Year 12 Chemistry Module 7 · Organic Chemistry ⏱ ~45 min 5 MC · 3 Short Answer Lesson 9 of 23 Core

Structure & Properties of Alcohols

Discover how the -OH group transforms a hydrocarbon into a molecule that hydrogen-bonds, dissolves in water, and behaves completely differently depending on where that hydroxyl sits on the carbon chain.

Today's hook: Methanol and ethanol are one carbon apart and have the same functional group, so why does a tablespoon of methanol cause permanent blindness while a glass of ethanol does not?

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Connect: predict what the hydroxyl group changes

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

Methanol, ethanol, propan-1-ol, and butan-1-ol are all primary alcohols in the same homologous series. Methanol is a colourless liquid that is fatally toxic, a single tablespoon can cause permanent blindness, and a few tablespoons can kill. Ethanol is the same functional group, one carbon longer, and is consumed in billions of litres per year by humans without moderate acute toxicity. Propan-1-ol and butan-1-ol are used as industrial solvents.

How can four consecutive members of the same homologous series have such dramatically different biological effects? Write your hypothesis, what changes as the chain gets longer, and why might methanol be so specifically toxic?

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Alcohol Structure, Classification & Hydrogen Bonding

R-CH₂-OH

Primary (1°) alcohol C-OH bonded to 1 other carbon

R-CH(OH)-R'

Secondary (2°) alcohol C-OH bonded to 2 other carbons

R-C(OH)(R')(R'')

Tertiary (3°) alcohol C-OH bonded to 3 other carbons

O-H ··· :O (neighbour)

Hydrogen bond ~20–25 kJ/mol Explains high BP vs comparable alkane

Classification rule: count the carbons directly bonded to the C-OH carbon, 1 = primary, 2 = secondary, 3 = tertiary. The -OH group is the same in all three; the difference is the carbon skeleton around it.

Learning Intentions

+15 XP

Know

The structural definition of primary, secondary, and tertiary alcohols
That alcohols form hydrogen bonds via O-H (donor) and lone pairs on O (acceptor)
The boiling point order: primary > secondary > tertiary for constitutional isomers

Understand

Why alcohols have dramatically higher boiling points than comparable alkanes
Why branching lowers boiling point (surface area → dispersion forces)
Why short-chain alcohols are water-miscible but long-chain alcohols are not

Can do

Classify any alcohol from its structural formula in one step
Rank alcohols by boiling point using IMF reasoning
Explain solubility trends in water and organic solvents using chain length and -OH arguments

Scan these before reading

+10 XP

alcoholAn organic compound containing a hydroxyl group (-OH) bonded directly to a carbon with only single bonds (tetrahedral geometry, ~109.5°). General formula R-OH.

hydroxyl group (-OH)The functional group of alcohols. The oxygen has two lone pairs (H-bond acceptor) and the O-H bond makes H a strong H-bond donor. Highly polar.

hydrogen bondAn electrostatic attraction between a δ+ H bonded to a highly electronegative atom (O, N, or F) and a lone pair on O, N, or F of a neighbouring molecule. ~20–25 kJ/mol for O-H···O.

primary alcohol (1°)The C-OH carbon is bonded to one other carbon (or zero, as in methanol). General structure R-CH₂OH.

secondary alcohol (2°)The C-OH carbon is bonded to two other carbons. General structure R-CH(OH)-R'.

tertiary alcohol (3°)The C-OH carbon is bonded to three other carbons. No H on the C-OH carbon. General structure R-C(OH)(R')(R'').

📚 Core Content

Explain: hydroxyl polarity and hydrogen bonding

The Hydroxyl Group, Structure and Bonding

The -OH group is the defining feature of alcohols, it is small, polar, and capable of hydrogen bonding, and those three characteristics together explain almost every physical property that separates alcohols from the hydrocarbons they are built on.

The carbon bearing –OH has only single bonds, tetrahedral geometry, ~109.5° bond angles. The oxygen in -OH has two bonding pairs (one to C, one to H) and two lone pairs. The O-H bond is highly polar (O is much more electronegative than H, δ⁻ on O, δ⁺ on H).

This combination, a strong H-bond donor (O-H) and a strong H-bond acceptor (lone pairs on O), means alcohol molecules form extensive hydrogen bond networks with each other and with water.

