Year 12 Chemistry Module 7 · Organic Chemistry ⏱ ~45 min 5 MC · 3 Short Answer Lesson 12 of 23 Core

Reactions of Alcohols, Dehydration, Substitution & Oxidation

Master the three core transformations of alcohols: eliminate water to make alkenes, swap –OH for a halogen, or strip electrons to build aldehydes, ketones and carboxylic acids, and learn exactly which conditions control which product.

Today's hook: A breathalyser turns orange dichromate green when you breathe into it, but why does the same oxidant stay orange when tested on a tertiary alcohol?

0/5TASKS

You’re here

Connect: predict the alcohol reaction

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions

Before You Read

Three Industries, One Molecule

A forensic breathalyser works by oxidising ethanol in breath to ethanoic acid using potassium dichromate, the orange dichromate turns green as it is reduced. A winemaker monitors the souring of wine into vinegar (ethanol → ethanoic acid) when wine is exposed to air. A polymer chemist dehydrates ethanol to ethene, which is then polymerised to polyethylene.

Before you read on: Write down what you think is the structural difference between the product of oxidation (ethanoic acid, CH₃COOH) and the product of dehydration (ethene, CH₂=CH₂). What bonds have been added to, or removed from, the ethanol molecule in each case?

Learning Intentions

By the end of this lesson you will:

Know

Three reactions of alcohols: dehydration, substitution with HX, oxidation
Oxidation outcomes: 1° → aldehyde (distillation) or COOH (reflux) · 2° → ketone · 3° → no reaction
Colour changes: K₂Cr₂O₇/H⁺ orange→green (oxidation occurred) or stays orange (no oxidation)
HX reactivity order: HI > HBr > HCl

Understand

Why tertiary alcohols cannot be oxidised, no H on C–OH carbon
Why distillation gives aldehyde but reflux gives carboxylic acid from the same primary alcohol
Why concentrated (not dilute) acid drives dehydration
Why H₂O is always a product of alcohol substitution with HX

Can Do

Write balanced equations for all three reaction types with correct conditions
Classify an unknown alcohol as 1°/2°/3° from oxidation test results
Select conditions to produce a specific oxidation product from a primary alcohol
Construct multi-step synthesis routes using alcohols as intermediates

Scan these before reading

Key Terms

DehydrationAn elimination reaction removing H₂O from an alcohol to form an alkene; requires H₂SO₄ catalyst at ~170°C.

Nucleophilic substitutionReaction of an alcohol with HX to replace –OH with a halogen; produces a halogenoalkane.

Oxidation of primary alcoholPrimary alcohols oxidise to aldehydes (gentle oxidant) and then to carboxylic acids (excess oxidant).

Oxidation of secondary alcoholSecondary alcohols oxidise to ketones; no further oxidation under normal conditions.

Tertiary alcohol oxidationTertiary alcohols resist oxidation because there is no C–H bond adjacent to the –OH group.

Acidified dichromate testK₂Cr₂O₇/H₂SO₄ changes from orange to green in the presence of oxidisable alcohols (primary or secondary).

📚 Core Content

Explain: dehydration forms alkenes

Dehydration of Alcohols, Making Alkenes

Dehydration is the exact reverse of hydration, water is eliminated across the C–OH bond and an adjacent C–H bond, regenerating the C=C double bond, and the conditions that drive this reversal are the opposite of those that drive hydration.

Dehydration is an elimination reaction in which an alcohol loses a water molecule (H from one carbon, OH from the adjacent carbon) to form an alkene. It is the reverse of alkene hydration (L06, L10).

R–CH₂–CH₂OH → R–CH=CH₂ + H₂O

Worked Equations

Ethanol → ethene:
CH₃CH₂OH → CH₂=CH₂ + H₂O
Conditions: conc. H₂SO₄, ~170°C

Propan-1-ol → propene:
CH₃CH₂CH₂OH → CH₃CH=CH₂ + H₂O
Conditions: conc. H₂SO₄ or conc. H₃PO₄, heat

Secondary and Tertiary Alcohols, Two Possible Products

When a secondary or tertiary alcohol is dehydrated, the H can be removed from either adjacent carbon, giving two possible alkene products. The major product is the more substituted alkene (more alkyl groups on the C=C carbons). For HSC, NESA accepts any alkene product, but the more substituted alkene is preferred.

