What is an alcohol? Classification
An alcohol has an –OH (hydroxyl) group joined to a carbon that has only single bonds (an sp³ carbon). General formula: R–OH.
By number of –OH groups: monohydric (one –OH, e.g. ethanol), dihydric (two, e.g. ethane-1,2-diol, the glycol in antifreeze) and trihydric (three, e.g. glycerol).
By the carbon that holds –OH (the carbinol carbon): count how many other carbons touch it.
- Primary (1°): one carbon – e.g. ethanol CH₃CH₂OH.
- Secondary (2°): two carbons – e.g. propan-2-ol (CH₃)₂CHOH.
- Tertiary (3°): three carbons – e.g. 2-methylpropan-2-ol (CH₃)₃COH.
Special cases: allylic alcohols (–OH on the carbon next to C=C) and benzylic alcohols (–OH on the carbon next to a benzene ring). If –OH is on a C=C carbon it is a vinylic alcohol (unstable), and if it is on the ring itself it is a phenol – a different family.
Nomenclature: naming alcohols
Common names: alkyl group + 'alcohol' (methyl alcohol, isopropyl alcohol). IUPAC names follow four steps:
- Pick the longest chain that holds the C–OH carbon.
- Number the chain from the end that gives –OH the lowest number.
- Drop the final 'e' of the alkane and add 'ol': propane → propan-1-ol or propan-2-ol.
- Add branches with their numbers in alphabetical order: 2-methylpropan-2-ol.
More than one –OH: keep the 'e' and use diol, triol – ethane-1,2-diol, propane-1,2,3-triol (glycerol). Ring alcohols: cyclohexanol.
Structure
The O in an alcohol has two bonds and two lone pairs. The C–O–H angle is about 109° – a little less than a perfect tetrahedron because the lone pairs push the bonds closer.
Preparation of alcohols
1. From alkenes
Acid-catalysed hydration: alkene + water with H⁺. H goes to the carbon that already has more H atoms; OH goes to the other one (Markovnikov). Propene → propan-2-ol. The steps: (i) H⁺ adds to C=C and a carbocation forms, (ii) water attacks the carbocation, (iii) a H⁺ is lost.
Hydroboration–oxidation: alkene + diborane (B₂H₆), then H₂O₂ / NaOH. OH ends up on the carbon with more H atoms (looks anti-Markovnikov). Propene → propan-1-ol.
2. Reduction of carbonyl compounds
Aldehydes → 1° alcohols; ketones → 2° alcohols (H₂ with Pd/Ni, or NaBH₄, or LiAlH₄). Carboxylic acids need the strong LiAlH₄ to give 1° alcohols; industry first turns acids into esters and reduces them with H₂.
3. From Grignard reagents
RMgX adds to a C=O and water then gives the alcohol. Methanal (HCHO) → 1°; any other aldehyde → 2°; a ketone → 3°. The new C–C bond lets you build a bigger carbon chain.
Physical properties
Boiling point: much higher than alkanes, haloalkanes and ethers of similar mass, because of intermolecular hydrogen bonds. Boiling point rises with chain length, and falls with branching (less surface contact).
Solubility: small alcohols (methanol, ethanol, propanol) mix with water in any ratio because they make H-bonds with water. As the carbon chain (the oily part) gets longer, solubility falls.
Chemical reactions of alcohols
A. O–H bond breaks (alcohol as a weak acid)
With Na or K: 2R–OH + 2Na → 2R–O⁻Na⁺ + H₂. Acidity order: 1° > 2° > 3° (alkyl groups push electrons towards O). Alcohols are weaker acids than water. Esterification: R–OH + R′COOH ⇌ R′COOR + H₂O (acid catalyst; remove water to push it forward). Acid chlorides and anhydrides also give esters.
B. C–O bond breaks
With HX: R–OH + HX → R–X + H₂O. Reactivity 3° > 2° > 1°. Lucas test (conc. HCl + anhydrous ZnCl₂): 3° turns cloudy at once, 2° in about 5 minutes, 1° stays clear at room temperature. Also PCl₃, PCl₅ and SOCl₂ give alkyl halides.
Dehydration (conc. H₂SO₄ or H₃PO₄, heat): ethanol at 443 K → ethene. Ease: 3° > 2° > 1°, because the step through a carbocation is easiest for 3°.
