Alcohols, phenols and ethers

In short

Alcohols (R-OH), phenols (Ar-OH) and ethers (R-O-R') are the three families of oxygen-containing organic compounds in which oxygen is singly bonded. They are the gateway functional groups of organic chemistry: alcohols link hydrocarbons to aldehydes, ketones and acids; phenols show how an aromatic ring changes the chemistry of the same -OH group; ethers are the standard solvents (diethyl ether, THF) in which most synthesis is done. B.Sc papers, CSIR-UGC NET/JRF, GATE Chemistry (CY), IIT-JAM and placement tests for pharma, agrochemical, dyes and specialty-chemical firms examine this topic heavily, usually as distinguishing tests, named reactions and acidity ordering. Industrially these are ethanol (sugar/molasses distilleries), phenol and cumene (petrochemical), glycerol (oleochemical) and polyether/epoxy intermediates.

Explained

Structure, classification and nomenclature essentials

• Alcohol: -OH attached to an sp³ carbon. Phenol: -OH attached directly to an sp² carbon of an aromatic ring. Enol: -OH on an sp² carbon of an alkene (usually unstable, tautomerises).
• Classification by the carbon bearing -OH: primary (1°) R-CH₂OH, secondary (2°) R₂CH-OH, tertiary (3°) R₃C-OH. Also monohydric, dihydric (glycols), trihydric (glycerol), polyhydric.
• Benzyl alcohol (C₆H₅CH₂OH) is a primary alcohol, not a phenol - the -OH is on an sp³ carbon.
• Ethers: R-O-R'. Simple/symmetrical (diethyl ether), mixed/unsymmetrical (ethyl methyl ether). Cyclic ethers: THF, 1,4-dioxane, epoxides (oxiranes), crown ethers.
• Geometry: C-O-H bond angle in methanol ≈ 108.9°, C-O-C in dimethyl ether ≈ 111° - oxygen is sp³ with two lone pairs. In phenol the C-O bond is shorter (≈136 pm vs ≈143 pm in alcohols) because of partial double-bond character from resonance.
• IUPAC: alcohols are "-ol" (propan-2-ol); ethers are named as alkoxyalkanes (methoxyethane) or by the "alkyl alkyl ether" style; phenol derivatives use retained names - cresol, catechol (1,2), resorcinol (1,3), hydroquinone (1,4), picric acid (2,4,6-trinitrophenol).

Physical properties and hydrogen bonding

• Alcohols and phenols are intermolecularly hydrogen bonded, so their boiling points are far above those of alkanes, haloalkanes and ethers of comparable mass: ethanol 78 °C versus dimethyl ether −24 °C (both C₂H₆O).
• Ethers have no O-H, so no self-association; they boil like comparable alkanes but still dissolve in water a little because oxygen accepts hydrogen bonds. This makes ethers excellent aprotic solvents for Grignard and hydride reactions.
• Boiling point order among isomeric alcohols: 1° > 2° > 3° (branching reduces surface area and effective H-bonding): butan-1-ol 118 °C, butan-2-ol 100 °C, 2-methylpropan-2-ol 82 °C.
• Water solubility falls as the chain lengthens: C₁-C₃ alcohols are miscible; solubility drops sharply beyond C₄-C₅.
• *o*-Nitrophenol is more volatile and less water-soluble than *p*-nitrophenol because of intramolecular hydrogen bonding (chelation) which prevents association with water.
• Glycerol (propane-1,2,3-triol) is viscous and very hygroscopic owing to three -OH groups; ethylene glycol is used as a coolant/antifreeze.