Butan-2-ol dehydration:

CH₃CHOHCH₂CH₃ → CH₃CH=CHCH₃ + H₂O (but-2-ene, major, more substituted)
CH₃CHOHCH₂CH₃ → CH₂=CHCH₂CH₃ + H₂O (but-1-ene, minor, less substituted)

Concentrated sulfuric acid and heat eliminate water from butan-2-ol to form but-2-ene.

Dehydration of butan-2-ol, H removed from C3 and OH removed from C2 combine to form H₂O; C=C forms between C2 and C3 giving but-2-ene (major, more substituted product)

Hydration vs. Dehydration, Opposite Conditions

HYDRATION (alkene → alcohol)

dilute acid · high P · high T

adds H₂O across C=C

DEHYDRATION (alcohol → alkene)

conc. acid · atmospheric P · heat

removes H₂O from C–OH and adjacent C–H

Conditions, Dehydration:
Reagent: conc. H₂SO₄ or conc. H₃PO₄ (acid catalyst + dehydrating agent)
Temperature: ~170°C for ethanol; ~230°C+ for higher alcohols
Pressure: Atmospheric
Equipment: Distillation (the product is allowed to evaporate from the heated mixture and is collected separately, used when the product needs to be removed as it forms), removes volatile alkene product as formed

Must Do: The acid in dehydration is CONCENTRATED, not dilute. Dilute acid + high pressure drives hydration (the reverse reaction). Writing "dilute H₂SO₄" for dehydration is wrong, concentrated acid at higher temperature drives dehydration. The concentration of the acid determines the reaction direction.

Common Error: Writing "water is added in dehydration." This is the exact opposite. DEhydration means removing water (–de = removal, hydration = water). The –OH from the alcohol and an H from the adjacent carbon leave together as H₂O, the molecule gets smaller. If water appears as a reactant in your dehydration equation, you have written a hydration reaction instead.

Quick check: Which acid and conditions are needed for dehydration of an alcohol?

Represent: substitution forms haloalkanes

Substitution of Alcohols with HX, Making Haloalkanes

The reverse of the haloalkane substitution from L10, instead of converting a haloalkane to an alcohol with NaOH, you can convert an alcohol to a haloalkane with HX, and the conditions and halide reactivity order are what HSC specifically tests.

When an alcohol is treated with a hydrogen halide (HX), the –OH group is replaced by the halogen, producing a haloalkane and water:

R–OH + HX → R–X + H₂O

Halide Reactivity Order

HI

fastest
C–I easiest to form

HBr

intermediate

HCl

slowest
C–Cl hardest to form

Worked Equations:

CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O (bromoethane)
CH₃CH₂CH₂OH + HI → CH₃CH₂CH₂I + H₂O (1-iodopropane)

Comparison, reverse reactions:

R–OH + HX → R–X + H₂O (alcohol → haloalkane, this lesson)
R–X + NaOH(aq) → R–OH + NaX (haloalkane → alcohol, L10)

Conditions, Substitution of Alcohol with HX:
Reagent: HCl, HBr, or HI (gas or concentrated aqueous solution)
Catalyst: None (ZnCl₂ sometimes used with HCl for primary alcohols)
Conditions: Heat under reflux (the reaction mixture is heated with a condenser returning vapour to the flask, allowing the reaction to proceed at high temperature without losing volatile reactants or products)
Equipment: Reflux condenser; fume cupboard (HX gases are corrosive)

Must Do: Always include H₂O as a product. R–OH + HBr → R–Br is incomplete, the equation is unbalanced without H₂O. Check: the –OH from the alcohol and the H from HX combine on the right side to give H₂O. Missing H₂O is one of the most commonly penalised equation errors.

Common Error: Confusing R–OH + HX → R–X + H₂O (alcohol substitution) with alkene + HX → haloalkane (hydrohalogenation from L06, addition). Both use HX and produce haloalkanes. Key differences: (1) starting material (alcohol vs alkene); (2) whether H₂O is also produced (yes for alcohol substitution; no for addition); (3) reaction type (substitution vs addition). The presence of H₂O as a product instantly identifies alcohol substitution.

