Functional group chemistry
IB Chemistry SL· Reactivity 3.2: Functional group chemistry· 6 min read
1. Identification of Common Functional Groups★★☆☆☆⏱ 15 min
Functional Group
A specific arrangement of atoms within an organic molecule that gives the molecule consistent characteristic chemical properties, regardless of the size of the attached carbon chain.
Example:
All alcohols contain an -OH group, which reacts in the same way during oxidation regardless of the length of the alkyl chain.
IB SL requires you to recognize and name 10 core functional groups from structural formulas. Each functional group defines a distinct class of organic compounds.
Class | General Structure | Example |
|---|---|---|
Alkene | C=C | Ethene |
Alcohol | R-OH | Ethanol |
Halogenoalkane | R-X (X=Cl, Br, I) | Chloroethane |
Aldehyde | R-CHO | Ethanal |
Ketone | R-CO-R' | Propanone |
Carboxylic acid | R-COOH | Ethanoic acid |
Ester | R-COO-R' | Methyl ethanoate |
Amine | R-NH₂ | Ethanamine |
Identify all functional groups present in the molecule HOCH₂CH₂COOCH₃
- 1
Step 1: Break the molecule into distinct structural parts: HO-CH₂-CH₂-COO-CH₃
- 2
Step 2: The -OH group bonded to an alkyl chain is an alcohol functional group.
- 3
Step 3: The R-COO-R' group is a distinct ester functional group.
- 4
Conclusion: The molecule contains two functional groups: alcohol and ester.
Test your identification skills
What functional group is present in CH₃CH₂COCH₃?
Aldehyde
Ketone
Carboxylic acid
Alcohol
Reveal answer
Ketone —Correct! The carbonyl C=O group is bonded to two alkyl groups, so it is a ketone.
Which functional group has the general formula RCOOH?
Ester
Aldehyde
Carboxylic acid
Alcohol
Reveal answer
Carboxylic acid —Right! The carboxyl -COOH group defines all carboxylic acids.
2. Functional Groups and Physical Properties★★★☆☆⏱ 20 min
Functional groups determine physical properties (like boiling point and solubility) by changing the strength of intermolecular forces between molecules. The biggest effect comes from the ability to form hydrogen bonds.
Explain why propanoic acid (C₂H₅COOH) has a higher boiling point than methyl acetate (CH₃COOCH₃), even though they have the same molecular formula C₃H₆O₂.
- 1
Step 1: Identify the functional groups: propanoic acid is a carboxylic acid (-COOH), methyl acetate is an ester (-COO-).
- 2
Step 2: The -COOH group of propanoic acid has an O-H bond, so it can form hydrogen bonds between molecules.
- 3
Step 3: Esters have no O-H or N-H bond, so they cannot form intermolecular hydrogen bonds, only weaker dipole-dipole interactions.
- 4
Conclusion: Hydrogen bonds are stronger than dipole-dipole interactions, so more energy is needed to separate propanoic acid molecules, leading to a higher boiling point.
3. Characteristic Reactivity by Functional Group★★★☆☆⏱ 20 min
All molecules with the same functional group undergo the same types of reactions. For IB SL, you need to remember these core reaction patterns to predict products in organic synthesis questions:
Alkenes (C=C) undergo addition reactions across the double bond
Alcohols undergo oxidation, substitution, and elimination (dehydration)
Halogenoalkanes undergo nucleophilic substitution
Aldehydes oxidize easily to carboxylic acids; ketones do not oxidize under mild conditions
Carboxylic acids react with alcohols to form esters (esterification)
Amines act as Brønsted-Lowry bases, accepting protons
Predict the product of the reaction between ethene (C₂H₄) and hydrogen bromide (HBr), and name the functional group in the product.
- 1
Step 1: Ethene is an alkene with a C=C double bond, which undergoes addition reactions with hydrogen halides.
- 2
Step 2: The double bond breaks, and H and Br add across the two carbon atoms:
- 3
- 4
Step 3: The product bromoethane has a bromine atom bonded to an alkyl chain, which is the halogenoalkane functional group.
Exam tip:
Always name the specific functional group when asked; avoid generic terms like 'halogen' or 'oxygen group'.