Ethanol forms hydrogen bonds and boils high; ethane has only weak dispersion forces and boils low.

Hydrogen bonding in ethanol vs dispersion forces only in ethane, explains the 167°C difference in boiling point despite similar molecular mass.

Exam Rule: When drawing the full structural formula of an alcohol, always show the O-H bond as two separate atoms (C-O-H), not as a block "-OH" stuck to the end. The marker needs to see that O and H are distinct atoms with distinct bonding, especially in questions about hydrogen bonding where the donor atom (H bonded to O) must be identified.

Common Error: "The C-OH bond is the hydrogen bond donor." Wrong. The hydrogen bond is donated by the H atom bonded to the oxygen. It is the δ⁺ H on the O-H bond that forms the electrostatic interaction with the lone pair of a neighbouring O. Writing "the oxygen donates the hydrogen bond" confuses the donor and acceptor roles, the oxygen is the acceptor (via its lone pairs); the H is the donor.

Key Insight: Methanol's toxicity is not about chain length or IMF, it is about metabolic fate. Both methanol and ethanol are metabolised by alcohol dehydrogenase. Ethanol forms ethanal and then ethanoic acid. Methanol forms methanal and then methanoic acid; the accumulated formate causes metabolic acidosis and damages the retina and optic nerve. Same enzyme pathway, radically different consequences because the products differ.

Quick check: In alcohol molecules, which atom acts as the hydrogen bond DONOR?

Represent: classify primary, secondary and tertiary alcohols

Primary, Secondary, and Tertiary Alcohols

The classification of an alcohol as primary, secondary, or tertiary is not just a naming convention, it determines every reaction the alcohol can undergo, from what oxidation products it gives to whether it undergoes elimination or substitution, and how quickly.

The classification is determined by the carbon bearing the -OH group, specifically, how many OTHER carbon atoms that carbon is directly bonded to.

1°

Primary

C-OH bonded to 1 other C
General: R-CH₂OH

Examples:
ethanol: CH₃CH₂OH
propan-1-ol: CH₃CH₂CH₂OH

2°

Secondary

C-OH bonded to 2 other C
General: R-CH(OH)-R'

Examples:
propan-2-ol: CH₃CHOHCH₃
butan-2-ol: CH₃CHOHCH₂CH₃

3°

Tertiary

C-OH bonded to 3 other C
No H on C-OH
General: R-C(OH)(R')(R'')

Example:
2-methylpropan-2-ol: (CH₃)₃COH

Classification: count the carbons directly bonded to the C-OH carbon (highlighted). 1 = primary, 2 = secondary, 3 = tertiary.

Alcohol	Formula	C-OH bonds to	Class
Methanol	CH₃OH	0 other C (3H)	Primary (1°) by convention
Ethanol	CH₃CH₂OH	1 other C	Primary (1°)
Propan-2-ol	CH₃CHOHCH₃	2 other C	Secondary (2°)
Butan-2-ol	CH₃CHOHCH₂CH₃	2 other C	Secondary (2°)
2-methylpropan-2-ol	(CH₃)₃COH	3 other C	Tertiary (3°)

Exam Rule: The classification step is always: find the carbon with -OH → count how many OTHER carbons are directly bonded to that carbon → that number (1, 2, or 3) is the classification. Do not count along the chain, do not count H atoms, do not count the oxygen. One step, one count.

Common Error: Students classify methanol as "zeroth order" or "not classifiable" because its C-OH carbon has no other carbon neighbours. By convention, methanol is classified as a PRIMARY alcohol.

True or False: 2-methylpropan-2-ol is classified as a secondary alcohol because it has two methyl groups.

Respond: explain boiling-point trends

Boiling Points of Alcohols, IMF in Detail

The boiling points of alcohols are dramatically higher than those of comparable hydrocarbons, and the reason is hydrogen bonding, but the variation in boiling point within the alcohol series, and between primary, secondary, and tertiary isomers, reveals additional layers of IMF subtlety.

Within a Primary Alcohol Series, Increasing Chain Length

Boiling point increases steadily with chain length: longer chains have greater surface area → stronger dispersion forces in addition to the H-bonding from -OH.

