Cambridge Syllabus Notes

Alcohols: Properties, Reactions, Preparation

1. What are Alcohols?

Alcohols are organic compounds that contain a hydroxyl group ($$-OH$$) attached to a saturated carbon atom. The general formula is $$C_nH_{2n+1}OH$$ (for straight‑chain alcohols). They are classified as primary, secondary, or tertiary depending on how many carbon atoms are attached to the carbon bearing the $$-OH$$ group.

2. Physical Properties

Boiling points: Higher than alkanes of similar size due to hydrogen bonding. Example: ethanol $$(C_2H_5OH)$$ boils at $$78^\circ C$$ vs. ethane $$(C_2H_6)$$ at $$-89^\circ C$$.
Solubility: Small alcohols (methanol, ethanol, propanol) are miscible with water because they can hydrogen‑bond with water molecules. Solubility decreases as the carbon chain length increases.
Density: Usually less than water (≈0.79 g cm⁻³ for ethanol).
Odour: Many have characteristic smells (e.g., ethanol – “alcoholic”, isopropanol – “rubbing alcohol”).

3. Chemical Reactions

Reaction Type	Typical Conditions	Products	Example Equation
Oxidation (primary)	$$[O]$$ (e.g., K₂Cr₂O₇/H₂SO₄, heat)	Aldehyde → further oxidation to carboxylic acid	$$CH_3CH_2OH \xrightarrow{[O]} CH_3CHO \xrightarrow{[O]} CH_3COOH$$
Oxidation (secondary)	$$[O]$$ (e.g., PCC, Na₂Cr₂O₇/H₂SO₄)	Ketone (no further oxidation)	$$CH_3CHOHCH_3 \xrightarrow{[O]} CH_3COCH_3$$
Dehydration	Conc. $$H_2SO_4$$, 170‑180 °C	Alkene + water	$$CH_3CH_2OH \xrightarrow{H_2SO_4,170^\circ C} CH_2=CH_2 + H_2O$$
Esterification	Carboxylic acid + conc. $$H_2SO_4$$ (reflux)	Ester + water	$$CH_3CH_2OH + CH_3COOH \rightleftharpoons CH_3COOCH_2CH_3 + H_2O$$
Nucleophilic substitution (with HX)	HCl, HBr, HI (often with ZnCl₂ catalyst for Lucas test)	Alkyl halide + water	$$R-OH + HCl \rightarrow R-Cl + H_2O$$

4. Preparation of Alcohols

Hydration of alkenes: $$C_nH_{2n} + H_2O \xrightarrow{H^+} C_nH_{2n+1}OH$$ (Markovnikov addition). Example: ethene + steam → ethanol.
Reduction of carbonyl compounds: Aldehydes or ketones reduced with $$NaBH_4$$ or $$LiAlH_4$$ give primary or secondary alcohols respectively. $$RCHO + 2[H] \rightarrow RCH_2OH$$.
Hydrolysis of alkyl halides: $$R-X + OH^- \rightarrow R-OH + X^-$$ (SN1 or SN2 depending on substrate).
Fermentation: Yeast converts sugars (e.g., glucose) to ethanol and carbon dioxide: $$C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$$.
Oxymercuration‑demercuration of alkenes: Gives alcohols following Markovnikov rule without rearrangements.

5. Important Points to Remember

Hydrogen bonding → higher boiling points & water solubility for short‑chain alcohols.
Oxidation: primary → aldehyde → acid; secondary → ketone; tertiary resists oxidation.
Dehydration favours the more substituted alkene (Zaitsev’s rule).
Lucas test: tertiary alcohols react instantly with HCl/ZnCl₂ (cloudy), secondary ~5 min, primary no reaction at room temperature.
Ethanol is a renewable fuel and solvent; methanol is toxic.

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Alcohols: properties, reactions, preparation