Alcohols: Properties, Reactions and Uses

What you'll learn

Alcohols form a crucial organic chemistry topic in the CXC CSEC Chemistry syllabus, tested regularly through structure identification, property comparisons, and reaction prediction questions. This revision guide covers the homologous series of alcohols, their distinctive physical and chemical properties, key reactions including combustion and oxidation, and their important industrial and domestic uses throughout the Caribbean region.

Key terms and definitions

Alcohol — an organic compound containing one or more hydroxyl (-OH) functional groups attached to a saturated carbon atom.

Homologous series — a family of organic compounds with the same general formula, similar chemical properties, and successive members differing by CH₂.

Functional group — a specific group of atoms within a molecule that determines the characteristic chemical reactions of that compound; in alcohols, this is the hydroxyl group (-OH).

Primary alcohol — an alcohol where the carbon atom bearing the -OH group is bonded to only one other carbon atom (or no other carbon atoms, as in methanol).

Secondary alcohol — an alcohol where the carbon atom bearing the -OH group is bonded to two other carbon atoms.

Tertiary alcohol — an alcohol where the carbon atom bearing the -OH group is bonded to three other carbon atoms.

Oxidation — a chemical reaction involving the loss of electrons or gain of oxygen; alcohols can be oxidized to aldehydes, ketones, or carboxylic acids depending on their structure.

Fermentation — the anaerobic biological process by which sugars are converted to ethanol and carbon dioxide using yeast enzymes.

Core concepts

Structure and nomenclature of alcohols

The general formula for alcohols is CₙH₂ₙ₊₁OH or CₙH₂ₙ₊₂O. The hydroxyl functional group (-OH) defines this homologous series and determines its characteristic properties.

The first four members of the alcohol homologous series are:

• Methanol: CH₃OH (CH₄O)
• Ethanol: C₂H₅OH (C₂H₆O)
• Propanol: C₃H₇OH (C₃H₈O)
• Butanol: C₄H₉OH (C₄H₁₀O)

Alcohols are named by replacing the final '-e' in the corresponding alkane name with '-ol'. The position of the -OH group is indicated by a number when necessary (e.g., propan-1-ol and propan-2-ol are structural isomers).

Classification by structure:

Primary alcohols have the -OH group on a carbon atom attached to at most one other carbon. Examples include methanol, ethanol, and propan-1-ol. Their general structure can be represented as RCH₂OH.

Secondary alcohols have the -OH group on a carbon attached to two other carbons, with general structure R₂CHOH. Propan-2-ol is a common example.

Tertiary alcohols have the -OH group on a carbon attached to three other carbons (R₃COH). 2-methylpropan-2-ol is an example found at CSEC level.

Physical properties of alcohols

Boiling points:

Alcohols have significantly higher boiling points than alkanes of similar molecular mass due to hydrogen bonding between alcohol molecules. The oxygen atom in the -OH group is highly electronegative, creating a polar O-H bond. The partially positive hydrogen can form intermolecular hydrogen bonds with the partially negative oxygen of another alcohol molecule.

For example:

• Ethanol (C₂H₅OH): boiling point 78°C
• Propane (C₃H₈): boiling point -42°C (despite higher molecular mass)

Within the alcohol series, boiling point increases with molecular size due to increasing van der Waals forces between larger molecules.

Solubility:

Lower alcohols (methanol, ethanol, propanol) are completely miscible with water because they can form hydrogen bonds with water molecules. The hydroxyl group is described as hydrophilic (water-loving).

As the carbon chain length increases, alcohols become less soluble in water. The hydrocarbon portion is hydrophobic (water-repelling), and larger molecules have a greater proportion of non-polar hydrocarbon chain relative to the polar -OH group. Butanol and higher alcohols show limited water solubility.

State at room temperature:

Methanol through dodecanol are liquids at room temperature due to hydrogen bonding. Higher alcohols (above C12) are waxy solids.

Chemical reactions of alcohols

Combustion:

Alcohols burn in oxygen to produce carbon dioxide and water, releasing energy. This makes them valuable fuels. The combustion is exothermic.

Complete combustion of ethanol: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

Incomplete combustion occurs with insufficient oxygen, producing carbon monoxide or carbon (soot): C₂H₅OH(l) + 2O₂(g) → 2CO(g) + 3H₂O(l)

Methanol combustion: 2CH₃OH(l) + 3O₂(g) → 2CO₂(g) + 4H₂O(l)

The flame colour is typically blue, and alcohols burn cleanly compared to alkanes of similar size.

Oxidation reactions:

The oxidation of alcohols is a key reaction tested in CXC CSEC Chemistry examinations. The products depend on whether the alcohol is primary, secondary, or tertiary.

