# Types of organic reactions

> IB Chemistry HL · IB Chemistry HL
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u6-types-of-organic-reactions/

This module covers classification of core organic reaction types for IB Chemistry HL. You will learn to identify reaction types from reactants, products and reagents, and avoid common classification errors in exams.

**Prerequisites:** [Functional group identification](https://www.owlsprep.com/study/ib-chemistry-hl-u5-functional-groups/); [Introduction to organic reaction mechanisms](https://www.owlsprep.com/study/ib-chemistry-hl-u6-introduction-mechanisms/)

## Learning objectives

- Classify common organic reactions by their reaction type
- Identify reaction types from reactants, products and reaction conditions
- Distinguish the role of reagents in different reaction types
- Predict products for a given starting material and reaction type

## Polar Reaction Types: Substitution and Addition

**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

**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. 2. 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. 3. 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.

## Elimination, Condensation and Hydrolysis

**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

**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. 2. Products: but-2-ene has a new carbon-carbon double bond, and water is formed as a byproduct.
3. 3. 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.

## Organic Oxidation and Reduction

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.

**Organic Redox** — Oxidation increases the oxidation state of carbon, common oxidizing agents are acidified $K_2Cr_2O_7$ and $KMnO_4$. Reduction decreases the oxidation state of carbon, common reducing agents are $NaBH_4$ and $LiAlH_4$.

**Worked example:** Classify the conversion of ethanol ($CH_3CH_2OH$) to ethanoic acid ($CH_3COOH$) using acidified potassium dichromate.

1. 1. Count C-O bonds on the terminal carbon of ethanol: it has 1 C-O single bond.
2. 2. 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. 3. The number of C-O bonds has increased, so carbon is oxidized.
4. Conclusion: This is an oxidation reaction.

> **info**
>
> Primary alcohols oxidize to aldehydes then carboxylic acids, secondary alcohols oxidize to ketones, and tertiary alcohols do not oxidize under standard conditions.

## Reaction Type Recognition in Exams

**Exam command terms**

Common command terms for this topic in IB exams have clear expectations:

- **Classify** — State the reaction type and justify your answer with reference to reactants or products *(Classify the conversion of 1-bromobutane to butan-1-ol)*

- **Predict the product** — Given the starting material and reaction type, draw the structure of the major organic product *(Predict the product of elimination of HCl from 2-chlorobutane)*

**Check your understanding**

Test your understanding:

1. Which reaction type describes formation of an amide from a carboxylic acid and amine, with loss of water?

   - Addition
   - Condensation
   - Elimination
   - Substitution

   *Why:* Correct! Two molecules join to form a larger product with loss of a small water byproduct, which is condensation.

2. The conversion of propanal to propan-1-ol is what type of reaction?

   - Oxidation
   - Reduction
   - Substitution
   - Elimination

   *Why:* Correct! Propanal gains two hydrogen atoms to form propan-1-ol, which is reduction for organic compounds.

## Common pitfalls

- **Wrong:** Confusing substitution and elimination of halogenoalkanes
  - Why it fails: (Both use halogenoalkane starting materials and similar reagents, so students often mix them up)
  - Correct: Check for a new alkene double bond: if present it is elimination, if the product is still saturated it is substitution.
- **Wrong:** Calling esterification an addition reaction
  - Why it fails: Students see one large product and assume it is addition, but forget the byproduct
  - Correct: Any reaction that joins two molecules and loses a small byproduct (like water) is a condensation reaction, including esterification and amide formation.
- **Wrong:** Thinking all reactions with water are hydrolysis
  - Why it fails: Water can act as a nucleophile in substitution or a base in elimination, not just for hydrolysis
  - Correct: Hydrolysis only occurs when a larger molecule is split into two smaller molecules by water, it is always the reverse of condensation.
- **Wrong:** Calling oxidation of a secondary alcohol a carboxylic acid formation
  - Why it fails: Students mix up oxidation products of primary and secondary alcohols
  - Correct: Secondary alcohols only oxidize to ketones, only primary alcohols oxidize to aldehydes then carboxylic acids.
- **Wrong:** Assuming all reactions starting with alkenes are addition
  - Why it fails: Alkenes can undergo oxidation (cleavage) or polymerization, which are not addition
  - Correct: Always check the number of products and any byproducts to confirm the reaction type, do not just rely on the starting functional group.

## 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 |

## 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.

- [Redox reactions and electrochemistry basics](https://www.owlsprep.com/study/ib-chemistry-hl-u6-redox-reactions-and-electrochemistry-basics/)
- [Oxidation numbers and voltaic cells](https://www.owlsprep.com/study/ib-chemistry-hl-u6-oxidation-numbers-and-voltaic-cells/)
- [Reaction mechanism basics](https://www.owlsprep.com/study/ib-chemistry-hl-u6-reaction-mechanism-basics/)

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