of organic compounds

1 further official linking question target a Tool, Inquiry or Nature of Science strand and are not drawn as edges on the map:

Editorial The decision to surface these off-graph questions here is our editorial judgement — the IB does not prescribe where they should appear.

  1. Official IB

    How useful are 3D models (real or virtual) to visualize the invisible?

    Structure 3.2.3 → Nature of Science

Bridge: GCSE → IB HL

Derived. These are the bridge items tagged to this sub-topic— places where the GCSE model gets redefined, genuinely new territory, or carry-over strengths. The tagging is this site’s editorial judgement.

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