Organic Chemistry (SL) — IB Chemistry SL SL Study Guide

For: IB Chemistry SL candidates sitting IB Chemistry SL.

Covers: functional groups and homologous series, IUPAC naming of organic compounds, core reactions of alkanes, alkenes and alcohols, and basic addition polymerisation.

You should already know: IGCSE Chemistry, basic algebra.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Chemistry SL style for educational use. They are not reproductions of past IBO papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official IBO mark schemes for grading conventions.

1. What Is Organic Chemistry (SL)?

Organic chemistry (SL) is the study of carbon-containing compounds (excluding simple inorganic carbon species like carbonates, oxides, and cyanides) and their structure, properties, and reactions. Carbon’s ability to form four stable covalent bonds with other carbon atoms and a wide range of other elements creates millions of unique organic compounds, many of which are critical to biological systems, industrial processes, and everyday materials. For IB Chemistry SL, this topic makes up 15% of your final assessment, and focuses on simple aliphatic and aromatic compounds with up to six carbon atoms, with no complex stereochemistry or reaction mechanisms required beyond the core content covered in this guide.

2. Functional groups and homologous series

A homologous series is a group of organic compounds that share four key characteristics:

1. The same functional group
2. The same general molecular formula
3. Similar chemical reactivity
4. A gradual, predictable trend in physical properties (e.g. boiling point, solubility, melting point) as the number of carbon atoms increases Consecutive members of a homologous series differ by a single -CH₂- (methylene) unit, which adds 14 g/mol to the molar mass of each subsequent compound.

A functional group is a specific atom or group of atoms attached to the carbon backbone of an organic molecule that dictates all of its characteristic chemical properties. The non-polar carbon backbone of most organic compounds is relatively unreactive, so all reactions are driven by the functional group present. The core SL functional groups are listed below:

Worked example

Q: Explain why the boiling point of pentane ($36^{\circ }\text{C}$) is significantly higher than the boiling point of ethane ($-89^{\circ }\text{C}$). A: Both alkanes are non-polar, so the only intermolecular forces between molecules are London dispersion forces. As the number of carbon atoms increases, the molar mass and surface area of the molecule increases, leading to stronger London dispersion forces. More energy is required to overcome these stronger forces, so boiling point rises with chain length. Examiners require explicit mention of intermolecular forces for full marks on this common question.

3. Naming organic compounds (IUPAC)

IB SL only requires IUPAC naming for non-cyclic compounds with up to six carbon atoms, and one functional group plus simple alkyl/halogen substituents. Follow these step-by-step rules to avoid mistakes:

1. Identify the parent chain: Find the longest continuous carbon chain that contains the highest priority functional group. Priority order for SL: carboxylic acid > ester > alcohol > alkene > alkane. Parent chain prefixes are: 1C = meth-, 2C = eth-, 3C = prop-, 4C = but-, 5C = pent-, 6C = hex-.
2. Number the parent chain: Start numbering from the end closest to the highest priority functional group, to assign the lowest possible number to the functional group.
3. List substituents: Identify any side chains (methyl = -CH₃, ethyl = -C₂H₅) or halogen substituents (chloro = -Cl, bromo = -Br) and note their position on the parent chain using the numbering from step 2. List substituents in alphabetical order (e.g. bromo before methyl).
4. Combine components: Format your final name as: [substituent position]-[substituent name] [parent chain prefix][functional group suffix]. Add a number before the suffix if required to show the position of the functional group (e.g. but-2-ene, propan-2-ol).

Worked examples

1. Name the compound with the condensed formula ${\text{CH}}_3{\text{CH}}_2\text{CH}(\text{OH}){\text{CH}}_3$

• Step 1: Longest chain is 4C, highest priority functional group is alcohol, so parent prefix = but-, suffix = -ol
• Step 2: Number from the end closest to the -OH group: the -OH is on carbon 2
• Step 3: No substituents present
• Final name: butan-2-ol

2. Name the compound with the condensed formula ${\text{CH}}_3\text{CBr}({\text{CH}}_3)\text{CH}={\text{CH}}_2$

• Step 1: Longest chain containing the alkene group is 4C, parent prefix = but-, suffix = -ene
• Step 2: Number from the end closest to the double bond: double bond is on carbon 1
• Step 3: Methyl substituent on C2, bromo substituent on C2, ordered alphabetically as bromo before methyl
• Final name: 2-bromo-2-methylbut-1-ene

Exam tip: Always double-check your numbering by counting from both ends of the chain. If you get the same functional group position from both ends, choose the numbering that gives the lowest number to the substituent.

4. Reactions of alkanes, alkenes, alcohols

All organic reactions in the SL syllabus require you to state reaction conditions, reactants, and products for full marks.

