Alcohols: properties, reactions, preparation

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Alcohols – Definition, Classification, Physical & Chemical Properties, and Preparation (Cambridge IGCSE/A‑Level 16.2)

16.2.1 Definition & Classification

Alcohols are organic compounds that contain one or more hydroxyl (‑OH) groups attached to a saturated carbon atom. The syllabus recognises the following classes:

Class General formula Typical example(s)
Primary (1°) alcohol R–CH₂–OH Methanol, ethanol, 1‑propanol
Secondary (2°) alcohol R–CH(OH)–R′ 2‑propanol, cyclohexanol
Tertiary (3°) alcohol R₃C–OH tert‑butanol, 2‑methyl‑2‑propanol
Diol (glycol) HO–CH₂–CH₂–OH (or longer chains) Ethylene glycol
Phenol (aromatic alcohol) Ar–OH Phenol (covered in the “Phenols” sub‑section of the syllabus)

16.2.2 Physical Properties

  • Hydrogen‑bonding – each –OH can donate and accept H‑bonds, giving alcohols relatively high boiling points and densities compared with alkanes of similar molecular weight.
  • Boiling‑point trend – rises with increasing chain length and with the number of –OH groups. Example values (°C):
    Methanol 65, Ethanol 78, 1‑Propanol 97, 1‑Butanol 118, 1‑Pentanol 138, 1‑Hexanol 157.
  • Water‑solubility trend – complete miscibility up to four carbon atoms; solubility falls sharply for longer chains.
  • Density – most low‑molecular‑weight alcohols are denser than water (e.g., methanol ρ = 0.79 g cm⁻³, ethanol ρ = 0.79 g cm⁻³, 1‑butanol ρ ≈ 0.81 g cm⁻³).
  • IR spectrum – broad O–H stretch centred near 3400 cm⁻¹ (often “U‑shaped”) and a C‑O stretch around 1050 cm⁻¹.
Selected aliphatic alcohols – boiling points, water‑solubility and density
Alcohol #C atoms Boiling point (°C) Water solubility Density (g cm⁻³)
Methanol 1 65 Miscible 0.79
Ethanol 2 78 Miscible 0.79
1‑Propanol 3 97 Miscible 0.80
1‑Butanol 4 118 Fully miscible (≈ 7 % w/w at 20 °C) 0.81
1‑Pentanol 5 138 Limited (≈ 22 % w/w at 20 °C) 0.82
1‑Hexanol 6 157 Very limited (≈ 5 % w/w at 20 °C) 0.82

16.2.3 Acidic Character

  • Alcohols are weak acids with pKₐ ≈ 15–18. Deprotonation occurs only with strong bases.
  • Typical deprotonation reactions:
    • With a strong, non‑nucleophilic base (e.g., NaH):
      R‑OH + NaH → R‑O⁻Na⁺ + H₂↑
    • With aqueous NaOH (much less vigorous):
      R‑OH + NaOH ⇌ R‑O⁻Na⁺ + H₂O
  • In water the equilibrium lies far to the left; only phenols (pKₐ ≈ 10) show appreciable ionisation under aqueous conditions.

16.2.4 Chemical Reactions

4.1 Oxidation (16.2.4.1)

The product depends on the degree of substitution of the carbon bearing the –OH group.

Alcohol type Controlled oxidising agents (stop at) Primary product Further oxidation (stronger reagents)
Primary PCC, CrO₃/H₂SO₄ (mild), Na₂Cr₂O₇/H₂SO₄ (cold) Aldehyde (R‑CHO) Aldehyde → carboxylic acid (R‑COOH) with hot KMnO₄, hot CrO₃, or Na₂Cr₂O₇/H₂SO₄ (conc.)
Secondary Na₂Cr₂O₇/H₂SO₄, KMnO₄ (cold, neutral), Jones reagent Ketone (R‑CO‑R′) Ketones are generally resistant under syllabus conditions.
Tertiary — (no oxidation under normal conditions) Resistant; only very strong oxidisers (e.g., hot conc. H₂CrO₄) cleave C–C bonds. —

Examples

  • Oxidation of ethanol to acetaldehyde (PCC, CH₂Cl₂, 0 °C):
    CH₃CH₂OH + [O] → CH₃CHO + H₂O
  • Oxidation of 2‑propanol to acetone (acidic dichromate):
    (CH₃)₂CHOH + [O] → (CH₃)₂CO + H₂O

4.2 Dehydration (E1 Elimination) (16.2.4.2)

  • Acid‑catalysed removal of water gives alkenes.
  • Reaction order of ease: tertiary > secondary > primary. Primary alcohols require high temperature and give poor yields.
  • Follows Zaitsev’s rule** – the more substituted alkene is favoured.
  • General equation (example with a secondary alcohol):
    R‑CH(OH)‑R′ —H₂SO₄, 140 °C→ R‑CH=CH‑R′ + H₂O

4.3 Nucleophilic Substitution (SN1 / SN2) (16.2.4.3)

Conversion of an alcohol into an alkyl halide (most commonly bromide) using phosphorus tribromide or hydrobromic acid.

