Alcohols: Classification and Properties

Alcohols are one of the most important homologous series in organic chemistry. They are the gateway to many other functional groups — you can oxidise them to carbonyls and carboxylic acids, dehydrate them to alkenes, substitute them to haloalkanes, and esterify them with carboxylic acids. Understanding their physical properties and classification is therefore essential before we tackle their reactions.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 4.2.1: classification of alcohols as primary, secondary or tertiary, and the effect of hydrogen bonding on their physical properties.

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1. The Alcohol Functional Group

An alcohol is a compound containing a hydroxyl group (–OH) bonded directly to a saturated (sp³) carbon atom. The general formula for a saturated aliphatic alcohol is:

$C_nH_{2n+1}OH$Cn​H2n+1​OH

or equivalently $C_nH_{2n+2}O$Cn​H2n+2​O.

Key Definition — Alcohol: An organic compound containing the hydroxyl functional group (–OH) bonded to a carbon atom that is itself saturated.

Note that phenol (C₆H₅OH) is not classified as a simple alcohol in OCR A-Level terms — the –OH is attached to an aromatic sp² carbon. This gives phenol markedly different properties and is treated separately at A2.

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2. Classification: Primary, Secondary, Tertiary

Alcohols are classified according to the number of carbon atoms bonded to the carbon carrying the –OH group.

Class	Carbons on C–OH carbon	Structural cue
Primary (1°)	0 or 1	R–CH₂–OH
Secondary (2°)	2	R₂CH–OH
Tertiary (3°)	3	R₃C–OH

graph TD
  A[Alcohol R-OH] --> B{How many C atoms bonded to the C-OH carbon?}
  B -->|0 or 1| C[Primary 1 degrees<br/>e.g. methanol, ethanol, propan-1-ol]
  B -->|2| D[Secondary 2 degrees<br/>e.g. propan-2-ol, butan-2-ol]
  B -->|3| E[Tertiary 3 degrees<br/>e.g. 2-methylpropan-2-ol]

Examples

Alcohol	Structure	Class
Methanol	CH₃–OH	Primary
Ethanol	CH₃–CH₂–OH	Primary
Propan-1-ol	CH₃–CH₂–CH₂–OH	Primary
Propan-2-ol	(CH₃)₂CH–OH	Secondary
Butan-2-ol	CH₃–CH(OH)–CH₂–CH₃	Secondary
2-Methylpropan-2-ol	(CH₃)₃C–OH	Tertiary

Tip: Methanol is officially counted as primary even though there are zero carbons on the C–OH carbon. Some textbooks call it "special"; OCR mark schemes accept primary.

Why does the classification matter?

The class of an alcohol controls how it reacts with oxidising agents. As we shall see in Lesson 2, primary alcohols oxidise to aldehydes then carboxylic acids, secondary alcohols oxidise to ketones, and tertiary alcohols resist oxidation under normal lab conditions. Identifying the class is therefore the first step in predicting the product of an oxidation.

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3. Physical Properties — The Role of Hydrogen Bonding

The hydroxyl group is both a hydrogen bond donor (because the O–H hydrogen is highly deshielded) and a hydrogen bond acceptor (because oxygen has two lone pairs). This gives alcohols much stronger intermolecular forces than alkanes of comparable molecular mass.

3.1 Boiling Points

The boiling point of an alcohol reflects the energy required to break the intermolecular forces holding the molecules together in the liquid. Alcohols have:

• London forces (induced dipole–induced dipole): present in all molecules, strength scales with molecular size.
• Permanent dipole–dipole forces: because the C–O and O–H bonds are polar.
• Hydrogen bonds: the dominant interaction, arising because O–H is polar enough for the H to interact strongly with the lone pair on a neighbouring O.

Compare the boiling points of some short molecules of similar molecular mass:

Molecule	Mᵣ	Boiling point (°C)	Strongest IMF
Ethane, C₂H₆	30	–89	London
Methanal, HCHO	30	–19	Permanent dipole
Methanol, CH₃OH	32	65	Hydrogen bonding
Propane, C₃H₈	44	–42	London
Ethanal, CH₃CHO	44	20	Permanent dipole
Ethanol, C₂H₅OH	46	78	Hydrogen bonding

Ethanol boils ~167 °C higher than propane despite being almost the same size. That difference is entirely down to hydrogen bonding.

graph LR
  A[Ethane -89 C<br/>London only] --> B[Methanal -19 C<br/>Dipole-dipole]
  B --> C[Methanol +65 C<br/>Hydrogen bonds]

3.2 Water Solubility

Short-chain alcohols (methanol, ethanol, propan-1-ol, propan-2-ol) are miscible with water in all proportions because their –OH group can hydrogen bond directly with water molecules. As the carbon chain lengthens, the hydrophobic alkyl "tail" dominates and solubility drops sharply.