4. Distinguishing Functional Groups With Chemical Tests★★★★☆⏱ 15 min
Common exam questions ask you to distinguish between two functional groups with similar formulas using simple chemical tests. IB SL requires you to know the tests below:
Functional Group | Test | Positive Result |
|---|---|---|
Alkene (C=C) | Add bromine water | Orange solution decolorizes |
Aldehyde | Add Tollens' reagent | Silver mirror forms |
Aldehyde | Add Fehling's solution | Red precipitate forms |
Carboxylic acid | Add sodium carbonate | Bubbles of CO₂ form |
Primary/secondary alcohol | Add acidified potassium dichromate | Orange turns green |
Describe a chemical test to distinguish between propanal (aldehyde) and propanone (ketone).
- 1
Step 1: Recall the key reactivity difference: aldehydes are easily oxidized, ketones are not oxidized under mild conditions.
- 2
Step 2: Add Tollens' reagent (silver nitrate in aqueous ammonia) to each sample and warm gently.
- 3
Result for propanal: A silver mirror forms on the test tube wall, as the aldehyde reduces Ag⁺ ions to metallic silver.
- 4
Result for propanone: No visible change occurs, as ketones cannot be oxidized by Tollens' reagent.
5. Common Pitfalls
Wrong move:
Confusing aldehydes and ketones, classifying R-CHO as a ketone
Why:
Both contain a carbonyl C=O group, so students mix up their classification
Correct move:
Aldehydes have the carbonyl at the end of the chain (bonded to one alkyl + one H), ketones have it in the chain (bonded to two alkyl groups)
Wrong move:
Claiming all carbonyl-containing compounds can form intermolecular hydrogen bonds
Why:
Students assume any oxygen-containing group can form hydrogen bonds
Correct move:
Only groups with O-H or N-H bonds form intermolecular hydrogen bonds; ketones/esters have C=O but no O-H so cannot
Wrong move:
Forgetting that a single molecule can have multiple functional groups
Why:
Students stop after identifying the first obvious group, missing other groups in the molecule
Correct move:
Always scan the entire molecular structure after identifying your first functional group to check for others
Wrong move:
Classifying the -OH in a carboxylic acid as an alcohol functional group
Why:
The -OH group is present, but it is part of a distinct functional group with different reactivity
Correct move:
When -OH is bonded directly to a carbonyl carbon (as part of -COOH), classify it as a carboxylic acid, not an alcohol
Wrong move:
Mixing up ester and carboxylic acid structures
Why:
Both have two oxygen atoms, leading to confusion
Correct move:
Esters are R-COO-R' (non-carbonyl oxygen bonded to an alkyl group), carboxylic acids are R-COOH (non-carbonyl oxygen bonded to hydrogen)
6. Quick Reference Cheatsheet
Class | General Structure | Key Property/Reaction | |
|---|---|---|---|
Alkene | C=C | Undergoes addition reactions | |
Alcohol | R-OH | Forms hydrogen bonds, oxidizes | Oxidizes to aldehyde/carboxylic acid |
Halogenoalkane | R-X | Undergoes nucleophilic substitution | |
Aldehyde | R-CHO | Oxidizes to carboxylic acid | |
Ketone | R-CO-R' | No oxidation under mild conditions | |
Carboxylic acid | R-COOH | Esterification with alcohols | |
Ester | R-COO-R' | Formed from esterification | |
Amine | R-NH₂ | Acts as a base |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 · 1
Identify aldehyde functional group
- 2024 · 2
Predict alcohol oxidation product
- 2023 · 1
Distinguish carboxylic acid from ester
Going deeper
What's Next
Functional group chemistry is the foundation for all organic chemistry topics in IB Chemistry SL. Mastery of identification and reactivity is essential for all organic synthesis, mechanism, and testing questions that appear on both Paper 1 and Paper 2. This topic connects to every other organic subtopic, since reactivity is always determined by the functional groups present in a molecule. Building on this core concept, you can explore specific reaction patterns for individual functional groups and reaction mechanisms in more detail, both of which are heavily weighted in the final IB SL exam.