Types of organic reactions

IB Chemistry HL· R3: What are the mechanisms of chemical change?· 25 min read

1. Polar Reaction Types: Substitution and Addition★★☆☆☆⏱ 8 min

📘 Definition

Substitution Reaction

A reaction where one atom or functional group on a substrate is replaced by a different atom or group from the reagent, with no change to the carbon skeleton saturation.

Example:

Replacement of bromine in bromoethane by hydroxyl to form ethanol

📘 Definition

Addition Reaction

A reaction that occurs across an unsaturated multiple bond, where two reagent atoms/groups add to the bonded atoms, forming a single saturated product with no byproducts.

Example:

Addition of HBr across ethene to form bromoethane

📐 Worked Example

Classify the reaction between propene and HBr that forms 2-bromopropane

1
1. Identify the starting functional group: propene contains a carbon-carbon double bond, so it is unsaturated.
2
1. Identify the product: 2-bromopropane is saturated, with no double bonds. All atoms from HBr are incorporated into the product, no byproducts are formed.
3
1. HBr has added across the double bond of propene to form one single product.
4
Conclusion: This is an addition reaction.

Exam tip:

Addition reactions always start with an unsaturated starting material (alkene/alkyne) and form a single saturated product.

2. Elimination, Condensation and Hydrolysis★★★☆☆⏱ 8 min

📘 Definition

Elimination Reaction

A reaction where two adjacent groups are removed from a saturated substrate to form a new carbon-carbon double bond, plus a small stable byproduct.

Example:

Elimination of HBr from 2-bromopropane to form propene and HBr

📘 Definition

Condensation & Hydrolysis

Condensation joins two smaller molecules into one larger molecule, with elimination of a small byproduct. Hydrolysis is the reverse: it splits a large molecule into two smaller molecules using water.

Example:

Condensation: ester formation; Hydrolysis: ester breakdown

📐 Worked Example

Butan-2-ol reacts with concentrated sulfuric acid to form but-2-ene and water. Classify this reaction.

1
1. Starting material: butan-2-ol is fully saturated, with no double bonds.
2
1. Products: but-2-ene has a new carbon-carbon double bond, and water is formed as a byproduct.
3
1. Two groups (hydroxyl from carbon 2 and hydrogen from carbon 3) have been removed to form the new double bond.
4
Conclusion: This is an elimination reaction.

Exam tip:

Concentrated reagents (acid/base) favor elimination, while dilute reagents favor substitution or hydrolysis.

3. Organic Oxidation and Reduction★★★☆☆⏱ 6 min

In organic chemistry, oxidation and reduction are defined by changes to carbon-oxygen and carbon-hydrogen bonds, rather than just oxidation numbers. Oxidation increases the number of C-O bonds or decreases the number of C-H bonds. Reduction does the opposite.

📘 Definition

Organic Redox

Oxidation increases the oxidation state of carbon, common oxidizing agents are acidified $K_{2} C r_{2} O_{7}$ and $K M n O_{4}$ . Reduction decreases the oxidation state of carbon, common reducing agents are $N a B H_{4}$ and $L i A l H_{4}$ .

📐 Worked Example

Classify the conversion of ethanol ( $C H_{3} C H_{2} O H$ ) to ethanoic acid ( $C H_{3} C O O H$ ) using acidified potassium dichromate.

1
1. Count C-O bonds on the terminal carbon of ethanol: it has 1 C-O single bond.
2
1. Count C-O bonds on the same terminal carbon of ethanoic acid: it has 3 C-O bonds (1 single to -OH, 2 from the double bond to O).
3
1. The number of C-O bonds has increased, so carbon is oxidized.
4
Conclusion: This is an oxidation reaction.

4. Reaction Type Recognition in Exams★★★★☆⏱ 3 min

✓ Quick check

Test your understanding:

Which reaction type describes formation of an amide from a carboxylic acid and amine, with loss of water?
- Addition
- Condensation
- Elimination
- Substitution
Reveal answer
Condensation —
Correct! Two molecules join to form a larger product with loss of a small water byproduct, which is condensation.
The conversion of propanal to propan-1-ol is what type of reaction?
- Oxidation
- Reduction
- Substitution
- Elimination
Reveal answer
Reduction —
Correct! Propanal gains two hydrogen atoms to form propan-1-ol, which is reduction for organic compounds.

5. Common Pitfalls

Wrong move:

Confusing substitution and elimination of halogenoalkanes

Why:

(Both use halogenoalkane starting materials and similar reagents, so students often mix them up)

Correct move:

Check for a new alkene double bond: if present it is elimination, if the product is still saturated it is substitution.

Wrong move:

Calling esterification an addition reaction

Why:

Students see one large product and assume it is addition, but forget the byproduct

Correct move:

Any reaction that joins two molecules and loses a small byproduct (like water) is a condensation reaction, including esterification and amide formation.

Wrong move:

Thinking all reactions with water are hydrolysis

Why:

Water can act as a nucleophile in substitution or a base in elimination, not just for hydrolysis

Correct move:

Hydrolysis only occurs when a larger molecule is split into two smaller molecules by water, it is always the reverse of condensation.

Wrong move:

Calling oxidation of a secondary alcohol a carboxylic acid formation

Why:

Students mix up oxidation products of primary and secondary alcohols

Correct move:

Secondary alcohols only oxidize to ketones, only primary alcohols oxidize to aldehydes then carboxylic acids.

Wrong move:

Assuming all reactions starting with alkenes are addition

Why:

Alkenes can undergo oxidation (cleavage) or polymerization, which are not addition

Correct move:

Always check the number of products and any byproducts to confirm the reaction type, do not just rely on the starting functional group.

6. Quick Reference Cheatsheet

Reaction Type	Key Identifying Feature	Common Example
Substitution	One group replaces another, saturated product	Halogenoalkane → alcohol
Addition	Add across unsaturated bond, no byproduct	Alkene + HBr → bromoalkane
Elimination	Forms new double bond, small byproduct	Alcohol → alkene + water
Condensation	Join two molecules, lose small byproduct	Carboxylic acid + alcohol → ester + water
Hydrolysis	Split molecule with water	Ester → carboxylic acid + alcohol
Organic Oxidation	Increase C-O bonds / decrease C-H	Primary alcohol → carboxylic acid
Organic Reduction	Increase C-H bonds / decrease C-O	Aldehyde → primary alcohol

7. Frequently Asked

Do HL and SL have different requirements for this topic?

HL requires full mastery of all reaction types covered here including redox of organic compounds, while SL only requires core knowledge of substitution, addition and elimination.

How do I distinguish substitution from elimination?

Substitution retains a saturated carbon skeleton and replaces a functional group, while elimination forms a new unsaturated double bond and produces a small byproduct.

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
Classify reaction from structures
2024 · 2
Identify reaction type for esterification
2023 · 3
Classify oxidation of alcohol

Going deeper

syllabusIB Chemistry HL R3 SyllabusOfficial syllabus reference

What's Next

Classifying reaction types is the foundational step for learning specific organic reaction mechanisms, a high-weight topic for IB Chemistry HL Paper 2 and Paper 3. Mastering classification now makes it easier to predict products, draw mechanism curly arrows and answer extended response questions correctly. This topic also underpins organic synthesis, where you will sequence multiple reaction types to produce a target molecule from a simple starting material, a common exam question.