Methanol (C1) 65°C

65°C

Ethanol (C2) 78°C

78°C

Propan-1-ol (C3) 97°C

97°C

Butan-1-ol (C4) 118°C

118°C

Pentan-1-ol (C5) 138°C

138°C

Comparing Primary, Secondary, Tertiary Isomers (same formula)

For constitutional isomers (same molecular formula), the boiling point order is: Primary > Secondary > Tertiary.

All three classes have the same -OH group, the H-bonding contribution to BP is approximately equal for all three. The difference arises from dispersion forces: tertiary alcohols are the most branched, the most compact shape, which minimises surface area → weaker dispersion forces → lower BP.

Butan-1-ol (1°) 118°C

118°C, straight chain, max surface area

2-methylpropan-1-ol (1°) 108°C

108°C, primary but branched

Butan-2-ol (2°) 100°C

100°C, secondary

2-methylpropan-2-ol (3°) 83°C

83°C, most branched, min surface area

Exam Rule: When comparing 1°/2°/3° boiling points, state TWO reasons: (1) all three have equivalent H-bonding from the -OH group; (2) the difference arises from dispersion forces, more branching → less surface area → weaker dispersion forces → lower BP.

Common Error: "Tertiary alcohols have higher BPs because they have three alkyl groups." Three alkyl groups means MORE branching → LESS surface area → WEAKER dispersion forces → LOWER BP. The ranking is primary > secondary > tertiary.

Which one does NOT belong with the others in terms of what it explains about alcohol boiling points?

Explain: balance polar and non-polar regions

Solubility of Alcohols, Water and Organic Solvents

Short-chain alcohols dissolve in water because the -OH group H-bonds with water, but chain length is a limit, and understanding where solubility ends and why gives you the framework to predict solubility for every functional group in Module 7.

Solubility comparison of ethanol vs hexan-1-ol in water as a two-column parallel list.

Solubility in water decreases with chain length: -OH H-bonding compensates for short chains; large non-polar chains overwhelm the -OH contribution.

Solubility in Water

Methanol, fully miscible
Ethanol, fully miscible
Propan-1-ol, fully miscible
Butan-1-ol, partially miscible
Pentan-1-ol, sparingly soluble
Hexan-1-ol, practically insoluble

️ Solubility in Hexane (non-polar)

Methanol, poorly miscible
Ethanol, poorly miscible
Butan-1-ol, miscible
Hexan-1-ol, miscible
Branching increases water solubility

Branching and solubility: Branched-chain alcohols are generally MORE soluble in water than their straight-chain isomers because branching reduces the effective size of the non-polar region. 2-methylpropan-2-ol (tertiary, 4C) is fully miscible with water; butan-1-ol (primary, 4C, straight chain) is only partially miscible.

Exam Rule: Any solubility explanation for alcohols needs three components: (1) name the IMF between the alcohol and the solvent; (2) explain how chain length shifts the balance between the polar -OH and non-polar chain; (3) conclude.

Common Error: "Butan-1-ol is insoluble in water because it has four carbons." There is no single sharp cut-off carbon number; solubility decreases gradually.

Complete: As carbon chain length increases, water solubility of alcohols _______ because the growing non-polar chain disrupts _______ more than the single -OH can compensate.

Feedback: compare structures using a complete IMF chain

🧠 Worked Examples

Classification & Boiling Point Ranking

Problem: Classify each alcohol and arrange in order of decreasing boiling point: (a) 2-methylbutan-2-ol, (b) pentan-1-ol, (c) pentan-2-ol, (d) 2-methylbutan-1-ol.

Classify each: (a) 2-methylbutan-2-ol: C-OH bonded to THREE carbons → Tertiary (3°). (b) Pentan-1-ol: C-OH at C1 bonded to ONE carbon → Primary (1°), straight chain. (c) Pentan-2-ol: C-OH at C2 bonded to C1 and C3 → Secondary (2°). (d) 2-methylbutan-1-ol: C-OH at C1 bonded to ONE carbon → Primary (1°), branched.

All four are C₅H₁₂O, constitutional isomers. All have one -OH group, equivalent H-bonding. BP differences come from dispersion forces → determined by branching/shape.

Rank by surface area (most extended → highest BP): Pentan-1-ol (straight, 1°) > 2-methylbutan-1-ol (1°, one branch) > pentan-2-ol (2°, compact) > 2-methylbutan-2-ol (3°, most compact).

