Chapter 11: Alcohols, Phenols and Ethers – Chemistry 2nd Year Notes

This chapter covers Alcohols, Phenols and Ethers from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Alcohols (R-OH) have a hydroxyl group attached to a carbon chain, phenols have a hydroxyl group attached directly to a benzene ring, and ethers (R-O-R) have an oxygen joining two carbon groups. Ethanol is among the most useful of all organic compounds.

Here you will learn how alcohols are classified and prepared, their reactions, how to distinguish primary, secondary and tertiary alcohols, why phenol is acidic, and how ethers are made.

Learning Objectives

• Define and classify alcohols as primary, secondary and tertiary.
• Describe the preparation of alcohols, including fermentation.
• Describe the important reactions of alcohols.
• Distinguish primary, secondary and tertiary alcohols by oxidation and the Lucas test.
• Explain why phenol is acidic and describe its reactions.
• Describe the preparation and properties of ethers.

Key Concepts

Alcohols and Their Classification

An alcohol contains the hydroxyl functional group. Alcohols are classified by the carbon that carries the hydroxyl group: in a primary alcohol that carbon is joined to one other carbon, in a secondary alcohol to two, and in a tertiary alcohol to three. This classification controls how the alcohol behaves on oxidation and in the Lucas test.

Preparation and Fermentation

Alcohols are prepared by hydration of alkenes, reduction of aldehydes and ketones, and hydrolysis of alkyl halides. Ethanol is also made on a large scale by fermentation, in which enzymes in yeast convert sugars into ethanol and carbon dioxide: glucose gives 2 ethanol plus 2 carbon dioxide.

Reactions of Alcohols

With sodium, alcohols release hydrogen and form alkoxides. With concentrated sulphuric acid they undergo dehydration, at about 170 degrees to give an alkene and at about 140 degrees to give an ether. With hydrogen halides or phosphorus pentachloride the hydroxyl is replaced by a halogen. With carboxylic acids they form sweet-smelling esters. On oxidation a primary alcohol gives an aldehyde and then a carboxylic acid, a secondary alcohol gives a ketone, and a tertiary alcohol is not easily oxidised. The Lucas test (with zinc chloride and concentrated hydrochloric acid) also distinguishes them: tertiary react at once, secondary within minutes, and primary do not react at room temperature.

Phenols and Ethers

In phenol the hydroxyl is attached directly to the benzene ring. Unlike ordinary alcohols, phenol is weakly acidic: it reacts with sodium hydroxide to form sodium phenoxide and water, because the ring stabilises the phenoxide ion by delocalising its negative charge. Ethers (R-O-R) are fairly unreactive and are used as solvents; a common laboratory method of making them is the Williamson synthesis, in which a sodium alkoxide reacts with an alkyl halide.

Important Definitions

Formulas & Rules

Item	Fact
Classes	primary, secondary, tertiary by the carbon bearing the hydroxyl
Dehydration	conc. H2SO4 at 170 C gives an alkene; at 140 C gives an ether
Oxidation	primary gives aldehyde then acid; secondary gives ketone; tertiary resists
Lucas test	tertiary immediate, secondary in minutes, primary no reaction cold
Fermentation	glucose gives 2 ethanol plus 2 carbon dioxide

Diagrams & Illustrations

Reactions of alcohols: a map around R-OH showing reaction with sodium, dehydration to alkene or ether, replacement by halide, esterification and oxidation.

Oxidation distinguishes alcohols: a chart showing primary alcohol to aldehyde to carboxylic acid, secondary alcohol to ketone, and tertiary alcohol not easily oxidised.

Solved Examples & Numericals

Example 1: Oxidation

What is formed when ethanol (a primary alcohol) is oxidised? It is first oxidised to ethanal (an aldehyde) and then to ethanoic acid.

Example 2: Distinguishing alcohols

How can you distinguish a tertiary alcohol from a primary alcohol? Use the Lucas test; a tertiary alcohol gives immediate turbidity, while a primary alcohol does not react at room temperature.

Example 3: Acidity of phenol

Why is phenol acidic while ethanol is neutral? When phenol loses a proton, the benzene ring delocalises the negative charge of the phenoxide ion, stabilising it; ethanol alkoxide has no such stabilisation.

Short Questions & Answers

Long Questions & Answers

Q1: Describe the important reactions of alcohols.

The hydroxyl group makes alcohols react in several ways. With active metals such as sodium they release hydrogen and form alkoxides, showing that the oxygen to hydrogen bond is weakly acidic. With concentrated sulphuric acid they undergo dehydration: at about 170 degrees an alcohol loses water to form an alkene, while at about 140 degrees two molecules combine with loss of water to give an ether. With hydrogen halides or phosphorus halides the hydroxyl is replaced by a halogen to give an alkyl halide. With carboxylic acids they form sweet-smelling esters, the reverse being hydrolysis. Finally, alcohols can be oxidised: a primary alcohol gives first an aldehyde and then a carboxylic acid, a secondary alcohol gives a ketone, and a tertiary alcohol resists oxidation.

Q2: How are primary, secondary and tertiary alcohols distinguished?

They can be told apart by two methods. The first is oxidation: with acidified potassium dichromate a primary alcohol is oxidised first to an aldehyde and then to a carboxylic acid, a secondary alcohol is oxidised to a ketone, and a tertiary alcohol is not oxidised because the carbon carrying the hydroxyl has no hydrogen to remove. The second is the Lucas test, in which the alcohol is shaken with the Lucas reagent (anhydrous zinc chloride in concentrated hydrochloric acid): a tertiary alcohol produces cloudiness immediately, a secondary alcohol within about five minutes, and a primary alcohol not at room temperature. Together these tests reliably classify an unknown alcohol.

Q3: Compare alcohols and phenols.

Alcohols and phenols both contain the hydroxyl group but differ in where it is attached and how it behaves. In an alcohol the hydroxyl is joined to a saturated carbon of a chain, whereas in a phenol it is joined directly to a benzene ring. This gives them different acidity: ordinary alcohols are essentially neutral and do not react with sodium hydroxide, while phenol is weakly acidic and reacts with sodium hydroxide to form sodium phenoxide, because the benzene ring delocalises and stabilises the negative charge of the phenoxide ion. Phenols also undergo electrophilic substitution on the ring more readily than benzene, and give a characteristic violet colour with neutral ferric chloride, a test that alcohols do not give.

MCQs with Answers

Quick Revision Summary

• Alcohol R-OH; classify primary, secondary, tertiary by the carbon bearing the hydroxyl.
• Prepared by hydration of alkenes, reduction and fermentation.
• Reactions: with sodium, dehydration, replacement by halide, esterification, oxidation.
• Oxidation: primary gives aldehyde then acid; secondary gives ketone; tertiary resists.
• Lucas test: tertiary immediate, secondary minutes, primary no reaction cold.
• Phenol is weakly acidic; ethers are made by the Williamson synthesis.

Exam Tips

• Draw the alcohol-reactions map and label the conditions (140 versus 170 degrees for sulphuric acid).
• Learn oxidation products for each class.
• Lucas test order (tertiary greater than secondary greater than primary) is a favourite short question.
• Explain phenol acidity via delocalisation of the phenoxide charge.
• Remember the fermentation equation and the enzyme in yeast.