IB Chemistry organic chemistry is best understood as a small set of connected, testable ideas: functional groups, nomenclature, isomerism, reaction pathways, mechanisms, physical properties, and structure determination. Exams rarely reward isolated memorization. They ask you to recognize a structural feature, predict its consequences, and communicate the answer using precise chemical formulas, equations, observations, or mechanisms.
The current IB Chemistry course, first assessed in 2025, is organized around structure and reactivity rather than the old numbered Topic 10 and Topic 20 system. Organic chemistry therefore appears across the course and can be combined with bonding, energetics, acids and bases, redox chemistry, and analytical techniques. This guide explains the central ideas, how examiners phrase questions, and how to convert your knowledge into marks.
The organizing idea: structure determines reactivity
Carbon forms four covalent bonds and can produce chains, branches, rings, multiple bonds, and bonds to heteroatoms such as oxygen, nitrogen, and halogens. The number of possible compounds is enormous, but their behavior becomes predictable once you identify the functional group.
A functional group is an atom or group of atoms responsible for a molecule's characteristic reactions. The carbon skeleton affects properties such as boiling point and solubility, while the functional group usually determines the reaction family. This is why the same reasoning can be applied to ethanol, propan-1-ol, and other alcohols.
| Family | Characteristic feature | Typical chemistry |
|---|---|---|
| Alkane | C-C single bonds only | Combustion, radical substitution |
| Alkene | C=C | Addition, addition polymerization |
| Halogenoalkane | C-X | Nucleophilic substitution, elimination |
| Alcohol | -OH | Oxidation, dehydration, esterification |
| Aldehyde | Terminal -CHO | Oxidation to a carboxylic acid |
| Ketone | Internal C=O | Resistant to mild oxidation |
| Carboxylic acid | -COOH | Acid-base reactions, esterification |
| Ester | -COO- | Formation from an alcohol and carboxylic acid |
| Amine | Nitrogen-containing amino group | Basic and nucleophilic behavior |
| Amide | -CONH₂ or substituted form | Polar molecules and condensation chemistry |
Use the RevisionDojo functional groups explanation if identifying these groups from skeletal structures is still slow.
Formula representations and nomenclature
IB questions may move between molecular, empirical, condensed structural, full structural, and skeletal formulas. In a skeletal formula, every unlabelled vertex or line ending represents a carbon atom. Hydrogens bonded to carbon are omitted, but heteroatoms and hydrogens attached to them must be shown.
For systematic naming, use a fixed procedure:
- Find the longest parent chain containing the principal functional group and relevant multiple bond.
- Number it from the end giving the important feature the lowest locant.
- Identify and locate branches and substituents.
- Select the correct suffix, such as -ane, -ene, -ol, -al, -one, or -oic acid.
- Check spelling, commas between numbers, and hyphens between numbers and words.
For example, CH₃CH(CH₃)CH₂OH is 2-methylpropan-1-ol. A frequent error is choosing the visually longest horizontal line instead of tracing the true longest chain containing the hydroxyl-bearing carbon.
Questions commonly say “name the compound,” “draw the structure,” “identify the functional group,” or “deduce the molecular formula.” These are precision tasks. A correct idea with an incorrect locant or missing bond may not earn the mark.
Isomerism and structure-property relationships
Structural isomers have the same molecular formula but different atom connectivity. They may differ by carbon skeleton, functional-group position, or functional-group type. Stereoisomers retain the same connectivity but differ in spatial arrangement, including geometrical and optical forms where required by the syllabus level.
When asked to draw all structural isomers, work systematically: change the carbon skeleton first, then move the functional group, and finally consider another compatible functional-group family. Count the atoms after every drawing and reject duplicates caused only by rotating or reversing a structure.
Structure also explains physical properties. Longer carbon chains generally have stronger London dispersion forces and higher boiling points. Branching usually lowers boiling point because it reduces surface contact, while hydrogen bonding raises the boiling points of alcohols and carboxylic acids relative to similar-sized non-polar compounds.
For solubility, compare the polar functional group with the non-polar hydrocarbon region. A short-chain alcohol can hydrogen-bond effectively with water, but solubility generally decreases as its hydrocarbon chain grows. The RevisionDojo isomerism guide provides further structural comparisons.
Build a reaction map instead of memorizing a list
Organic reactions become manageable when classified by what changes around the functional group.
| Reaction type | Structural change | Representative example |
|---|---|---|
| Addition | Atoms add across a multiple bond | Alkene + Br₂ produces a dibromoalkane |
| Substitution | One atom or group replaces another | Halogenoalkane + OH⁻ produces an alcohol |
| Elimination | A small molecule is removed and a multiple bond forms | Alcohol produces alkene + H₂O |
| Oxidation | More C-O or fewer C-H bonds | Primary alcohol to aldehyde to acid |
| Condensation | Molecules join and eliminate a small molecule | Alcohol + acid produces ester + water |
| Polymerization | Many monomers form a macromolecule | Alkene to addition polymer |
Learn each pathway with five pieces of information: reactant, reagent, conditions, product, and reaction type. For oxidation, also distinguish primary, secondary, and tertiary alcohols. A primary alcohol can form an aldehyde and then a carboxylic acid; a secondary alcohol forms a ketone; a tertiary alcohol does not undergo the corresponding mild oxidation because its hydroxyl-bearing carbon lacks the required hydrogen.
