Alcohols

Chemistry· 5 min read

1. Nomenclature and Classification★★☆☆☆⏱ 15 min

📘 Definition

Hydroxyl Group

-OH

The defining functional group of alcohols, bonded to a saturated sp³-hybridised carbon atom

Example:

Ethanol ( $C H_{3} C H_{2} O H$ ) has one hydroxyl group

Alcohols are named by replacing the -e suffix of the parent alkane with -ol. The position of the hydroxyl group is indicated by a number before the suffix. For alcohols with multiple hydroxyl groups, use suffixes like -diol or -triol.

📘 Definition

Alcohol Classification

Alcohols are classified by the number of alkyl groups bonded to the carbon that bears the hydroxyl group

Example:

1° = 1 alkyl group, 2° = 2 alkyl groups, 3° = 3 alkyl groups

📐 Worked Example

Classify 2-methylpropan-2-ol as primary, secondary or tertiary

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Step 1: Identify the carbon bonded to the hydroxyl group (the second carbon in the parent chain)
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Step 2: Count how many other carbon atoms are bonded to this carbon:
3
$(CH_3)_3COH$
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The hydroxyl-bearing carbon is bonded to 3 separate methyl groups (3 other carbon atoms)
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Step 3: Conclusion: 3 alkyl groups = tertiary (3°) alcohol

2. Preparation of Alcohols★★☆☆☆⏱ 15 min

There are four common preparation routes for alcohols regularly tested in CIE exams:

Nucleophilic substitution (hydrolysis) of halogenoalkanes with aqueous sodium hydroxide
Electrophilic addition (hydration) of alkenes with steam and acid catalyst (industrial)
Reduction of aldehydes/ketones with $N a B H_{4}$ to 1°/2° alcohols
Anaerobic fermentation of glucose to produce ethanol

📐 Worked Example

State reagents, conditions and the product for preparation of ethanol from ethene

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Step 1: This is an industrial hydration reaction of the alkene double bond
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Step 2: Reagents are ethene and steam. Reaction conditions:
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Temperature = 300°C, Pressure = 60-70 atm, Catalyst = concentrated $H_{3} P O_{4}$ on silica
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$CH_2=CH_2(g) + H_2O(g) \rightleftharpoons CH_3CH_2OH(g)$
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Step 3: Unreacted ethene is recycled to increase overall yield of ethanol

3. Key Reactions of Alcohols★★★☆☆⏱ 25 min

Alcohols undergo four core reaction classes tested in exams: combustion, oxidation, substitution to form halogenoalkanes, and elimination (dehydration) to form alkenes. Oxidation is the most frequently examined, with products dependent on alcohol classification.

📘 Definition

Oxidation of Alcohols

Uses acidified potassium dichromate(VI) ( $K_{2} C r_{2} O_{7}$ ) as oxidising agent. Products depend on alcohol class and reaction conditions

Example:

1° → aldehyde (distillation) → carboxylic acid (reflux); 2° → ketone (reflux); 3° no oxidation

📐 Worked Example

What product forms when butan-1-ol is heated under reflux with excess acidified $K_{2} C r_{2} O_{7}$ ?

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Step 1: Butan-1-ol is a primary alcohol, with the hydroxyl group on the terminal carbon
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Step 2: Heating under reflux with excess oxidising agent causes full oxidation to the carboxylic acid
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$CH_3CH_2CH_2CH_2OH \xrightarrow[reflux]{H^+ / K_2Cr_2O_7} CH_3CH_2CH_2COOH$
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If the product was distilled off as it formed, the intermediate aldehyde (butanal) would be collected instead
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Observation: Orange dichromate(VI) ions reduce to green chromium(III) ions

Other key reactions: Substitution with $P C l_{5}$ , $S O C l_{2}$ or $H C l / Z n C l_{2}$ replaces -OH with -Cl to form a halogenoalkane. Dehydration (elimination) with concentrated acid catalyst eliminates water to form an alkene, following Zaitsev's rule (more substituted alkene = major product).

4. Chemical Identification of Alcohols★★☆☆☆⏱ 10 min

Alcohols can be identified via simple chemical tests, as well as spectroscopic methods covered in other subtopics. The standard test for a hydroxyl group is reaction with sodium metal.

📐 Worked Example

Describe a test using sodium to distinguish between ethanol and ethane

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Step 1: Add a small piece of clean sodium metal to each test sample
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Step 2: Ethanol contains a reactive hydroxyl group that reacts with sodium
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$2CH_3CH_2OH + 2Na \rightarrow 2CH_3CH_2ONa + H_2$
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Result for ethanol: Effervescence of hydrogen gas is observed, and sodium dissolves
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Result for ethane: No reaction occurs, with no visible change, confirming it is not an alcohol

5. Common Pitfalls

Wrong move:

Claiming tertiary alcohols can be oxidized by acidified dichromate

Why:

Tertiary alcohols have no C-H bond on the hydroxyl-bearing carbon, so oxidation cannot occur without breaking the carbon skeleton

Correct move:

State that tertiary alcohols do not react with acidified potassium dichromate(VI) under standard conditions

Wrong move:

Drawing an aldehyde as product when 1° alcohol is refluxed with excess oxidant

Why:

Reflux with excess oxidising agent gives full oxidation to carboxylic acid; aldehyde is only collected if distilled off immediately

Correct move:

Product = carboxylic acid for 1° alcohol under reflux, aldehyde for distillation of 1° alcohol

Wrong move:

Confusing conditions for hydrolysis of halogenoalkanes vs elimination

Why:

Different conditions give completely different products, a common 1-2 mark exam question

Correct move:

Aqueous NaOH = substitution (alcohol product); ethanolic NaOH = elimination (alkene product)

Wrong move:

Claiming all alcohols turn acidified dichromate from orange to green

Why:

Only oxidisable alcohols cause the color change; tertiary alcohols do not react

Correct move:

Only 1° and 2° alcohols give an orange to green color change with acidified dichromate

6. Quick Reference Cheatsheet

Property	Primary (1°)	Secondary (2°)	Tertiary (3°)
OH-C bonded to	1 other C	2 other C	3 other C
K₂Cr₂O₇ (distill)	Aldehyde	Ketone	No reaction
K₂Cr₂O₇ (reflux)	Carboxylic acid	Ketone	No reaction
Reaction with Na	H₂ + sodium alkoxide	H₂ + sodium alkoxide	H₂ + sodium alkoxide
Dehydration product	Follows Zaitsev rule	Follows Zaitsev rule	Follows Zaitsev rule

7. Frequently Asked

How do I distinguish 1°, 2° and 3° alcohols in exams?

Use acidified potassium dichromate(VI): 1° and 2° oxidise (orange → green), 3° does not. 1° gives aldehyde (distillation) or carboxylic acid (reflux), 2° gives ketone only.

Why does dehydration of alcohols follow Zaitsev's rule?

More substituted alkenes are more thermodynamically stable due to hyperconjugation, so they form as the major product under standard reaction conditions.

What's Next

Alcohols are a foundational functional group that connects to almost all other organic chemistry topics in CIE A-Level. The oxidation reactions of alcohols covered here are the first step in learning about carbonyl compounds and carboxylic acids, while substitution reactions of alcohols are the basis for ester formation. Alcohols also feature prominently in multi-step organic synthesis questions, which make up a large share of extended response marks in Papers 2 and 4.