C. Oxidation
1° → aldehyde (PCC or hot Cu at 573 K) → carboxylic acid (strong KMnO₄/acidified K₂Cr₂O₇). 2° → ketone. 3° alcohols resist normal oxidation; hot Cu turns them into alkenes.
Commercially important alcohols
Methanol (wood spirit): once made by heating wood; now from CO + 2H₂ over a ZnO–Cr₂O₃ catalyst at 573–673 K and 200–300 atm. Used as a solvent, in making formaldehyde, and as a fuel. Very poisonous: small amounts cause blindness, large amounts death.
Ethanol: made by fermentation of sugar in molasses, grapes or grain. Yeast enzymes invertase (sucrose → glucose + fructose) and zymase (glucose → ethanol + CO₂) do the work, without air. Used in sanitisers, medicines, drinks and as a petrol blend. Ethanol for industry is denatured (made unfit to drink) by adding a little copper sulphate and pyridine.
Key formulas and definitions
- General formula of a saturated monohydric alcohol: CₙH₂ₙ₊₁OH
- Hydration: CH₃CH=CH₂ + H₂O (H⁺) → CH₃CH(OH)CH₃
- Hydroboration–oxidation: CH₃CH=CH₂ → (B₂H₆; H₂O₂/OH⁻) → CH₃CH₂CH₂OH
- 2R–OH + 2Na → 2R–ONa + H₂
- Lucas test speed: 3° > 2° > 1°
- Dehydration: C₂H₅OH (conc. H₂SO₄, 443 K) → CH₂=CH₂ + H₂O
- Oxidation: 1° → aldehyde → acid; 2° → ketone; 3° → no normal oxidation
- Grignard: HCHO → 1°, RCHO → 2°, R₂CO → 3° alcohol
Worked examples
1. Classify (CH₃)₂CH–OH, CH₃CH₂CH₂OH and (CH₃)₃C–OH.
Step 1: find the carbon holding –OH. Step 2: count carbons joined to it. (CH₃)₂CH–OH: 2 carbons → secondary. CH₃CH₂CH₂OH: 1 carbon → primary. (CH₃)₃C–OH: 3 carbons → tertiary.
2. Give the IUPAC name of CH₃–CH(OH)–CH₂–CH₃.
Longest chain = 4 carbons (butane). Numbering from the left gives –OH position 2; from the right it would be 3, so choose 2. Name: butan-2-ol.
3. Name (CH₃)₂CH–CH₂–CH₂OH.
Longest chain with the C–OH carbon = 4 C. Number from the –OH end: C1 holds OH, the CH₃ branch is on C3. Name: 3-methylbutan-1-ol.
4. Which alcohol forms from propene by (a) acid-catalysed hydration, (b) hydroboration–oxidation?
(a) H⁺ adds to the end CH₂, making the more stable secondary carbocation on C2; water attacks it → propan-2-ol. (b) Boron joins the end carbon, then H₂O₂/OH⁻ replaces B by OH → propan-1-ol.
5. Which Grignard reagent and carbonyl compound give 2-methylpropan-2-ol?
The product is a 3° alcohol, so we need a ketone. (CH₃)₃C–OH has three CH₃ groups on the carbinol carbon: use propanone (CH₃)₂C=O + CH₃MgBr, then water. CH₃ adds to the C=O carbon and O becomes OH.
6. Three unlabelled bottles hold butan-1-ol, butan-2-ol and 2-methylpropan-2-ol. How will you tell them apart?
Add Lucas reagent (conc. HCl + ZnCl₂) to each at room temperature. Cloudy at once → 2-methylpropan-2-ol (3°). Cloudy in about 5 minutes → butan-2-ol (2°). Stays clear → butan-1-ol (1°). The cloudiness is the insoluble alkyl chloride.
Common mistakes
- Counting H atoms instead of carbon atoms on the C–OH carbon when classifying 1°/2°/3°.
- Calling a compound with –OH on a benzene ring an alcohol – that is a phenol. Benzyl alcohol (C₆H₅CH₂OH) is an alcohol because OH is on the CH₂.
- Saying 3° alcohols are the most acidic. It is the opposite: acidity 1° > 2° > 3°, but reactivity with HX and ease of dehydration are 3° > 2° > 1°.
- Mixing up the products from propene: acid hydration gives propan-2-ol; hydroboration–oxidation gives propan-1-ol.