True or False: When ethanol reacts with HBr, water is produced as a co-product.

Respond: classify oxidation products

Oxidation of Alcohols, The Classification-Dependent Reaction

Oxidation of alcohols is the single most important reaction for understanding how the organic functional group classes connect, primary alcohols become aldehydes and then carboxylic acids, secondary alcohols become ketones, and tertiary alcohols refuse to react, and the reason is structural, not arbitrary.

Why Oxidation Depends on Alcohol Classification

Oxidation of an alcohol involves removing two hydrogen atoms, one H from the C–OH carbon and one H from the –OH oxygen, to form the C=O (carbonyl) bond. For this to be possible, the C–OH carbon must have at least one H atom available to be removed.

1°

C–OH has 2 H atoms → gives aldehyde (distillation) → can further oxidise to carboxylic acid (reflux)

2°

C–OH has 1 H atom → gives ketone. Ketone C=O is flanked by two carbons, no H on carbonyl carbon → cannot be further oxidised

3°

C–OH has 0 H atoms (bonded to 3 carbons) → no H to remove → NO REACTION with K₂Cr₂O₇/H⁺

Controlling Product from Primary Alcohols, Distillation vs. Reflux

Distillation → Aldehyde

Aldehyde has lower BP than alcohol, collected as it forms before excess oxidant can oxidise it further. Removes product from reaction mixture immediately.

R–CH₂OH + [O] → R–CHO + H₂O

Reflux → Carboxylic Acid

All components stay in flask; excess oxidant has maximum contact time to fully oxidise aldehyde intermediate through to carboxylic acid.

R–CH₂OH + 2[O] → R–COOH + H₂O

Oxidation Summary Table

Alcohol Class	Oxidant	Equipment	Product	Colour Change
Primary (1°)	K₂Cr₂O₇/H⁺	Distillation	Aldehyde	Orange → Green
Primary (1°)	K₂Cr₂O₇/H⁺ (excess)	Reflux	Carboxylic acid	Orange → Green
Secondary (2°)	K₂Cr₂O₇/H⁺	Reflux	Ketone	Orange → Green
Tertiary (3°)	K₂Cr₂O₇/H⁺	Any	No reaction	Stays orange

Must Do, Two Conditions Required: For any alcohol oxidation answer, always state: (1) the oxidising agent (K₂Cr₂O₇/H⁺, always acidified); AND (2) the equipment (distillation for aldehyde; reflux for carboxylic acid or ketone). Stating only the reagent without the equipment earns partial marks. The equipment choice determines the product.

Safety: Acidified potassium dichromate contains chromium(VI), which is toxic, corrosive and a recognised carcinogen, wear goggles and gloves, avoid skin contact and inhalation, and collect the spent green chromium(III) solution as heavy-metal waste rather than discarding it down the sink. The acid is concentrated sulfuric acid (used here and in dehydration), which is highly corrosive and reacts exothermically with water, always add acid to water, never the reverse, and handle hot reflux/distillation glassware with care because the organic reactants and products are flammable. Schools normally run dichromate oxidations as a small-scale teacher demonstration with full PPE and ventilation.

Common Error: "Tertiary alcohols give a ketone with KMnO₄." Tertiary alcohols do NOT undergo oxidation, the solution stays orange. This is the diagnostic test for tertiary alcohols.

Common Error: "KMnO₄ goes purple → orange." KMnO₄ goes purple → colourless (Mn²⁺) in acidic solution. Orange is the colour of dichromate. KMnO₄ NEVER turns orange.

A student oxidises butan-2-ol with K₂Cr₂O₇/H₂SO₄ under reflux. What is the product and colour change?

Feedback: control products with conditions

Alcohols at the Centre, The Reaction Map Hub

Putting dehydration, substitution, and oxidation together with the production reactions from L10 reveals that alcohols are the central hub of the organic reaction map, every arrow into and out of alcohol connects to a different functional group class.

Alcohols are produced from alkenes, haloalkanes and glucose, and react on to alkenes, haloalkanes and carbonyls.