Answer: (a) 3°, (b) 1°, (c) 2°, (d) 1°. Decreasing BP: pentan-1-ol (138°C) > 2-methylbutan-1-ol (129°C) > pentan-2-ol (119°C) > 2-methylbutan-2-ol (102°C). All have equivalent -OH H-bonding; BP differences driven by branching → surface area → dispersion forces.

Explaining Solubility Using IMF

Problem: (a) Explain why ethanol is fully miscible with water but hexan-1-ol is practically insoluble. (b) Explain why hexan-1-ol dissolves readily in hexane but ethanol does not.

(a) Both have -OH: Both can form H-bonds with water. The difference is the non-polar chain.

Ethanol (C2): The 2-carbon chain creates a small non-polar region. The -OH interaction with water is energetically sufficient to compensate for disrupting water's H-bond network around the small ethyl group. Fully miscible.

Hexan-1-ol (C6): The 6-carbon chain must be accommodated within water's H-bond network. Disrupting water-water H-bonds to accommodate six carbons requires more energy than the single -OH can compensate for. Practically insoluble.

(b) Hexane is non-polar. Hexan-1-ol in hexane: the 6-carbon non-polar chain is compatible with hexane via dispersion forces. Ethanol in hexane: ethanol's dominant -OH makes it polar, hexane cannot compensate for breaking ethanol-ethanol H-bonds. Poorly miscible.

Answer: (a) Ethanol: short 2C chain, -OH H-bonding compensates → miscible. Hexan-1-ol: 6C chain disrupts more water H-bonds than -OH compensates → insoluble. (b) Hexan-1-ol: long non-polar chain compatible with hexane (dispersion). Ethanol: polar -OH incompatible with non-polar hexane.

Extended Response, C4H10O Isomers (7 marks)

Problem: Three C₄H₁₀O compounds: A = CH₃CH₂CH₂CH₂OH (BP 118°C, partially miscible in water); B = CH₃CH(OH)CH₂CH₃ (BP 100°C, miscible in water); C = (CH₃)₃COH (BP 83°C, fully miscible in water). (a) Name and classify each. (b) Explain BP trend A > B > C. (c) Explain why C is fully water-miscible while A is only partially miscible.

(a): A = butan-1-ol (1°, C-OH at C1, one carbon neighbour). B = butan-2-ol (2°, C-OH at C2, two carbon neighbours). C = 2-methylpropan-2-ol (3°, C-OH bonded to three CH₃ groups).

(b) BP trend: All three are C₄H₁₀O with one -OH, equivalent H-bonding. Differences from dispersion forces. A (straight chain): maximum surface area, strongest dispersion, highest BP. C (three methyl groups): most compact, minimum surface area, weakest dispersion, lowest BP.

(c) Water solubility C > A: Both have the same -OH. C's compact branched structure exposes less non-polar surface to water than A's extended straight chain, less disruption to water's H-bond network. The -OH compensates more effectively in C than in A despite the same carbon count.

Answer: (a) A = butan-1-ol (1°); B = butan-2-ol (2°); C = 2-methylpropan-2-ol (3°). (b) Equal -OH H-bonding; BP decreases A→B→C due to increasing branching → decreasing surface area → decreasing dispersion forces. (c) C's compact shape disrupts fewer water H-bonds than A's extended chain, same -OH compensates more effectively.

⚠️ High-Frequency Misconceptions

"Tertiary alcohols have higher boiling points than primary because they have more alkyl groups." Wrong. More alkyl groups = more branching = LOWER BP.
"Tertiary alcohols cannot form hydrogen bonds." Wrong. All alcohols have -OH and form H-bonds regardless of classification.
"Methanol is classified as a 0° (zeroth) alcohol." By convention, methanol is PRIMARY.
"The oxygen donates the hydrogen bond in alcohols." Wrong. The HYDROGEN (on the O-H bond, δ⁺) is the donor; the oxygen's lone pairs are the acceptors.

🔬 Predict, Then Reveal +8 XP

Three alcohols: methanol (CH₃OH), propan-2-ol ((CH₃)₂CHOH), and 2-methylpropan-2-ol ((CH₃)₃COH). Predict which has the highest boiling point and explain using intermolecular forces.

Your predictionExpert answerCompare

Apply: classify, rank and explain alcohol properties

Complete the Learn phase to unlock Practice.

