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3.5.1Physical Chemistry

Classifying Alcohols — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 82 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5What Makes an Alcohol

What Makes an Alcohol
Functional group
  • Functional group Alcohols contain the hydroxyl group, –OH, attached to a saturated carbon atom of an alkyl chain.
General formula
  • General formula Alcohols with one –OH group and no rings or double bonds have the general formula CₙH₂ₙ₊₁OH.
Naming
  • Naming The suffix -ol is used, with a number showing the carbon carrying –OH, as in propan-2-ol.
More than one –OH
  • More than one –OH Compounds such as ethane-1,2-diol have two hydroxyl groups, and each one is classified separately.

Part 2 of 5Primary, Secondary and Tertiary

The class depends on how many carbon groups are attached to the carbon that carries the –OH group.

Primary (1°)
  • Primary (1°) The carbon bonded to –OH is attached to one other carbon group, for example propan-1-ol, CH₃CH₂CH₂OH.
Secondary (2°)
  • Secondary (2°) The carbon bonded to –OH is attached to two other carbon groups, for example propan-2-ol, CH₃CH(OH)CH₃.
Tertiary (3°)
  • Tertiary (3°) The carbon bonded to –OH is attached to three other carbon groups, for example 2-methylpropan-2-ol, (CH₃)₃COH.
AlcoholClassCarbon groups on C–OH
EthanolPrimary1
Butan-2-olSecondary2
2-Methylpropan-2-olTertiary3

Part 3 of 5How to Classify

1
Locate the hydroxyl group in the displayed or structural formula.
2
Identify the single carbon atom that the –OH group is bonded to.
3
Count the carbon atoms or alkyl groups attached directly to that carbon, ignoring hydrogens.
4
One carbon group means primary, two means secondary and three means tertiary.
5
For a molecule with two –OH groups, repeat the count for each hydroxyl carbon.

Part 4 of 5Class and Oxidation

Class and Oxidation
Primary
  • Primary Oxidised by warm acidified potassium dichromate(VI), first to an aldehyde and then to a carboxylic acid.
Secondary
  • Secondary Oxidised to a ketone only, because no further oxidation is possible without breaking a C–C bond.
Tertiary
  • Tertiary Not oxidised by acidified dichromate, because the hydroxyl carbon has no hydrogen atom to lose.
Colour change
  • Colour change For primary and secondary alcohols the orange Cr₂O₇²⁻ ions turn green as Cr³⁺ forms;
    • tertiary alcohols leave it orange.

Part 5 of 5Exam Checklist & Tips

3.5.1 – Key Points for the Exam

  • Primary alcohol An alcohol in which the carbon bonded to –OH is attached to one other carbon group.
  • Secondary alcohol An alcohol in which the carbon bonded to –OH is attached to two other carbon groups.
  • Tertiary alcohol An alcohol in which the carbon bonded to –OH is attached to three other carbon groups.
  • Hydroxyl group The –OH functional group that defines alcohols.

Common Mistakes

  • Counting the position of the –OH along the chain instead of counting carbon groups on the hydroxyl carbon.
  • Calling propan-2-ol a primary alcohol because it is a short molecule.
  • Saying a tertiary alcohol turns dichromate green; it stays orange.
  • Counting hydrogen atoms as groups when classifying the hydroxyl carbon.

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