Acidity and basicity

• Order of acidity: carboxylic acid > phenol > water > alcohol > terminal alkyne. Typical pKa: acetic acid ≈ 4.8, phenol ≈ 10.0, water 15.7, ethanol ≈ 16, t-butanol ≈ 18.
• Why phenol is more acidic than alcohols: the phenoxide ion is resonance-stabilised (negative charge delocalised onto *ortho* and *para* carbons) and the sp² carbon is more electronegative; an alkoxide has no such delocalisation.
• Among alcohols, acidity falls 1° > 2° > 3° in solution (electron-releasing alkyl groups destabilise the alkoxide and steric bulk hinders solvation).
• Substituent effect on phenols: electron-withdrawing groups (-NO₂, -CN, -X) increase acidity, strongest at *o*/*p* where resonance also operates; electron-donating groups (-CH₃, -OCH₃, -NH₂) decrease it. Picric acid (pKa ≈ 0.4) is as strong as a mineral acid; *p*-nitrophenol pKa ≈ 7.2, *p*-cresol ≈ 10.3.
• Phenol dissolves in NaOH but not in NaHCO₃ (it is weaker than carbonic acid) - this is the standard way to separate a phenol from a carboxylic acid in a mixture. Alcohols dissolve in neither.
• Basicity/nucleophilicity: the O of alcohols and ethers is protonated by strong acids to give oxonium ions; ethers form Lewis acid-base complexes (BF₃·OEt₂) and solvate Grignard reagents.

Preparation

Alcohols:

1. Hydration of alkenes - acid-catalysed, Markovnikov, via carbocation (rearrangement possible).
2. Hydroboration-oxidation (B₂H₆ then H₂O₂/OH⁻) - anti-Markovnikov, *syn* addition, no rearrangement.
3. Oxymercuration-demercuration - Markovnikov without rearrangement.
4. Reduction of carbonyls: NaBH₄ (selective, aldehydes/ketones), LiAlH₄ (also esters, acids, amides), catalytic H₂/Ni.
5. Grignard route: RMgX + HCHO → 1°; + other aldehyde → 2°; + ketone → 3°; + ester (2 eq) → 3°; + epoxide → alcohol two carbons longer.
6. Hydrolysis of haloalkanes (aq. KOH) and fermentation of sugars (industrial ethanol: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ by zymase).

Phenols:

1. Cumene process (dominant industrial route): benzene + propene → cumene; air oxidation → cumene hydroperoxide; acid cleavage → phenol + acetone.
2. From chlorobenzene (Dow process, NaOH, 350 °C/high pressure); from benzenesulphonic acid (fused NaOH); from diazonium salts (warm water); from aniline via diazotisation.

Ethers:

1. Williamson synthesis: R-X + R'O⁻Na⁺ → R-O-R' (Sₙ2). Use the less hindered alkyl halide and the more hindered alkoxide; 3° halides give alkene by E2 instead. Aryl ethers (anisole) come from ArO⁻ + CH₃I, never ArX + RO⁻.
2. Acid dehydration of alcohols - good for symmetrical 1° ethers (ethanol/H₂SO₄ at 413 K → diethyl ether; at 443 K → ethene).
3. Epoxides from alkene + peroxyacid, or from halohydrin + base.

Reactions of alcohols

• With active metals: 2R-OH + 2Na → 2R-ONa + H₂ (shows acidic H).
• Substitution of OH: HX (reactivity HI > HBr > HCl), PCl₅, PCl₃, SOCl₂ (Darzens - gives gaseous by-products, cleanest), or conversion to a tosylate. 1° go Sₙ2, 3° go Sₙ1; the Lucas test exploits this.
• Dehydration (E1): conc. H₂SO₄ or Al₂O₃/623 K; ease 3° > 2° > 1°; follows Saytzeff (Zaitsev) rule giving the more substituted alkene; carbocation rearrangements common.
• Oxidation: 1° → aldehyde (PCC, PDC, Collins, Cu/573 K, MnO₂) → acid (KMnO₄, K₂Cr₂O₇/H⁺, Jones); 2° → ketone; 3° resists (only dehydrates/cleaves). Dehydrogenation over hot Cu: 1° → aldehyde, 2° → ketone, 3° → alkene - a classic exam distinction.
• Esterification: R-OH + R'COOH ⇌ ester + H₂O (H₂SO₄, reversible); with acid chlorides or anhydrides it is irreversible (use pyridine).
• Victor Meyer test (classification by colour of the nitrolic product: red for 1°, blue for 2°, colourless for 3°) and Lucas test (3° cloudy at once, 2° in ~5 min, 1° only on heating).
• Glycol-specific: oxidative cleavage by HIO₄ (Malaprade) or Pb(OAc)₄; pinacol-pinacolone rearrangement under acid (1,2-diol → ketone via carbocation and methyl migration).
• Haloform test with I₂/NaOH: ethanol and any CH₃-CH(OH)- alcohol give yellow iodoform - distinguishes methyl carbinols.