✏️ Activities

Classification and Boiling Point Ranking

For each of the following alcohols: (i) classify as 1°, 2°, or 3°; (ii) identify the C-OH carbon and count its carbon neighbours.

CH₃CH₂C(OH)(CH₃)CH₂CH₃
(CH₃)₂CHCH₂OH
CH₃CH(OH)CH(CH₃)CH₃

Now rank all three in order of decreasing boiling point and justify your ranking.

Solubility Predictions and Explanations

A. Would you expect octan-1-ol (C8) to dissolve in water? Explain using IMF reasoning.

B. Compare the water solubility of 2-methylpropan-2-ol (tertiary, 4C) vs butan-1-ol (primary, 4C). Which is more water-soluble and why?

C. A student says "ethanol is soluble in water because it has an -OH group." Identify what this explanation is missing.

📝 Check Your Understanding

Randomised Question Bank

Complete five questions drawn from ten items aligned to alcohol structure, classification, boiling point and solubility.

Short Answer

1. (3 marks) Explain why butan-1-ol has a much higher boiling point than butane (C₄H₁₀), which has a similar molecular mass.

2. (4 marks) Describe and explain the trend in solubility as the carbon chain length increases from C1 to C6. Include reference to the relevant intermolecular forces.

3. (5 marks) Compare and contrast the boiling points and water solubility of butan-1-ol and 2-methylpropan-2-ol. Both have the molecular formula C₄H₁₀O. Explain how molecular shape determines both properties.

Show All Answers

MC reasoning reminders

Classification. Count the carbon atoms directly bonded to the carbon bearing the hydroxyl group.

Boiling point. Compare hydrogen bonding first, then molecular size, surface area and dispersion forces.

Solubility. Balance the polar hydroxyl group against the growing non-polar carbon chain.

Short Answer Model Answers

Q1 (3 marks): Butan-1-ol contains a hydroxyl group (-OH) [1]. The O-H bond is highly polar, making the H a hydrogen bond donor (δ⁺) and the oxygen's lone pairs hydrogen bond acceptors (δ⁻). Butan-1-ol molecules form hydrogen bonds with each other (~20–25 kJ/mol each) [1]. Butane has only C-H and C-C bonds, only weak London dispersion forces act between its molecules. Much more energy is required to overcome the H-bond network in butan-1-ol than to overcome dispersion forces in butane [1].

Q2 (4 marks): As chain length increases from C1 to C6, water solubility decreases [1]. Short-chain alcohols (C1–C3) are fully miscible, the -OH group can form H-bonds with water, and the small non-polar chain creates minimal disruption to water's H-bond network [1]. As chain length increases (C4+), the non-polar chain grows while the -OH remains constant. Accommodating the longer chain within water's H-bond network requires breaking more water-water H-bonds than the single -OH can compensate for [1]. Beyond ~C5, the non-polar chain dominates molecular character, water solubility becomes negligible [1].

Q3 (5 marks): Both have one -OH group, both form hydrogen bonds of similar strength [1]. Butan-1-ol (1°, straight chain) has a higher boiling point (118°C vs 83°C) because its extended chain maximises surface area, enabling stronger London dispersion forces [1]. 2-methylpropan-2-ol (3°) has compact branched structure, minimum surface area, weaker dispersion, lower BP [1]. 2-methylpropan-2-ol is fully water-miscible while butan-1-ol is only partially miscible, despite the same carbon count [1]. The compact spherical shape of 2-methylpropan-2-ol exposes less non-polar surface area to water, causing less disruption to water's H-bond network [1].

How did your thinking change?

Return to your hypothesis about methanol's specific toxicity. Methanol is metabolised to methanal and then methanoic acid. Accumulated formate causes metabolic acidosis and damages the retina and optic nerve. The toxicity is not explained by IMF or chain length alone; it follows from metabolic products.

Review: retrieve the structure-property links

Check what actually stuck

🏆 Review

What is a primary alcohol, and give one example?

Why do alcohols have much higher boiling points than alkanes of similar molecular mass?

Why does the boiling point order for C4H10O isomers follow: primary > secondary > tertiary?

Explain why ethanol is fully miscible in water but hexan-1-ol is practically insoluble.

In alcohol-alcohol hydrogen bonding, identify the donor and acceptor species.