Alcohols in Organic Chemistry

Introduction

Alcohols are a class of organic compounds containing one or more hydroxyl (–OH) groups attached to a saturated carbon atom. They play vital roles in industrial applications, biological systems, and laboratory synthesis.

General Characteristics

• Functional Group: –OH (hydroxyl group)
• General Formula: C_nH_2n+1OH
• Nature: Polar due to the presence of hydrogen bonding

Classification

Based on Number of –OH Groups

• Monohydric: One –OH group (e.g., Ethanol)
• Dihydric: Two –OH groups (e.g., Ethylene glycol)
• Trihydric: Three –OH groups (e.g., Glycerol)

Based on Carbon Attachment

• Primary (1°): –OH attached to a primary carbon
• Secondary (2°): –OH attached to a secondary carbon
• Tertiary (3°): –OH attached to a tertiary carbon

Nomenclature

• Common Names: Methyl alcohol, Isopropyl alcohol
• IUPAC Names: Methanol, Ethanol (CH₃CH₂OH)

Physical Properties

• High boiling points due to strong intermolecular hydrogen bonding
• Lower alcohols are miscible in water
• Volatile and flammable in nature

Preparation Methods

• Hydration of Alkenes: Alkene + H₂O → Alcohol
• Reduction of Aldehydes/Ketones: Using LiAlH₄ or NaBH₄
• Hydrolysis of Alkyl Halides: R–X + H₂O → R–OH
• Fermentation: Glucose → Ethanol + CO₂

Chemical Reactions

• Reaction with Sodium: Produces alkoxide and hydrogen gas
• Dehydration: Forms alkenes using concentrated H₂SO₄
• Oxidation: Converts to aldehydes, ketones, or acids depending on class
• Esterification: Reacts with carboxylic acids to form esters

Applications

• Used in beverages (ethanol)
• Solvent in pharmaceutical and cosmetic industries
• Key ingredient in sanitizers and antiseptics
• Used as a biofuel (ethanol blended with petrol)
• Intermediate in synthesis of dyes, plastics, and drugs

Did you know? Glycerol, a trihydric alcohol, is widely used in skincare products due to its excellent moisturizing properties and is also a byproduct of biodiesel production.

Quick MCQ Quiz

1. Which of the following is a trihydric alcohol?
   a) Methanol
   b) Ethanol
   c) Glycerol ✅
   d) Propanol
2. Primary alcohols on oxidation give:
   a) Ketones
   b) Aldehydes ✅
   c) Carboxylic acids
   d) Esters
3. Which reagent is used for the reduction of aldehydes to alcohols?
   a) H₂SO₄
   b) LiAlH₄ ✅
   c) NaCl
   d) KMnO₄
4. The functional group of alcohol is:
   a) –COOH
   b) –OH ✅
   c) –CHO
   d) –C≡N
5. Ethanol can be produced by:
   a) Fermentation ✅
   b) Combustion
   c) Hydrogenation
   d) Neutralization

True/False

1. Alcohols have lower boiling points than alkanes of similar molecular weight.
   ❌ False – They have higher boiling points due to hydrogen bonding.
2. All alcohols are soluble in water.
   ❌ False – Only lower alcohols are miscible with water.
3. Ethanol is used as a biofuel.
   ✅ True