Chapter 7 - Alcohols, Phenols and Ethers

Alcohols, Phenols and Ethers studies three oxygen-containing families with different structures and chemical behaviour. Alcohols contain a hydroxyl group attached to a saturated carbon, phenols have a hydroxyl group directly attached to an aromatic ring, and ethers contain an oxygen atom between two carbon groups.

The chapter shows how a small structural change can alter acidity, boiling point and reaction mechanism. It also builds a useful network of organic conversions connecting haloalkanes, alkenes, carbonyl compounds, carboxylic acids and aromatic compounds.

Classification, nomenclature and structure

Alcohols are classified as primary, secondary or tertiary according to the carbon bearing the hydroxyl group. They may also be mono-, di- or trihydric according to the number of hydroxyl groups. Phenols can carry one or more hydroxyl groups, while ethers may be symmetrical or unsymmetrical.

Hydrogen bonding strongly affects alcohols and phenols. Their boiling points are generally higher than those of comparable hydrocarbons or ethers. Lower alcohols are soluble in water, but solubility decreases as the non-polar carbon chain grows.

Preparation of alcohols

Alcohols can be prepared by hydration of alkenes, hydrolysis of haloalkanes and reduction of aldehydes, ketones or carboxylic-acid derivatives. Grignard reagents react with suitable carbonyl compounds followed by hydrolysis to produce different classes of alcohol.

For conversion questions, students should identify which carbon atoms are already present and whether the chosen reagent changes the carbon skeleton.

Reactions of alcohols

The oxygen-hydrogen bond allows alcohols to react with active metals and form alkoxides. The carbon-oxygen bond is involved when alcohols form haloalkanes, dehydrate to alkenes or undergo other substitution reactions.

Primary alcohols can be oxidised to aldehydes and then carboxylic acids, while secondary alcohols form ketones. Tertiary alcohols resist ordinary oxidation because the carbon bearing –OH has no hydrogen. Dehydrogenation over heated copper also gives products determined by alcohol class.

Dehydration is affected by temperature, alcohol structure and reaction conditions. When more than one alkene is possible, the more substituted product is commonly favoured under the standard rule.

Methanol and ethanol

Methanol and ethanol have major industrial and everyday uses, but their biological effects differ sharply. Methanol is highly toxic and can cause blindness or death. Ethanol is used as a solvent, fuel component and chemical feedstock; excessive consumption is harmful. Denatured alcohol contains additives that make industrial ethanol unfit for drinking.

Phenol: preparation and acidity

Phenol can be prepared from haloarenes, benzene sulphonic acid, diazonium salts and cumene. Phenol is more acidic than ordinary alcohols because the phenoxide ion is resonance-stabilised. Electron-withdrawing substituents generally increase phenol acidity, while electron-donating groups generally decrease it.

Phenol is less acidic than carboxylic acids because the carboxylate ion distributes negative charge more effectively over two electronegative oxygen atoms.

Reactions of phenol

The hydroxyl group strongly activates the benzene ring toward electrophilic substitution and directs substitution to the ortho and para positions. Phenol undergoes bromination, nitration and sulphonation under suitable conditions.

The Kolbe reaction introduces a carboxyl group, giving salicylic acid after acidification, while the Reimer-Tiemann reaction introduces a formyl group mainly at the ortho position. These named reactions should be learned through the reagent, structural change and product rather than the name alone.

Preparation and reactions of ethers

Ethers can be prepared by intermolecular dehydration of alcohols under controlled conditions and by Williamson ether synthesis. Williamson synthesis is an SN2 reaction between an alkoxide ion and an alkyl halide; primary haloalkanes usually work best because secondary and tertiary substrates may eliminate.

Ethers are comparatively unreactive, but concentrated hydrogen iodide or hydrogen bromide can cleave the carbon-oxygen bond. In aryl-alkyl ethers, cleavage usually occurs at the alkyl-oxygen bond because the aryl-oxygen bond has partial double-bond character and the aromatic carbon does not undergo ordinary SN2 attack.

Anisole undergoes electrophilic substitution mainly at the ortho and para positions because the methoxy group donates electron density by resonance.

What can students study on Teachoo?

• classification and IUPAC naming;

• hydrogen bonding, boiling point and solubility;

• preparation of alcohols;

• reactions and oxidation of primary, secondary and tertiary alcohols;

• dehydration mechanisms and product prediction;

• methanol, ethanol and denatured alcohol;

• preparation and acidity of phenol;

• bromination, nitration, Kolbe and Reimer-Tiemann reactions;

• ether preparation and Williamson synthesis;

• ether cleavage by hydrogen halides;

• anisole reactions; and

• multi-step organic conversions.

Common mistakes to avoid

• Do not use “primary phenol” or “tertiary phenol”; that classification applies to alcohols based on the carbon bearing –OH.

• Do not assume all compounds containing –OH have the same acidity.

• Remember that tertiary alcohols do not give simple aldehyde or ketone products on mild oxidation.

• Williamson synthesis is most reliable with a primary alkyl halide.

• In an aryl-alkyl ether, do not break the aryl-oxygen bond by an assumed SN2 pathway.

• Distinguish the Kolbe reaction of phenol from other reactions also known by the Kolbe name.

Best way to study Alcohols, Phenols and Ethers

Study the three families in separate reaction maps. For alcohols, organise reactions by O–H bond cleavage, C–O bond cleavage, dehydration and oxidation. For phenol, connect resonance to acidity and ring activation before memorising substitutions. For ethers, focus on Williamson synthesis and cleavage rules. Finish by practising conversions that cross between the three families.

Frequently asked questions

Why do alcohols have relatively high boiling points?

Their molecules form intermolecular hydrogen bonds, so more energy is needed to separate them.

Why does alcohol solubility in water decrease with chain length?

The non-polar hydrocarbon portion becomes increasingly dominant while the hydrogen-bonding hydroxyl group remains one small part of the molecule.

Why is phenol more acidic than ethanol?

The phenoxide conjugate base is stabilised by resonance, whereas the ethoxide ion does not have comparable resonance stabilisation.

How can primary, secondary and tertiary alcohols be distinguished conceptually?

Count how many carbon groups are attached to the carbon bearing the hydroxyl group: one for primary, two for secondary and three for tertiary.

What is Williamson ether synthesis?

It is the reaction of an alkoxide or phenoxide ion with a suitable alkyl halide to form an ether, generally through an SN2 mechanism.

Why is anisole ortho/para directing?

The oxygen lone pair donates electron density into the ring by resonance, particularly increasing electron density at the ortho and para positions.

Which reactions of phenol are especially important?

Students should understand bromination, nitration, the Kolbe reaction, the Reimer-Tiemann reaction and reactions showing phenol's acidity.

Learn Alcohols, Phenols and Ethers with Teachoo through connected reaction maps, named-reaction explanations and stepwise NCERT solutions.