Alcohol reaction hub, dashed arrows show production routes (L10); solid arrows show this lesson's reactions. Alcohol is the central node connecting every major functional group class.

Must Do: The orange→green colour change confirms oxidation occurred, it does NOT identify the specific product. To determine the product, you need: (1) the conditions (distillation vs. reflux); AND (2) a secondary test on the product (Tollens' for aldehyde vs. litmus for acid). Never write "the product is an aldehyde because the dichromate went green", the colour change identifies the class (primary/secondary) but not the specific carbonyl product.

Complete: To convert a primary alcohol to a carboxylic acid, use _____ oxidant under _____ conditions (equipment).

📋 Worked Examples

Predicting Products of Alcohol Reactions

Problem: For each reaction below, write the balanced equation, name the product, and state all conditions: (a) dehydration of propan-1-ol; (b) reaction of butan-1-ol with HBr; (c) oxidation of propan-2-ol to its maximum oxidation product.

(a) Dehydration of propan-1-ol:
Propan-1-ol: CH₃CH₂CH₂OH. Remove H from C2 and –OH from C1 → propene + H₂O.
CH₃CH₂CH₂OH → CH₃CH=CH₂ + H₂O
Product: propene | Conditions: conc. H₂SO₄ (or conc. H₃PO₄) · heat (~170–230°C) · distillation

(b) Butan-1-ol + HBr:
CH₃CH₂CH₂CH₂OH + HBr → CH₃CH₂CH₂CH₂Br + H₂O
Product: 1-bromobutane | Conditions: HBr · heat under reflux · fume cupboard

(c) Oxidation of propan-2-ol (maximum product):
Propan-2-ol: CH₃CHOHCH₃, secondary alcohol. Maximum oxidation product = ketone. Ketones cannot be further oxidised.
CH₃CHOHCH₃ + [O] → CH₃COCH₃ + H₂O
Product: propanone | Conditions: K₂Cr₂O₇/H₂SO₄ · reflux · Colour change: orange → green

Summary:
(a) CH₃CH₂CH₂OH → CH₃CH=CH₂ + H₂O (propene). Conc. H₂SO₄, heat, distillation.
(b) CH₃CH₂CH₂CH₂OH + HBr → CH₃CH₂CH₂CH₂Br + H₂O (1-bromobutane). HBr, reflux.
(c) CH₃CHOHCH₃ + [O] → CH₃COCH₃ + H₂O (propanone). K₂Cr₂O₇/H₂SO₄, reflux. Orange → green.

Identifying Unknown Alcohols Using Oxidation Tests

Problem: Three unlabelled alcohols, P, Q, R, all have molecular formula C₄H₁₀O. When treated with acidified K₂Cr₂O₇: P stays orange. Q turns green; distillation of the product gives a compound that produces a silver mirror with Tollens' reagent. R turns green; the product does NOT give a silver mirror and does NOT turn litmus red. (a) Classify P, Q, R as 1°/2°/3°. (b) Identify each compound. (c) Write the oxidation equation for Q under reflux conditions.

(a) Classify:
P: no colour change → no oxidation → tertiary alcohol
Q: orange → green; positive Tollens' → aldehyde formed → primary alcohol
R: orange → green; neither aldehyde nor carboxylic acid → ketone formed → secondary alcohol

(b) Identify (all C₄H₁₀O isomers):
P (tertiary, C₄): only one tertiary C₄ alcohol → 2-methylpropan-2-ol ((CH₃)₃COH)
Q (primary, C₄): most common exam assumption → butan-1-ol (CH₃CH₂CH₂CH₂OH)
R (secondary, C₄): only one secondary C₄H₁₀O alcohol → butan-2-ol (CH₃CHOHCH₂CH₃)

(c) Oxidation of Q (butan-1-ol) under reflux, maximum product:
Primary alcohol + excess oxidant + reflux → carboxylic acid.
CH₃CH₂CH₂CH₂OH + 2[O] → CH₃CH₂CH₂COOH + H₂O
Product: butanoic acid. Conditions: excess K₂Cr₂O₇/H₂SO₄, reflux. Orange → green.