Primary alcohols can be oxidized in two stages:

1. First to an aldehyde: RCH₂OH → RCHO + H₂O
2. Then to a carboxylic acid: RCHO → RCOOH

For ethanol:

• Stage 1: C₂H₅OH → CH₃CHO (ethanal) + H₂O
• Stage 2: CH₃CHO → CH₃COOH (ethanoic acid)

Common oxidizing agents include:

• Acidified potassium dichromate(VI) solution (K₂Cr₂O₇/H⁺)
• Acidified potassium manganate(VII) solution (KMnO₄/H⁺)

The orange potassium dichromate turns green (Cr³⁺ ions) during oxidation, providing a visual test. Purple potassium manganate(VII) becomes colourless (Mn²⁺ ions).

Secondary alcohols are oxidized to ketones only: R₂CHOH → R₂CO + H₂O

For propan-2-ol: (CH₃)₂CHOH → (CH₃)₂CO (propanone) + H₂O

Ketones resist further oxidation under normal conditions.

Tertiary alcohols are resistant to oxidation by common oxidizing agents because the carbon bearing the -OH group has no hydrogen atoms attached to it that can be removed.

Reaction with sodium metal:

Alcohols react with sodium metal to produce a sodium alkoxide salt and hydrogen gas. This reaction is less vigorous than the reaction of sodium with water.

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂(g) (ethanol + sodium → sodium ethoxide + hydrogen)

2CH₃OH + 2Na → 2CH₃ONa + H₂(g)

The hydrogen gas produced can be tested with a lighted splint, which gives a 'pop' sound.

Esterification:

Alcohols react with carboxylic acids in the presence of a concentrated sulfuric acid catalyst to form esters and water. This is a condensation reaction.

C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O (ethanol + ethanoic acid ⇌ ethyl ethanoate + water)

The reaction is reversible and reaches equilibrium. Esters have distinctive fruity odours and are used in flavourings and perfumes.

Production of ethanol

Fermentation:

In the Caribbean, fermentation is particularly important for rum production. Yeast enzymes convert glucose from sugar cane or molasses into ethanol and carbon dioxide under anaerobic conditions.

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g) (glucose → ethanol + carbon dioxide)

Conditions for fermentation:

• Temperature: 25-37°C (optimum around 30°C)
• Absence of oxygen (anaerobic)
• Enzyme: zymase in yeast
• pH: slightly acidic (around pH 4-5)

Above 15-18% alcohol concentration, the ethanol kills the yeast, stopping fermentation. Distillation is required for higher concentrations.

Catalytic hydration of ethene:

Industrial ethanol production uses the hydration of ethene derived from crude oil:

C₂H₄(g) + H₂O(g) → C₂H₅OH(g)

Conditions:

• Temperature: 300°C
• Pressure: 60-70 atmospheres
• Catalyst: concentrated phosphoric acid (H₃PO₄) on silica

This process produces pure ethanol directly and is faster than fermentation, but requires non-renewable crude oil as the ethene source.

Uses of alcohols

Ethanol:

• Alcoholic beverages: rum production in Trinidad, Barbados, Jamaica, and other Caribbean territories; beers, wines
• Solvent: in perfumes, aftershaves, paints, varnishes, and ink manufacture
• Fuel: gasohol (ethanol-petrol mixture) used in vehicles; burns cleanly with less pollution than pure petrol
• Disinfectant: 70% ethanol solutions used as antiseptics in hospitals and homes
• Raw material: for producing ethanoic acid, esters, and other organic chemicals

Methanol:

• Solvent: in industrial processes and paint strippers
• Fuel: racing car fuel; converted to biodiesel
• Raw material: manufacturing formaldehyde for plastics and resins
• Antifreeze: in windscreen washer fluids

Warning: Methanol is highly toxic and can cause blindness or death if consumed. Cases of methanol poisoning from adulterated alcoholic drinks have occurred in the Caribbean region.

Propan-2-ol (isopropanol):

• Rubbing alcohol: disinfectant for skin before injections
• Solvent: in hand sanitizers and cosmetics
• Cleaning agent: for electronic equipment and optical lenses

Worked examples

Example 1: Structure and classification

Question: Draw the structural formula of propan-2-ol and state whether it is a primary, secondary, or tertiary alcohol. Explain your answer. [4 marks]

Solution:

Structural formula:

    OH
    |
CH₃-CH-CH₃

or CH₃CH(OH)CH₃ [1 mark]

Classification: Secondary alcohol [1 mark]

Explanation: The carbon atom bearing the -OH group is bonded to two other carbon atoms [1 mark]. This makes it a secondary alcohol by definition [1 mark].

Example 2: Oxidation reactions

Question: A student oxidizes ethanol using acidified potassium dichromate(VI) solution.

(a) Name the organic product formed in the first stage of oxidation. [1 mark]

(b) Write the formula of this product. [1 mark]

(c) Describe the colour change observed. [2 marks]

(d) If the oxidation continues, what is the final organic product? [1 mark]

Solution:

(a) Ethanal [1 mark]

(b) CH₃CHO [1 mark]

(c) The acidified potassium dichromate changes from orange [1 mark] to green [1 mark] due to reduction of Cr₂O₇²⁻ (orange) to Cr³⁺ (green).