  • Reagents: PBr₃ (preferred in the laboratory) or HBr.
  • Mechanistic outcome:
    • Primary alcohol – SN2 (single step, inversion of configuration).
    • Secondary alcohol – mixture of SN1 and SN2 (depends on temperature, solvent).
    • Tertiary alcohol – SN1 (carbocation intermediate; possible rearrangements).
  • Example (PBr₃):
    3 R‑OH + PBr₃ → 3 R‑Br + H₃PO₃

4.4 Esterification (Acid‑catalysed condensation) (16.2.4.4)

  • Alcohol + carboxylic acid (or anhydride) → ester + water.
  • Equilibrium reaction; removal of water (or use of excess alcohol) drives it to the right.
  • General equation:
    R‑OH + R′COOH ⇌ R′COOR + H₂O (H₂SO₄ cat.)

4.5 Other Important Reactions

  • Formation of alkoxides (strong base deprotonation):
    R‑OH + NaH → R‑ONa + H₂↑
  • Reaction with Grignard reagents – alcohols act as proton donors; Grignard reagents add to carbonyl compounds to give secondary or tertiary alcohols after acidic work‑up (see 5.4).

16.2.5 Preparation of Alcohols

5.1 Hydration of Alkenes (Markovnikov addition)

  1. Acid‑catalysed addition of water (usually dilute H₂SO₄).
  2. Typical conditions: 60–80 °C, excess water, reflux.

Example (propene → 2‑propanol):
CH₂=CH‑CH₃ + H₂O —H₂SO₄, 70 °C→ CH₃CH(OH)CH₃

5.2 Reduction of Carbonyl Compounds

  • Aldehydes → primary alcohols (NaBH₄, LiAlH₄).
  • Ketones → secondary alcohols (NaBH₄, LiAlH₄).

Example (acetaldehyde to ethanol with NaBH₄):
CH₃CHO + NaBH₄ → CH₃CH₂OH

5.3 From Alkyl Halides

  • Reaction with aqueous NaOH or KOH.
  • Mechanism depends on the substrate:
    • Primary – SN2 (good yields).
    • Secondary – mixture of SN1/SN2 (depends on conditions).
    • Tertiary – SN1 (requires heat; possible rearrangements).

General equation:
R‑X + NaOH → R‑OH + NaX

5.4 From Grignard Reagents (Carbon‑Carbon bond formation)

  1. Prepare RMgX (R = alkyl or aryl, X = halide) in dry ether under inert atmosphere.
  2. Add the Grignard reagent to a carbonyl compound (aldehyde or ketone).
  3. Acidic work‑up (H₃O⁺) furnishes the corresponding alcohol.

Overall example (secondary alcohol):
R′CHO + RMgX → R′CH(OMgX)R —H₃O⁺→ R′CH(OH)R

5.5 Fermentation (Biological method)

Yeast catalyses the anaerobic conversion of glucose to ethanol and carbon dioxide.

C₆H₁₂O₆ → 2 CH₃CH₂OH + 2 CO₂ (yeast, 30 °C, anaerobic)

Summary of Key Reactions (Outcome 16.2.4)

Reaction type Typical reagents / conditions Main product(s) Key syllabus notes
Oxidation (primary) PCC, CrO₃/H₂SO₄ (cold), Na₂Cr₂O₇/H₂SO₄ (cold) Aldehyde → (stronger oxidiser) Carboxylic acid Controlled oxidation stops at aldehyde (PCC); stronger reagents give acid.
Oxidation (secondary) Na₂Cr₂O₇/H₂SO₄, KMnO₄ (cold, neutral), Jones reagent Ketone No further oxidation under syllabus conditions.
Dehydration (E1) Conc. H₂SO₄, 140 °C Alkene + H₂O Follows Zaitsev’s rule; tertiary > secondary > primary.
Halogenation (SN1/SN2) PBr₃ or HBr Alkyl bromide Primary – SN2; tertiary – SN1 (carbocation rearrangements possible).
Esterification R′COOH, H₂SO₄ (cat.) – remove water Ester + H₂O Equilibrium; removal of water drives reaction forward.
Hydration of alkene H₂O, dilute H₂SO₄, 60–80 °C Markovnikov alcohol Useful for preparing secondary alcohols.
Reduction of carbonyls NaBH₄ (mild), LiAlH₄ (strong) Primary (from aldehydes) or secondary (from ketones) alcohol LiAlH₄ reduces both aldehydes & ketones; NaBH₄ is selective for aldehydes.
Preparation from alkyl halides NaOH / KOH (aq.) Alcohol SN2 for primary, SN1 for tertiary (heat required).
Grignard addition RMgX + carbonyl, then H₃O⁺ Secondary or tertiary alcohol Carbon‑carbon bond formation; must be anhydrous.
Fermentation Yeast, 30 °C, anaerobic Ethanol + CO₂ Biological preparation of a primary alcohol.

Link to Cambridge Syllabus Outcomes

  • 16.2.1 – Definition & classification – covered in Section 1.
  • 16.2.2 – Physical properties – covered in Section 2 (boiling point, solubility, density, IR).
  • 16.2.3 – Acidic character – covered in Section 3.
  • 16.2.4 – Reactions – covered in Section 4 (oxidation, dehydration, substitution, esterification, other).
  • 16.2.5 – Preparation – covered in Section 5 (hydration, reduction, halide substitution, Grignard, fermentation).

These notes are designed for quick revision and for use as a checklist against the Cambridge IGCSE/A‑Level syllabus. Each outcome is explicitly marked, key trends are tabulated, and representative examples are provided to aid understanding and exam preparation.

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