# Functional group chemistry

> IB Chemistry SL · Reactivity 3: Equilibrium and Organic Chemistry
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u6-functional-group-chemistry/

This module covers identification of common IB SL functional groups, their effect on physical properties, characteristic reactivity patterns, and chemical tests for distinguishing functional groups, all required for IB SL exam assessments.

**Prerequisites:** [Organic nomenclature and classification](https://www.owlsprep.com/study/ib-chemistry-sl-u6-organic-nomenclature/); [Intermolecular forces](https://www.owlsprep.com/study/ib-chemistry-sl-u4-intermolecular-forces/)

## Learning objectives

- Identify common IB SL functional groups from structural formulas
- Predict how functional groups influence physical properties of organic molecules
- Recall characteristic reaction patterns for key functional groups
- Describe simple chemical tests to distinguish between common functional groups

## Identification of Common Functional Groups

**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 |

**Worked example:** 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.

**Check your understanding**

Test your identification skills

1. What functional group is present in CH₃CH₂COCH₃?

   - Aldehyde
   - Ketone
   - Carboxylic acid
   - Alcohol

   *Why:* Correct! The carbonyl C=O group is bonded to two alkyl groups, so it is a ketone.

2. Which functional group has the general formula RCOOH?

   - Ester
   - Aldehyde
   - Carboxylic acid
   - Alcohol

   *Why:* Right! The carboxyl -COOH group defines all carboxylic acids.

## Functional Groups and Physical Properties

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.

> **info**
>
> Only functional groups with a hydrogen atom bonded directly to oxygen (O-H) or nitrogen (N-H) can form intermolecular hydrogen bonds. These molecules have much higher boiling points than similar-sized non-polar molecules.

**Worked example:** 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.

## Characteristic Reactivity by Functional Group

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

**Worked example:** 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. $$H_2C=CH_2 + HBr \rightarrow CH_3CH_2Br$$
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'.

## Distinguishing Functional Groups With Chemical Tests

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 |

**Worked example:** 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.

## Common pitfalls

- **Wrong:** Confusing aldehydes and ketones, classifying R-CHO as a ketone
  - Why it fails: Both contain a carbonyl C=O group, so students mix up their classification
  - Correct: 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:** Claiming all carbonyl-containing compounds can form intermolecular hydrogen bonds
  - Why it fails: Students assume any oxygen-containing group can form hydrogen bonds
  - Correct: 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:** Forgetting that a single molecule can have multiple functional groups
  - Why it fails: Students stop after identifying the first obvious group, missing other groups in the molecule
  - Correct: Always scan the entire molecular structure after identifying your first functional group to check for others
- **Wrong:** Classifying the -OH in a carboxylic acid as an alcohol functional group
  - Why it fails: The -OH group is present, but it is part of a distinct functional group with different reactivity
  - Correct: When -OH is bonded directly to a carbonyl carbon (as part of -COOH), classify it as a carboxylic acid, not an alcohol
- **Wrong:** Mixing up ester and carboxylic acid structures
  - Why it fails: Both have two oxygen atoms, leading to confusion
  - Correct: Esters are R-COO-R' (non-carbonyl oxygen bonded to an alkyl group), carboxylic acids are R-COOH (non-carbonyl oxygen bonded to hydrogen)

## Cheatsheet

| Class | General Structure | Key Property/Reaction |
| --- | --- | --- |
| Alkene | C=C | Undergoes addition reactions |
| Alcohol | R-OH | Forms hydrogen bonds, oxidizes |
| 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 |

## 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.

- [Organic Reaction Mechanisms](https://www.owlsprep.com/study/ib-chemistry-sl-u6-organic-reaction-mechanisms/)
- [Practical and Investigative Chemistry](https://www.owlsprep.com/study/ib-chemistry-sl-u7-overview/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-chemistry-sl-u6-functional-group-chemistry/