Alkanes

Alkanes are saturated (only single C-C bonds) so they are very unreactive under standard conditions, with only two core reactions:

1. Combustion: Complete combustion in excess oxygen produces carbon dioxide and water, and releases large amounts of energy, making alkanes widely used as fuels. $${\text{CH}}_4(g)+2{\text{O}}_2(g)\to {\text{CO}}_2(g)+2{\text{H}}_2\text{O}(l)\Delta H=-890\text{ kJ/mol}$$ Incomplete combustion in limited oxygen produces toxic carbon monoxide or solid carbon (soot) instead of CO₂.
2. Free radical substitution: Alkanes react with halogens (Cl₂, Br₂) in the presence of UV light to form haloalkanes, replacing a hydrogen atom with a halogen atom. A mixture of substitution products forms as more hydrogen atoms are replaced. $${\text{CH}}_4(g)+{\text{Cl}}_2(g)\stackrel{\text{UV light}}{\to }{\text{CH}}_3\text{Cl}(g)+\text{HCl}(g)$$

Alkenes

Alkenes are unsaturated (contain a C=C double bond) so they undergo addition reactions, where the double bond breaks and two atoms/groups add across the bond:

1. Hydrogenation: Reacts with H₂ gas at high temperature with a nickel catalyst to form an alkane. This reaction is used to turn liquid vegetable oils into solid margarine. $${\text{C}}_2{\text{H}}_4(g)+{\text{H}}_2(g)\stackrel{\text{Ni, }180^{\circ }\text{C}}{\to }{\text{C}}_2{\text{H}}_6(g)$$
2. Halogenation: Reacts with halogens at room temperature with no catalyst to form a dihaloalkane. This is the standard test for unsaturation: orange/brown bromine water turns colourless when added to an alkene. $${\text{C}}_2{\text{H}}_4(g)+{\text{Br}}_2(aq)\to {\text{C}}_2{\text{H}}_4{\text{Br}}_2(aq)(\text{colourless})$$
3. Hydration: Reacts with steam at 300°C, 60 atm pressure, with a concentrated H₃PO₄ catalyst to form an alcohol. This is the industrial method for producing ethanol. $${\text{C}}_2{\text{H}}_4(g)+{\text{H}}_2\text{O}(g)\stackrel{{\text{H}}_3{\text{PO}}_4,300^{\circ }\text{C},60\text{ atm}}{\to }{\text{C}}_2{\text{H}}_5\text{OH}(l)$$

Alcohols

The three core alcohol reactions for SL are:

1. Combustion: Alcohols burn in excess oxygen to produce CO₂ and water, releasing large amounts of energy. Ethanol is widely used as a renewable biofuel. $${\text{C}}_2{\text{H}}_5\text{OH}(l)+3{\text{O}}_2(g)\to 2{\text{CO}}_2(g)+3{\text{H}}_2\text{O}(l)\Delta H=-1367\text{ kJ/mol}$$
2. Oxidation: Primary alcohols are oxidised first to aldehydes, then to carboxylic acids when heated with excess acidified potassium dichromate (K₂Cr₂O₇/H⁺) oxidising agent. The oxidising agent changes colour from orange (Cr₂O₇²⁻) to green (Cr³⁺) during the reaction. Secondary alcohols oxidise to ketones only, while tertiary alcohols have no oxidation reaction under these conditions. $${\text{C}}_2{\text{H}}_5\text{OH}(l)\stackrel{{\text{excess K}}_2{\text{Cr}}_2{\text{O}}_7/{\text{H}}^{+},\text{reflux}}{\to }{\text{CH}}_3\text{COOH}(l)$$
3. Esterification: Alcohols react with carboxylic acids when heated with a concentrated H₂SO₄ catalyst to form an ester and water, in a reversible reaction. Esters have sweet, fruity smells and are used in food flavourings and perfumes. $${\text{C}}_2{\text{H}}_5\text{OH}(l)+{\text{CH}}_3\text{COOH}(l)\stackrel{{\text{conc H}}_2{\text{SO}}_4,\text{heat}}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5(l)+{\text{H}}_2\text{O}(l)$$

5. Polymerisation basics

A polymer is a large macromolecule formed by joining hundreds or thousands of small repeating units called monomers. For SL, you only need to understand addition polymerisation, which uses unsaturated alkene monomers: During addition polymerisation, the C=C double bond in each alkene monomer breaks, and the monomers covalently bond to form a long, saturated polymer chain. No by-products are formed in this reaction.

Common addition polymers for SL include:

1. Poly(ethene): formed from ethene monomers, used for plastic bags, bottles, and food wrap $$n{\text{CH}}_2={\text{CH}}_2\to -[{\text{CH}}_2-{\text{CH}}_2{]}_n-$$
2. Poly(chloroethene) (PVC): formed from chloroethene monomers, used for plastic pipes, window frames, and electrical insulation $$n{\text{CH}}_2=\text{CHCl}\to -[{\text{CH}}_2-\text{CHCl}{]}_n-$$
3. Poly(propene): formed from propene monomers, used for ropes, food containers, and automotive parts

To draw a repeat unit from an alkene monomer: break the C=C double bond, add a single bond extending out from each end of the two main carbon atoms, wrap the structure in square brackets, and add a subscript n outside to show the unit repeats many times.

Exam tip: Never leave a double bond in an addition polymer repeat unit: this is one of the most common mark-losing mistakes on this topic.