In questions involving acidified dichromate(VI), state the observation precisely: orange to green. For bromine water reacting with an alkene, give orange or brown to colourless, rather than saying merely that the colour changes. If the question requests an equation, an observation alone is insufficient.
Mechanisms: show where the electrons move
A mechanism explains bond breaking and formation step by step. At HL in particular, examiners may require curly arrows, lone pairs, charges, intermediates, and the correct organic products.
A full-headed curly arrow represents movement of an electron pair. It must begin at the electron source, such as a lone pair or bond, and point toward the atom or position receiving the pair. A common mark-losing mistake is drawing an arrow from a positive atom rather than from the electron-rich species attacking it.
For nucleophilic substitution, the nucleophile donates an electron pair to an electron-deficient carbon bonded to a leaving group. For electrophilic addition, the electron-rich alkene π bond attacks an electrophile. Radical mechanisms instead use single-headed arrows and include initiation, propagation, and termination steps when those stages are requested.
Do not replace a requested mechanism with an overall equation. Before moving on, check that every atom is present, formal charges are consistent, and carbon has an acceptable number of bonds.
Organic structure determination
Structure-deduction questions require evidence to be combined rather than interpreted separately. Depending on the question and syllabus level, this may involve mass spectrometry, infrared spectroscopy, and proton NMR, together with chemical tests and molecular formulas.
A reliable order is:
- Use the molecular ion or supplied mass information to estimate molar mass.
- Use the molecular formula to restrict possible structures.
- Use IR absorptions to identify or exclude functional groups.
- Use chemical shifts, signal numbers, integrations, and splitting patterns from ¹H NMR where applicable.
- Draw a structure and verify that every piece of evidence fits.
The prescribed data booklet should be used actively rather than memorized indiscriminately. In an explanation, quote the relevant evidence and connect it to the conclusion: for example, a broad O-H absorption together with a carbonyl absorption supports a carboxylic acid more strongly than either observation alone.
How organic chemistry appears in current examinations
Under the assessment model first used in 2025, Paper 1A contains multiple-choice questions, Paper 1B contains data-based and experimental-work questions, and Paper 2 contains short-answer and extended-response questions. Organic chemistry can appear in any of these formats and may be integrated with other parts of the course.
Pay close attention to command terms:
- State or identify: give a concise answer without unnecessary explanation.
- Deduce: reach a conclusion from supplied information and show essential reasoning.
- Explain: connect the observation to bonding, structure, stability, or electron movement.
- Distinguish: give a test and the different result for each substance.
- Draw: use an unambiguous chemical representation with all requested details.
For a “distinguish” question, naming bromine water is not enough. State what happens with both compounds and ensure the test genuinely differentiates them. For a “predict and explain” question, separate the prediction from the chemical reason so both marking points are visible.
Common mistakes and a practical revision method
Students often lose marks by drawing impossible structures, omitting conditions, confusing aldehydes with ketones, or giving a reaction type when a mechanism was requested. Another recurring error is memorizing a reagent without understanding what structural transformation it causes.
Use this revision cycle:
- Create a one-page reaction map from memory.
- Practise naming and drawing five compounds daily.
- Complete short mixed sets from the organic chemistry Questionbank.
- Review weak theory through the organic chemistry notes and HL organic chemistry resources.
- Record each error as a knowledge, representation, mechanism, or command-term problem.
- Retest the same skill without notes within two days.
Most importantly, watch the method being applied to authentic question styles. The Past Papers area on the RevisionDojo IB Chemistry resources page provides per-question video walkthroughs, helping you see how structures are interpreted, mechanisms are drawn, and mark-scheme points are presented.
Conclusion
IB organic chemistry centres on recognizing functional groups, reading structures accurately, connecting reactions into pathways, and explaining reactivity through bonding and electron movement. Strong exam answers are chemically correct, visibly structured, and matched to the command term.
RevisionDojo can support this process with focused Study Notes, the Organic Chemistry Questionbank, and Jojo AI feedback. After reviewing the theory, the most useful next step is to attempt past-paper questions and compare your method with the per-question video solutions.
Sources and referenced URLs
- IB Chemistry curriculum overview
- Official IB Chemistry curriculum and assessment updates
- Official IB Chemistry specimen papers for first assessment 2025
- RevisionDojo IB Chemistry resources and past-paper videos
- RevisionDojo Organic Chemistry Questionbank
- RevisionDojo Organic Chemistry notes
- RevisionDojo HL Organic Chemistry resources
- RevisionDojo functional groups explanation
- RevisionDojo isomerism explanation
