Alcohols

Alcohols

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Alcohols — Key Facts for SLMC Medical (Sri Lanka)

• Alcohols contain the –OH functional group attached to a carbon atom
• Classification: Primary (1°) — OH on C with 1 R group; Secondary (2°) — OH on C with 2 R groups; Tertiary (3°) — OH on C with 3 R groups
• General formula: CₙH₂ₙ₊₁OH (mono-alcohol)
• Methanol (wood spirit): highly toxic — 10 mL can cause blindness, 100 mL causes death
• ⚡ Exam tip: Dehydration of alcohols to alkenes and oxidation of 1°/2° alcohols are high-yield for SLMC

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Alcohols — SLMC Medical (Sri Lanka) Study Guide

What Are Alcohols?

Alcohols are hydroxyl (-OH) functional group attached to a saturated carbon atom. They can be viewed as alkanes where one H is replaced by –OH. Their formula is R–OH where R is an alkyl group.

Classification

Type	Definition	Oxidation Product	Lucas Test (time)
Primary (1°)	–OH on carbon bonded to ONE other carbon	Aldehyde (→ carboxylic acid)	No reaction at RT
Secondary (2°)	–OH on carbon bonded to TWO other carbons	Ketone	5–10 min at RT
Tertiary (3°)	–OH on carbon bonded to THREE other carbons	No oxidation	Immediate reaction at RT

Lucas Test: Alcohol + Lucas reagent (conc. HCl + ZnCl₂) — 3° reacts immediately (cloudy), 2° reacts in 5-10 min, 1° reacts on heating.

Nomenclature

Common names: methyl alcohol, ethyl alcohol, isopropyl alcohol IUPAC: replace –e of alkane with –ol; position number before “ol”:

• Methanol (CH₃OH)
• Ethanol (C₂H₅OH)
• Propan-1-ol / Propan-2-ol
• Butan-1-ol, Butan-2-ol

Dihydroxy alcohols (glycols):

• Ethane-1,2-diol = ethylene glycol (antifreeze)
• Propane-1,2-diol = propylene glycol (food additive, pharmaceutical)

Trihydroxy:

• Glycerol / Glycerin = Propane-1,2,3-triol (sweet, viscous liquid)

Physical Properties

Property	Trend
Boiling point	Higher than analogous alkanes due to hydrogen bonding
Solubility	Lower alcohols (C₁–C₃) soluble in water (H-bonding); decreases with chain length
Density	Less than water (except glycerol)
Viscosity	Increases with molecular size and –OH count

Hydrogen bonding in alcohols:

• Alcohols can hydrogen-bond with themselves (O–H···O)
• Alcohols can hydrogen-bond with water (O–H···O from water)
• This explains their higher BP vs alkanes and water solubility for small R groups

Methanol: BP 64.7°C (vs methane: –161°C) Ethanol: BP 78.4°C (vs ethane: –88°C)

Chemical Properties

1. Oxidation

Alcohol	Oxidizing Agent	Product
1° alcohol	K₂Cr₂O₇/H₂SO₄ (cold, dilute)	Aldehyde (blue Cr³⁺ → green)
1° alcohol	KMnO₄/H₂SO₄ (hot)	Carboxylic acid
2° alcohol	K₂Cr₂O₇/H₂SO₄ or KMnO₄	Ketone
3° alcohol	Strong oxidizing agents	No reaction

Benedict’s/Fehling’s test: Aldehydes reduce Cu²⁺ (blue) → Cu⁺ (red precipitate of Cu₂O). Ketones do not react.

2. Dehydration

Alcohols lose water (dehydrate) to form alkenes or ethers under acidic conditions:

Elimination (E1) at higher temperature (>140°C):

• Ethanol + conc. H₂SO₄ (170°C) → Ethene + H₂O
• Rate: 3° > 2° > 1° (tertiary dehydrates most readily)

Substitution (Sn1) at lower temperature (<140°C):

• Ethanol + conc. H₂SO₄ (140°C) → Diethyl ether + H₂O

3. Esterification

Alcohol + carboxylic acid ⇌ ester + water (with conc. H₂SO₄ catalyst):

• CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (ethyl acetate)
• Esters are named: alkyl alkanoate (e.g., methyl acetate = methyl ethanoate)

4. Reaction with Sodium

2R–OH + 2Na → 2R–ONa + H₂↑ (alkoxide + hydrogen gas)

• Alkoxides are strong bases (stronger than NaOH)
• Used to test for the –OH group

5. Halogen Substitution

R–OH + HX → R–X + H₂O (with conc. HX or NaBr/H₂SO₄) Or: R–OH + PCl₅ → R–Cl + POCl₃ + HCl (for 1° and 2° alcohols)

6. Oxidation State

Methanol is metabolized in the liver by alcohol dehydrogenase:

• Methanol → Formaldehyde → Formic acid (toxic metabolites)
• Fomepizole (4-methylpyrazole) inhibits alcohol dehydrogenase — used in methanol poisoning treatment
• Ethanol competes with methanol for the same enzyme —basis for ethanol treatment in methanol poisoning

Industrial Preparation

Ethanol from Fermentation

• C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (yeast fermentation)
• Sugar source: sugarcane, grapes, cereals

Ethanol from Hydration of Ethene

• C₂H₄ + H₂O → C₂H₅OH (H₃PO₄ catalyst, 300°C, 60 atm)

Methanol from Synthesis Gas

• CO + 2H₂ → CH₃OH (ZnO/Cr₂O₃ catalyst, 200 atm, 400°C)
• Methanol is wood spirit — historically from destructive distillation of wood

Clinical and Medical Relevance

• Methanol poisoning: causes blindness (optic nerve damage) and metabolic acidosis; treat with ethanol or fomepizole
• Ethanol: CNS depressant, antidote for methanol/ethylene glycol poisoning, used in IV cannulae as antiseptic
• Glycerol: component of triglycerides; used as osmotic laxative and in topical formulations
• Isopropyl alcohol (IPA): 70% IPA used as disinfectant; more effective than ethanol against bacterial spores
• Propylene glycol: solvent in IV medications (e.g., diazepam, lorazepam); safe in humans but toxic to animals
• Ethylene glycol poisoning: antifreeze ingestion → metabolic acidosis → renal failure; treat with ethanol or fomepizole

Common SLMC Exam Traps

• 3° alcohols do NOT oxidize — they have no hydrogen on the carbon bearing –OH. This is a classic MCQ trick.
• Dehydration of alcohols: higher temperature favors alkene (elimination), lower temperature favors ether (substitution)
• Lucas test distinguishes alcohol classes by reactivity with HCl/ZnCl₂ — 3° is fastest
• Ethanol and methanol both produce CO₂ and H₂O on complete combustion, but only ethanol can be obtained by fermentation

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