6. Common Pitfalls (and how to avoid them)

• Wrong move: Naming compounds using the shortest carbon chain instead of the longest, or numbering the chain from the wrong end. Why it happens: Rushing through naming questions to save time. Correct move: Count all possible continuous carbon chains to find the longest, then number from both ends to confirm the highest priority functional group gets the lowest possible number.
• Wrong move: Writing CO as a product of complete combustion. Why it happens: Mixing up complete and incomplete combustion conditions. Correct move: Only use CO or C as products if the question explicitly states incomplete combustion or limited oxygen; complete combustion always produces CO₂ and H₂O.
• Wrong move: Stating alkanes decolourise bromine water. Why it happens: Confusing saturated alkanes with unsaturated alkenes. Correct move: Only alkenes (and other unsaturated compounds) decolourise bromine water at room temperature; alkanes have no reaction with bromine water without UV light.
• Wrong move: Omitting reaction conditions from your answers. Why it happens: Focusing only on writing reactants and products. Correct move: The IB mark scheme awards up to 50% of marks for correct conditions, so always add catalysts, temperature, pressure, or UV light as required for each reaction.
• **Wrong move: Drawing a double bond in addition polymer repeat units. Why it happens: Forgetting the double bond breaks during polymerisation. Correct move: All addition polymer repeat units have only single bonds between the two main chain carbon atoms that were part of the monomer double bond.

7. Practice Questions (IB Chemistry SL Style)

Question 1

(a) State three characteristics of a homologous series. [3 marks] (b) Explain why ethanol (boiling point $78^{\circ }\text{C}$) has a much higher boiling point than ethane (boiling point $-89^{\circ }\text{C}$), even though they have very similar molar masses. [2 marks]

Worked solution

(a) Any three of the following, 1 mark each, maximum 3 marks:

1. All members share the same functional group
2. All members follow the same general molecular formula
3. Consecutive members differ by a single -CH₂- unit
4. All members have similar chemical properties
5. Physical properties show a gradual trend with increasing carbon chain length (b) 1 mark for identifying that ethanol has a hydroxyl (-OH) group, allowing it to form strong hydrogen bonds between molecules. 1 mark for noting that ethane is non-polar, so only has weak London dispersion forces between molecules, which require much less energy to overcome.

Question 2

Give the IUPAC name for each of the following compounds: (a) ${\text{CH}}_3{\text{CH}}_2\text{CH}({\text{CH}}_3)\text{COOH}$ [2 marks] (b) ${\text{CH}}_3{\text{CH}}_2\text{C}({\text{CH}}_3)={\text{CHCH}}_3$ [2 marks]

Worked solution

(a) Step 1: Longest chain containing the carboxylic acid group is 4 carbon atoms, so parent prefix = but-, suffix = -oic acid. Step 2: Carboxylic acid carbon is always numbered as C1, so the methyl substituent is on C2. Final name: 2-methylbutanoic acid (1 mark for correct parent chain, 1 mark for correct substituent position). (b) Step 1: Longest chain containing the alkene group is 5 carbon atoms, parent prefix = pent-, suffix = -ene. Step 2: Number from the end closest to the double bond: double bond is between C2 and C3, methyl substituent on C2. Final name: 2-methylpent-2-ene (1 mark for correct double bond position, 1 mark for correct substituent position and name).

Question 3

Propene (${\text{CH}}_3\text{CH}={\text{CH}}_2$) undergoes addition polymerisation to form poly(propene). (a) Write a balanced equation for this reaction, using structural formulas. [2 marks] (b) State one common use of poly(propene). [1 mark] (c) Explain why most addition polymers are non-biodegradable. [1 mark]

Worked solution

(a) $$n{\text{CH}}_3\text{CH}={\text{CH}}_2\to -[{\text{CH(CH}}_3\text{)}-{\text{CH}}_2{]}_n-$$ (1 mark for correct monomer structure, 1 mark for correct single-bonded repeat unit with subscript n) (b) Any valid use: food packaging, ropes, plastic furniture, automotive parts, lab equipment (1 mark) (c) The polymer chain is held together by strong, non-polar C-C and C-H single bonds that cannot be broken down by enzymes or biological organisms in the environment (1 mark)

8. Quick Reference Cheatsheet

Homologous Series General Formulas

Key Reaction Conditions

Naming Priority Order

Carboxylic acid > ester > alcohol > alkene > alkane

9. What's Next

This SL organic chemistry content forms the foundation for multiple other parts of the IB syllabus: it connects to the energetics topic when calculating enthalpy changes of combustion for organic fuels, the redox topic when studying oxidation reactions of alcohols, and the optional materials chemistry topic if you choose to study it for your paper 3 exam. If you plan to progress to IB Chemistry HL, mastering this core content is critical, as you will build on it to learn more complex functional groups, reaction mechanisms, and stereochemistry.

If you are stuck on any part of SL organic chemistry, from naming tricky compounds to remembering reaction conditions, you can ask Ollie for personalised explanations, extra practice questions, or step-by-step walkthroughs of past paper problems anytime on the Ollie. Make sure to test your knowledge with official past IB Chemistry SL papers to get used to the exam format and mark scheme requirements before your test.