Alcohol	Solubility in water at 20 °C
Methanol	Miscible
Ethanol	Miscible
Propan-1-ol	Miscible
Butan-1-ol	~8 g per 100 cm³
Pentan-1-ol	~2 g per 100 cm³
Hexan-1-ol	~0.6 g per 100 cm³
Decan-1-ol	Essentially insoluble

graph TD
  A[Small alcohol e.g. ethanol] --> B[-OH group dominates]
  B --> C[H-bonds with water easily]
  C --> D[Fully miscible]
  E[Large alcohol e.g. decan-1-ol] --> F[Alkyl chain dominates]
  F --> G[Cannot disrupt water H-bonding<br/>enough to dissolve]
  G --> H[Insoluble]

3.3 Volatility, Viscosity and Density

• Volatility decreases as chain length increases, for the same reason boiling point rises.
• Viscosity increases with chain length — longer chains tangle and hydrogen bond more extensively (glycerol, propane-1,2,3-triol, is very viscous because it has three –OH groups).
• Density of liquid alcohols is lower than water (~0.79 g cm⁻³ for ethanol) — useful to remember when thinking about fractional distillation or density-based separation.

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4. Worked Example — Predicting Boiling Points

Place the following molecules in order of increasing boiling point, and justify your ranking: propane, propan-1-ol, propan-2-ol, propanal, propane-1,2,3-triol (glycerol).

Answer:

1. Propane (–42 °C) — only London forces.
2. Propanal (48 °C) — London + permanent dipole from C=O, no hydrogen bonds.
3. Propan-2-ol (82 °C) — hydrogen bonding, but secondary –OH is slightly shielded by the two methyl groups.
4. Propan-1-ol (97 °C) — hydrogen bonding, primary –OH more accessible.
5. Glycerol (290 °C) — three –OH groups per molecule, extensive three-dimensional hydrogen bond network.

Notice the dramatic jump from propanal to the alcohols (~50 °C per hydrogen bond), and an even bigger jump from propan-1-ol to glycerol because you gain three hydrogen bonds per molecule.

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5. Worked Example — Identifying the Class

Classify each alcohol as primary, secondary or tertiary:

(a) 2-Methylbutan-2-ol, (CH₃)₂C(OH)–CH₂CH₃ (b) 3-Methylbutan-1-ol, (CH₃)₂CH–CH₂–CH₂OH (c) Cyclohexanol

Answer:

(a) The C–OH carbon is bonded to two methyl groups and one ethyl group — three carbons — so this is tertiary. (b) The C–OH carbon is bonded to a single CH₂ group — one carbon — so this is primary. (c) In cyclohexanol the C–OH carbon is part of the ring and is bonded to two ring carbons — two carbons — so this is secondary.

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Exam Tips

• When asked why alcohols have higher boiling points than alkanes, the mark scheme wants hydrogen bonding stated and contrasted with the alkane's weaker London forces.
• Always say hydrogen bonds are broken, not "the O–H bond is broken" — the covalent bond inside the molecule is unchanged during boiling.
• To justify decreasing solubility with chain length, mention the non-polar alkyl tail cannot participate in hydrogen bonding with water.
• The words miscible and soluble mean different things — two liquids are miscible, a solid dissolves to form a solution.

Common Exam Mistakes

• Confusing hydrogen bonding (an intermolecular force between an H attached to N, O or F and a lone pair on another molecule) with a covalent O–H bond.
• Writing that "hydrogen bonds are broken when an alcohol evaporates" without saying this is why the boiling point is high.
• Classifying methanol as "not an alcohol" or as "zero-degree".
• Claiming tertiary alcohols have the highest boiling points — steric shielding of the –OH actually tends to lower the boiling point relative to their primary isomers.
• Saying alcohols are soluble "because of their polarity" — the specific reason is hydrogen bonding with water.

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Reference: OCR A-Level Chemistry A (H432), Module 4 — Core Organic Chemistry, section 4.2.1: alcohols as primary, secondary and tertiary; physical properties of alcohols including the influence of hydrogen bonding on boiling point and water solubility.