Science Class 12 - Alcohols Notes

Alcohols

Welcome to the chapter on Alcohols for Class 12. In this chapter, you will learn about the structure, classification, preparation, properties, and uses of alcohols. By the end of this chapter, you will understand the chemistry of alcohols and their importance in daily life and industry.

Introduction

Alcohols are organic compounds containing one or more hydroxyl (-OH) groups attached to a saturated carbon atom. The general formula for a simple alcohol is C_nH_2n+1OH.

Classification of Alcohols

• Monohydric alcohols: Contain one -OH group (e.g., ethanol).
• Dihydric alcohols (Glycols): Contain two -OH groups (e.g., ethylene glycol).
• Trihydric alcohols: Contain three -OH groups (e.g., glycerol).

• Primary (1°) alcohol: -OH group attached to a carbon with one alkyl group (e.g., ethanol).
• Secondary (2°) alcohol: -OH group attached to a carbon with two alkyl groups (e.g., isopropanol).
• Tertiary (3°) alcohol: -OH group attached to a carbon with three alkyl groups (e.g., tert-butanol).

Nomenclature

• Common names: Based on the alkyl group + "alcohol" (e.g., methyl alcohol).
• IUPAC names: Replace the "-e" of the alkane with "-ol" (e.g., methanol, ethanol, propan-1-ol).

Preparation of Alcohols

• From alkenes: Hydration of alkenes (e.g., ethene + H₂O → ethanol).
• From alkyl halides: Hydrolysis of alkyl halides with aqueous KOH/NaOH.
• From carbonyl compounds: Reduction of aldehydes and ketones.
• Fermentation: Glucose is converted to ethanol using yeast.

Physical Properties

• Lower alcohols are colorless liquids with a characteristic smell.
• Alcohols are soluble in water due to hydrogen bonding (solubility decreases with increasing chain length).
• They have higher boiling points than alkanes of similar molecular mass.

Chemical Properties

• Oxidation: Primary alcohols → aldehydes → acids; Secondary alcohols → ketones; Tertiary alcohols resist oxidation.
• Dehydration: Alcohols lose water to form alkenes (with conc. H₂SO₄).
• Reaction with sodium: Alcohols react with sodium to form sodium alkoxide and hydrogen gas.
• Esterification: Alcohols react with acids to form esters and water.
• Reaction with halogen acids: Alcohols react with HX to form alkyl halides.

Uses of Alcohols

• Ethanol is used in alcoholic beverages, as a solvent, and as a fuel (bioethanol).
• Methanol is used as an industrial solvent and antifreeze.
• Glycerol is used in medicines, cosmetics, and food products.
• Alcohols are used in the manufacture of perfumes, dyes, and disinfectants.

Practice Questions

1. Write the IUPAC name of CH₃CH₂OH.
2. How is ethanol prepared from glucose?
3. What happens when ethanol is heated with concentrated H₂SO₄?
4. Name one use of methanol.
5. What is the product when a primary alcohol is oxidized?

Challenge Yourself

• Explain why alcohols have higher boiling points than alkanes of similar mass.
• Describe the difference between primary, secondary, and tertiary alcohols with examples.

Did You Know?

• Ethanol is also called "grain alcohol" because it is made by fermenting grains.
• Methanol is poisonous and should never be consumed.

Glossary

• Alcohol: An organic compound with one or more -OH groups attached to carbon.
• Fermentation: The process of converting sugar to alcohol using yeast.
• Esterification: The reaction of an alcohol with an acid to form an ester.
• Oxidation: A chemical reaction involving the addition of oxygen or removal of hydrogen.

Answers to Practice Questions

1. Ethanol
2. By fermentation: C₆H₁₂O₆ (glucose) → 2 C₂H₅OH + 2 CO₂ (with yeast)
3. It gets dehydrated to form ethene (C₂H₄).
4. As an industrial solvent (or antifreeze).
5. It is first oxidized to an aldehyde, then to a carboxylic acid.

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Understanding alcohols is important for chemistry and many real-life applications!