Alcohols

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recall the reactions by which alcohols can be produced, including:

    • Electrophilic addition of steam to an alkene
    • Reaction of alkenes with cold dilute acidified potassium manganate(VII) to form a diol
    • Substitution of a halogenoalkane using NaOH(aq) and heat
    • Reduction of an aldehyde or ketone using NaBH₄ or LiAlH₄
    • Reduction of a carboxylic acid using LiAlH₄
    • Hydrolysis of an ester using dilute acid or dilute alkali and heat

  2. Describe the reactions of alcohols, including:

    • Combustion in oxygen
    • Substitution to form halogenoalkanes
    • Reaction with sodium metal
    • Oxidation with acidified dichromate or manganate(VII)
    • Dehydration to form alkenes
    • Formation of esters

  3. Classify alcohols as primary, secondary, and tertiary, and state characteristic distinguishing reactions

  4. Deduce the presence of a CH₃CH(OH)– group using the iodoform test

  5. Explain the acidity of alcohols compared with water

What Are Alcohols?

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Alcohols are organic compounds that contain at least one hydroxyl group (–OH). This –OH group is the functional group that gives alcohols their special chemical properties.

The general formula for simple alcohols is: CₙH₂ₙ₊₁OH

Examples of simple alcohols:

  • Methanol: CH₃OH
  • Ethanol: CH₃CH₂OH (also written as C₂H₅OH)
  • Propan-1-ol: CH₃CH₂CH₂OH
  • Propan-2-ol: CH₃CH(OH)CH₃

Classification of Alcohols

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Alcohols are classified based on how many carbon atoms are attached to the carbon bearing the –OH group.

Primary Alcohols (1°)

In a primary alcohol, the carbon atom attached to the –OH group is bonded to one other carbon atom (and two hydrogen atoms). Think of it as having one carbon "neighbor."

Examples:

  • Ethanol: CH₃CH₂OH
  • Propan-1-ol: CH₃CH₂CH₂OH

Exception: Methanol (CH₃OH) is also considered primary even though the carbon with –OH is only bonded to hydrogen atoms, not other carbons.

Secondary Alcohols (2°)

In a secondary alcohol, the carbon atom attached to the –OH group is bonded to two other carbon atoms (and one hydrogen atom). It has two carbon "neighbors."

Examples:

  • Propan-2-ol: CH₃CH(OH)CH₃
  • Butan-2-ol: CH₃CH₂CH(OH)CH₃

Tertiary Alcohols (3°)

In a tertiary alcohol, the carbon atom attached to the –OH group is bonded to three other carbon atoms (and no hydrogen atoms). It has three carbon "neighbors."

Examples:

  • 2-methylpropan-2-ol: (CH₃)₃COH

Alcohols with Multiple –OH Groups

Some molecules contain more than one hydroxyl group:

  • Diols have two –OH groups (example: ethane-1,2-diol)
  • Triols have three –OH groups (example: 2-methylbutan-1,2,3-triol)

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