Conclusion: P is tertiary, Q is primary and R is secondary. The oxidation evidence distinguishes the three classes under the stated acidified-dichromate conditions.

→

Key Equations, Reactions of Alcohols

Dehydration (Elimination)

R–CH₂–CH₂OH → R–CH=CH₂ + H₂O

conc. H₂SO₄ or conc. H₃PO₄ · heat (~170°C) · atmospheric pressure

Substitution with HX

R–OH + HX → R–X + H₂O

HCl, HBr, or HI · heat under reflux · reactivity: HI > HBr > HCl

Oxidation: 1° → Aldehyde

R–CH₂OH + [O] → R–CHO + H₂O

K₂Cr₂O₇/H⁺ · distillation · orange → green

Oxidation: 1° → Carboxylic Acid

R–CH₂OH + 2[O] → R–COOH + H₂O

excess K₂Cr₂O₇/H⁺ · reflux · orange → green

Oxidation: 2° → Ketone

R–CHOH–R' + [O] → R–CO–R' + H₂O

K₂Cr₂O₇/H⁺ · reflux · orange → green · cannot oxidise further

Tertiary → No Reaction

R–C(OH)(R')(R'') + [O] → no reaction

K₂Cr₂O₇/H⁺ stays ORANGE · no H on C–OH carbon

Interactive Tool, Alcohols Laboratory Open fullscreen ↗

In the Alcohol Properties tool, which type of alcohol CANNOT be oxidised to a carboxylic acid?

🔀 Sort the Steps +7 XP

Arrange the correct sequence of steps to oxidise ethanol all the way to ethanoic acid using acidified K₂Cr₂O₇:

Ethanol (primary alcohol) is first oxidised to ethanal (aldehyde)

Continued oxidation converts ethanal to ethanoic acid (carboxylic acid)

Observe colour change from orange (Cr₂O₇²⁻) to green (Cr³⁺) confirming oxidation

Mix ethanol with acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄)

Heat under reflux conditions (not distillation) to prevent loss of intermediate

Apply: conditions, evidence and pathways

Complete the Learn phase to unlock Practice.

✏️ Activities

Activity A, Conditions Grid: Fill the Gaps

For each transformation below, write: the reagent, the conditions (temperature/equipment), and the type of reaction.

Butan-1-ol → but-1-ene
Pentan-1-ol → pentanal
Pentan-1-ol → pentanoic acid
Propan-2-ol → propanone
Ethanol + HCl → chloroethane

Activity B, Diagnostic Reasoning: What Alcohol Is This?

An unknown alcohol X (molecular formula C₃H₈O) is tested with the following reagents:

K₂Cr₂O₇/H₂SO₄ (reflux): solution turns green; product has a sharp vinegar-like smell
Dehydration with conc. H₂SO₄: produces propene

Identify alcohol X, write its structural formula, and explain each piece of evidence. Then write the equation for the oxidation of X to its maximum oxidation product.

📝 Check Your Understanding

Randomised Question Bank

Complete five questions drawn from the ten aligned alcohol-reaction items.

Q6, Short Answer (4 marks)

Write balanced equations for three reactions of ethanol: (a) dehydration to ethene; (b) reaction with HBr to form a haloalkane; (c) oxidation to ethanoic acid. For each equation, state the reagents and conditions required.

Q7, Short Answer (5 marks)

The same primary alcohol, pentan-1-ol, can be converted to either pentanal or pentanoic acid using K₂Cr₂O₇/H₂SO₄. (a) Write the equation for each transformation. (b) State the specific conditions (including equipment) that produce each product. (c) Explain why using reflux rather than distillation changes the product from aldehyde to carboxylic acid.

Q8, Short Answer (6 marks)

A student performs a two-step synthesis: Step 1, ethene is reacted with steam to produce ethanol. Step 2, ethanol is converted to ethanoic acid. For each step, write the balanced equation, state the reagents and conditions, and explain the role of a specific condition in determining the product (including why reflux in step 2 gives acid rather than aldehyde).

Show All Answers

Multiple Choice Answers

Q1, B: Pentan-1-ol is a primary alcohol. Excess K₂Cr₂O₇/H⁺ under REFLUX gives carboxylic acid (pentanoic acid). Orange → green. A would result from DISTILLATION. C is wrong, pentan-1-one would require a secondary alcohol. D has wrong colour change, purple→colourless is KMnO₄.

