Alcohols: Structure, Properties, Reactions & Uses

What are Alcohols?

Alcohols are organic compounds containing one or more hydroxyl groups (–OH) attached to a carbon atom. They are classified based on the number of hydroxyl groups and the carbon atom to which they are attached.

Classification of Alcohols

classDiagram
    class Alcohol {
        <<abstract>>
        +OH group attached to carbon
    }
    class Monohydric {
        +One –OH group
        +Examples: Ethanol (C₂H₅OH)
    }
    class Dihydric {
        +Two –OH groups
        +Examples: Ethylene glycol (C₂H₄(OH)₂)
    }
    class Trihydric {
        +Three –OH groups
        +Examples: Glycerol (C₃H₅(OH)₃)
    }
    class Primary {
        +–OH attached to primary carbon (1°)
    }
    class Secondary {
        +–OH attached to secondary carbon (2°)
    }
    class Tertiary {
        +–OH attached to tertiary carbon (3°)
    }
    Alcohol <|-- Monohydric
    Alcohol <|-- Dihydric
    Alcohol <|-- Trihydric
    Monohydric <|-- Primary
    Monohydric <|-- Secondary
    Monohydric <|-- Tertiary

Nomenclature (IUPAC Rules)

1. Find the longest carbon chain containing the –OH group.
2. Number the chain to give the –OH group the lowest possible number.
3. Replace the ‘e’ in the alkane name with ‘ol’.
4. Indicate the position of the –OH group with a number.

Example:

• CH₃–CH₂–OH → Ethanol (not methyl alcohol, which is common name)
• CH₃–CH(OH)–CH₃ → Propan-2-ol

Physical Properties of Alcohols

Alcohols have higher boiling points than alkanes of similar mass due to hydrogen bonding between –OH groups.

Chemical Properties of Alcohols

1. Oxidation Reactions

Alcohols can be oxidized to aldehydes, ketones, or carboxylic acids depending on their class.

flowchart TD
    A["Primary Alcohol"] -->|"Oxidation"| B["Aldehyde"]
    B -->|"Further Oxidation"| C["Carboxylic Acid"]
    D["Secondary Alcohol"] -->|"Oxidation"| E["Ketone"]
    E -->|"No Further Oxidation"| E
    F["Tertiary Alcohol"] -->|"No Oxidation"| F

Example:

• Ethanol (Primary Alcohol) → Ethanal (Aldehyde) → Ethanoic Acid (Carboxylic Acid) Reaction:

CH₃CH₂OH (Ethanol) + [O] → CH₃CHO (Ethanal) + H₂O
CH₃CHO + [O] → CH₃COOH (Ethanoic Acid) + H₂O

2. Dehydration Reaction

Alcohols lose water (H₂O) to form alkenes when heated with concentrated sulfuric acid (H₂SO₄).

Example:

• Ethanol → Ethene

CH₃CH₂OH → CH₂=CH₂ + H₂O

3. Substitution Reaction (with Hydrogen Halides)

Alcohols react with HCl or HBr to form alkyl halides.

Example:

• Ethanol + HCl → Chloroethane

CH₃CH₂OH + HCl → CH₃CH₂Cl + H₂O

Preparation of Alcohols

1. From Alkenes (Hydration)

• Ethene + Water → Ethanol (Industrial method)

  CH₂=CH₂ + H₂O → CH₃CH₂OH
  

2. From Alkyl Halides (Hydrolysis)

• Chloroethane + NaOH → Ethanol

  CH₃CH₂Cl + NaOH → CH₃CH₂OH + NaCl
  

3. From Aldehydes/Ketones (Reduction)

• Ethanal + [H] → Ethanol

  CH₃CHO + 2[H] → CH₃CH₂OH
  

Uses of Alcohols

NEB-Style Solved Examples

Example 1: Nomenclature

Q: Name the following compound: CH₃–CH₂–CH(OH)–CH₃

Solution:

1. Longest chain: 4 carbons (butane)
2. –OH on 2nd carbon → Butan-2-ol

Answer: Butan-2-ol

Example 2: Oxidation Reaction

Q: What is the product when propan-2-ol is oxidized?

Solution:

• Propan-2-ol is a secondary alcohol → oxidized to a ketone.
• Product: Propanone (Acetone)

Answer: Propanone

Example 3: Dehydration

Q: Write the equation for the dehydration of propan-2-ol.

Solution:

CH₃–CH(OH)–CH₃ → CH₃–CH=CH₂ + H₂O

Answer: Propene + Water

NEB Board-Style Questions

Short Answer Questions

1. Why do alcohols have higher boiling points than alkanes?
2. Classify the following alcohols as primary, secondary, or tertiary:
   • CH₃–CH₂–CH₂OH
   • (CH₃)₂CHOH
   • (CH₃)₃COH
3. Write the IUPAC name of CH₃–CH(OH)–CH₂–CH₃.

Long Answer Questions

4. Explain the oxidation of ethanol with a balanced equation.
5. Describe the dehydration reaction of alcohols with an example.
6. How is glycerol used in the soap industry? Explain with a chemical reaction.

Exam Tip

✅ Focus on:

• Nomenclature (IUPAC names for alcohols).
• Oxidation reactions (primary → aldehyde → acid; secondary → ketone).
• Dehydration (alcohol → alkene).
• Preparation methods (from alkenes, alkyl halides, aldehydes).
• Uses of common alcohols (ethanol, methanol, glycerol).

❌ Avoid:

• Confusing primary, secondary, tertiary alcohols.
• Forgetting hydrogen bonding in physical properties.
• Mixing up oxidation vs. reduction reactions.

Good luck! Practice drawing reaction mechanisms and naming compounds to score full marks. 🚀