(d) Ethanoic acid [1 mark] (formula CH₃COOH or C₂H₄O₂)

Example 3: Fermentation calculation

Question: A rum distillery in Barbados ferments 900 kg of glucose to produce ethanol.

(a) Write the balanced equation for the fermentation of glucose. [2 marks]

(b) Calculate the maximum theoretical mass of ethanol that can be produced. [3 marks] (Relative atomic masses: C = 12, H = 1, O = 16)

Solution:

(a) C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g) [2 marks for correct equation with state symbols; 1 mark if missing state symbols or balancing error]

(b) Molar mass of glucose (C₆H₁₂O₆) = (6 × 12) + (12 × 1) + (6 × 16) = 180 g/mol Molar mass of ethanol (C₂H₅OH) = (2 × 12) + (6 × 1) + 16 = 46 g/mol

From equation: 1 mole glucose produces 2 moles ethanol [1 mark] 180 g glucose produces 2 × 46 = 92 g ethanol

900 kg glucose produces (92 × 900) ÷ 180 = 460 kg ethanol [1 mark for calculation]

Maximum theoretical mass = 460 kg [1 mark]

Common mistakes and how to avoid them

• Mistake: Confusing the general formula of alcohols with alkanes or alkenes (writing CₙH₂ₙ instead of CₙH₂ₙ₊₂O). Correction: Remember that alcohols contain oxygen and have the formula CₙH₂ₙ₊₁OH or CₙH₂ₙ₊₂O. The +2 in the hydrogen count reflects the saturated carbon chain with an added -OH group.

• Mistake: Stating that all alcohols oxidize to carboxylic acids. Correction: Only primary alcohols can be oxidized to carboxylic acids (via aldehyde intermediate). Secondary alcohols oxidize to ketones only. Tertiary alcohols resist oxidation by common oxidizing agents.

• Mistake: Writing incomplete combustion equations that don't balance or showing carbon dioxide as the product when insufficient oxygen is specified. Correction: In complete combustion, products are always CO₂ and H₂O. In incomplete combustion, products include CO and/or C (soot) plus H₂O. Always balance equations systematically and check oxygen atom counts.

• Mistake: Confusing fermentation conditions, particularly stating it requires oxygen or occurs at high temperatures. Correction: Fermentation is anaerobic (no oxygen) and occurs at moderate temperatures (25-37°C). High temperatures denature yeast enzymes. Use the mnemonic: "Yeast needs warmth without air."

• Mistake: Incorrectly explaining alcohol solubility patterns, stating that all alcohols are soluble in water or that larger alcohols are more soluble. Correction: Only lower alcohols (C1-C3) are completely miscible with water due to hydrogen bonding. As chain length increases, the hydrophobic hydrocarbon portion dominates, reducing water solubility. Butanol and higher alcohols show limited solubility.

• Mistake: Drawing structural formulae showing the -OH group bonded to two carbons or failing to show it attached to a carbon at all. Correction: The hydroxyl group consists of O-H bonded together, with the oxygen bonded to one carbon atom only. Always draw C-O-H in that sequence with correct bonding.

Exam technique for "Alcohols: Properties, Reactions and Uses"

• Command word recognition: "State" requires a short answer (1-2 words) for naming compounds or classifications. "Explain" requires you to give reasons why something occurs, linking cause and effect for full marks. "Describe and explain" questions about colour changes in oxidation must include both the colour observation (describe) and the species responsible (explain).

• Balanced equations: When asked to write equations for combustion, oxidation, or fermentation, always balance systematically. Show state symbols (s, l, g, aq) when specifically requested or when worth a mark. Check oxygen and hydrogen atoms carefully—these are where most balancing errors occur in alcohol reactions.

• Classification questions: When classifying alcohols as primary, secondary, or tertiary, the mark scheme awards points for both the correct classification and a clear explanation referencing the number of carbon atoms bonded to the carbon bearing the -OH group. Drawing the structure first helps avoid errors.

• Comparative property questions: Questions comparing boiling points or solubility of alcohols with alkanes test understanding of hydrogen bonding. Award-winning answers mention "hydrogen bonding between alcohol molecules" specifically, not just "stronger intermolecular forces." Reference the polar O-H bond and relate molecular size to property trends.

Quick revision summary

Alcohols form a homologous series with general formula CₙH₂ₙ₊₁OH, characterized by the hydroxyl (-OH) functional group. They display higher boiling points than comparable alkanes due to hydrogen bonding, with lower members being water-soluble. Classified as primary, secondary, or tertiary based on the carbon bearing -OH, alcohols undergo combustion producing CO₂ and H₂O, and oxidation producing aldehydes/acids (primary), ketones (secondary), or no reaction (tertiary). Ethanol is produced industrially via catalytic hydration of ethene or biologically through fermentation of glucose. Major uses include beverages (rum in Caribbean distilleries), fuels (gasohol), solvents, and disinfectants, making alcohols economically significant organic compounds.