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2.9.1Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Alcohols – Introduction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 51 of 53 Unit quiz

📚 IAL Edexcel Linear (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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Section 1 of 5Structure and Naming

Alcohols are a homologous series containing the hydroxyl functional group, –OH.

General formula
  • General formula Saturated alcohols with one –OH group fit the general formula CₙH₂ₙ₊₁OH, so each member differs by CH₂.
Naming
  • Naming The suffix –ol is added to the alkane stem, with a number to show which carbon carries the –OH group, as in propan-2-ol.
Diols
  • Diols A molecule with two –OH groups takes the ending –diol, for example ethane-1,2-diol, HOCH₂CH₂OH.
First four
  • First four Methanol CH₃OH, ethanol C₂H₅OH, propan-1-ol C₃H₇OH and butan-1-ol C₄H₉OH are the first four unbranched members.

Section 2 of 5Classifying Alcohols

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

Classifying Alcohols
Primary
  • Primary The C–OH carbon is attached to one alkyl group (or none, as in methanol), as in propan-1-ol, CH₃CH₂CH₂OH.
Secondary
  • Secondary The C–OH carbon is attached to two alkyl groups, as in propan-2-ol, CH₃CH(OH)CH₃.
Tertiary
  • Tertiary The C–OH carbon is attached to three alkyl groups, as in 2-methylpropan-2-ol, (CH₃)₃COH.
Why it matters
  • Why it matters The class decides how the alcohol is oxidised, so classification is tested together with oxidation reactions.
AlcoholStructural formulaClass
Propan-1-olCH₃CH₂CH₂OHPrimary
Propan-2-olCH₃CH(OH)CH₃Secondary
2-Methylpropan-2-ol(CH₃)₃COHTertiary

Section 3 of 5Physical Properties

Hydrogen bonding makes alcohols behave differently from alkanes of similar size.

1
The O–H bond is highly polar, with δ– on oxygen and δ+ on hydrogen, because oxygen is much more electronegative.
2
A hydrogen atom on one molecule is attracted to a lone pair on the oxygen of a neighbouring molecule, forming a hydrogen bond.
3
These hydrogen bonds need extra energy to overcome, so ethanol boils at 78 °C while ethane, of similar size, is a gas at room temperature.
4
Boiling points rise along the series, from methanol at 65 °C to butan-1-ol at 117 °C, because London forces grow with chain length.
5
Short-chain alcohols mix freely with water since they can hydrogen bond with it, and solubility falls as the hydrocarbon chain lengthens.

Section 4 of 5Uses and Safety

Fuel
  • Fuel Ethanol burns cleanly with a pale flame and is blended into petrol or burned directly as a renewable fuel.
Solvent
  • Solvent Alcohols dissolve both polar and non-polar substances, so ethanol is widely used in perfumes, inks and cleaning products.
Flammability
  • Flammability Alcohols are flammable, so they must be heated with a water bath or electric heater rather than a naked flame.
Toxicity
  • Toxicity Methanol is poisonous even in small amounts, so it must never be confused with the ethanol found in drinks.

Section 5 of 5Exam Checklist & Tips

2.9.1 – Key Points for the Exam

  • Hydroxyl group The –OH functional group that defines the alcohol family.
  • Primary alcohol An alcohol whose –OH carbon is bonded to only one other carbon group.
  • Tertiary alcohol An alcohol whose –OH carbon is bonded to three other carbon groups.
  • Hydrogen bond An attraction between a δ+ hydrogen bonded to O, N or F and a lone pair on a nearby O, N or F atom.

Common Mistakes

  • Counting the position of the –OH incorrectly: number from the end that gives the –OH group the lowest possible number.
  • Classifying by the number of OH groups instead of the number of alkyl groups attached to the C–OH carbon.
  • Explaining the high boiling point by breaking O–H covalent bonds rather than overcoming intermolecular hydrogen bonds.
  • Writing the general formula as CₙH₂ₙOH, which has one hydrogen too few in the alkyl part.

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