Q2, B: To produce the aldehyde from a primary alcohol, the aldehyde must be removed before excess oxidant can oxidise it further, achieved by DISTILLATION. A (excess oxidant, reflux) fully oxidises to ethanoic acid. D describes dehydration to ethene, not oxidation.

Q3, C: 2-methylpropan-2-ol is a tertiary alcohol, the C–OH carbon is bonded to three other carbons with no H atoms. K₂Cr₂O₇/H⁺ stays orange = no oxidation. This orange/no-change result is the diagnostic test for tertiary alcohols.

Q4, C: To stop at the aldehyde stage, the aldehyde must be removed from the reaction mixture as soon as it forms, before the oxidant can oxidise it further to butanoic acid. Distillation achieves this because butanal has a lower boiling point than butan-1-ol.

Q5, D: No reaction with K₂Cr₂O₇/H⁺ (stays orange) → not a primary or secondary alcohol and not an aldehyde. No positive Tollens' → not an aldehyde. C₄H₈O with a C=O that is not an aldehyde = ketone. Butan-2-one (CH₃COCH₂CH₃) is the C₄ ketone.

Short Answer Sample Answers

Q6 (4 marks): (a) CH₃CH₂OH → CH₂=CH₂ + H₂O. Conditions: conc. H₂SO₄, heat (~170°C), distillation. (b) CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O. Conditions: HBr, heat under reflux. (c) CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. Conditions: excess K₂Cr₂O₇/H₂SO₄, reflux.

Q7 (5 marks): (a) Pentanal: CH₃CH₂CH₂CH₂CH₂OH + [O] → CH₃CH₂CH₂CH₂CHO + H₂O. Pentanoic acid: + 2[O] → CH₃CH₂CH₂CH₂COOH + H₂O. (b) Pentanal: K₂Cr₂O₇/H₂SO₄, gentle heating, DISTILLATION. Pentanoic acid: excess K₂Cr₂O₇/H₂SO₄, REFLUX. (c) Pentanal has a lower boiling point than pentan-1-ol. Under distillation, pentanal boils off and is removed from the reaction mixture as soon as it forms, before excess oxidant can oxidise it further. Under reflux, all volatile components are condensed and returned to the flask, giving the oxidant maximum contact time to oxidise the aldehyde all the way to the carboxylic acid.

Q8 (6 marks): Step 1: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH. Conditions: H₂O (steam), H₃PO₄ catalyst, ~300°C, ~65 atm. High pressure used because the left side has 2 moles of gas, by Le Chatelier, high pressure shifts equilibrium right toward ethanol. Step 2: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. Conditions: excess K₂Cr₂O₇/H₂SO₄, reflux. Role of reflux: keeps all volatile components (including the aldehyde intermediate, ethanal) in the reaction flask, giving the excess oxidant maximum contact time to oxidise ethanal through to ethanoic acid. If distillation were used instead, ethanal would be removed before further oxidation.

How did your thinking change?

Return to your prediction about the structural difference between ethanoic acid (oxidation) and ethene (dehydration), both starting from ethanol. Oxidation: two H atoms removed, C=O and OH group added, functional group changes from –OH to –COOH. Dehydration: –OH and an H from adjacent carbon leave as H₂O, C=C double bond forms. How well did your structural prediction compare?

Coming up in Lesson 13: The oxidation products of primary alcohols (aldehydes) and secondary alcohols (ketones) become the focus, understanding their distinct structural properties, reactivity with Tollens' and Fehling's reagents, and how to distinguish between them experimentally.

Review: retrieve the alcohol reaction map

Check what actually stuck

🏆 Review

What reagent and equipment convert a primary alcohol to an aldehyde (stopping before carboxylic acid)?

Why do tertiary alcohols not react with K₂Cr₂O₇/H⁺?

Give the HX reactivity order and explain why HI is the fastest.

What acid concentration is required for dehydration and why does concentration matter?

A student observes that KMnO₄ turns orange when oxidising an alcohol. What is wrong with this observation?