Reactions of phenols

• O-H reactions: salt formation with NaOH; ether formation (Williamson); ester formation needs an acid chloride/anhydride, not the acid.
• Electrophilic substitution is strongly activated and *o*/*p*-directing: bromination in CS₂ gives mainly *p*-bromophenol, but in water gives 2,4,6-tribromophenol; nitration with dilute HNO₃ gives *o*- and *p*-nitrophenol, with conc. HNO₃ gives picric acid; sulphonation is temperature-reversible (*o* at low, *p* at high temperature); Friedel-Crafts works but gives poor yields (O coordinates AlCl₃).
• Kolbe-Schmitt: sodium phenoxide + CO₂ (4-7 atm, 400 K) then H⁺ → salicylic acid - route to aspirin.
• Reimer-Tiemann: phenol + CHCl₃/NaOH → salicylaldehyde (*ortho*), via dichlorocarbene as the electrophile.
• Fries rearrangement: aryl ester + AlCl₃ → *o*- or *p*-hydroxyaryl ketone (low temperature favours *para*, high favours *ortho*).
• Claisen rearrangement: allyl aryl ether heated (~200 °C) → *o*-allylphenol by a concerted [3,3]-sigmatropic shift; if both *ortho* positions are blocked, the *para* product forms by two successive shifts.
• Coupling and tests: FeCl₃ gives a violet/green colour (characteristic of phenols and enols); diazonium coupling gives azo dyes; Liebermann's nitroso test; Lederer-Manasse reaction with HCHO gives phenol-formaldehyde resin (bakelite).
• Reduction: Zn dust → benzene; hydrogenation (Ni) → cyclohexanol. Oxidation by Na₂Cr₂O₇ gives *p*-benzoquinone.

Reactions of ethers and epoxides

• Ethers are inert to bases, metals, oxidising and reducing agents - hence their use as solvents. Weaknesses: they form explosive peroxides on standing in air (test and never distil an old ether to dryness) and are highly flammable.
• Acid cleavage: R-O-R' + HI → alcohol + iodoalkane. Mechanism depends on structure: 1° alkyl groups go Sₙ2 (iodide attacks the less hindered carbon); 3° (or benzylic) groups go Sₙ1 (iodide ends up on the more substituted carbon). Anisole + HI → phenol + CH₃I, because aryl-O cannot be attacked.
• Zeisel method: quantitative estimation of methoxy groups by cleaving with HI and weighing CH₃I as AgI.
• Electrophilic substitution in anisole: -OCH₃ activates and directs *o*/*p*.
• Epoxides (strained three-membered ring) open readily: base/nucleophile → attack at the less hindered carbon, anti; acid → attack at the more substituted carbon (more carbocation-like). Hydrolysis gives *trans*-diols; RMgX opens an epoxide to give a homologated alcohol.
• Crown ethers (18-crown-6, 15-crown-5) chelate metal cations by cavity size (18-crown-6 for K⁺), generating "naked" anions and enabling phase-transfer catalysis; basis of host-guest chemistry.

Industrial and Indian regulatory context

• Ethanol: made from sugarcane molasses, juice, B-heavy syrup and damaged grain. India's Ethanol Blended Petrol (EBP) programme reached about 20% blending (E20) in petrol nationwide during 2025; the roadmap and targets are revised periodically by MoPNG/NITI Aayog, so check the current notification. Rectified spirit ≈ 95.6% ethanol (azeotrope with water, bp 78.1 °C); absolute alcohol is made by azeotropic distillation with benzene/cyclohexane or by molecular sieves. Denatured/methylated spirit contains methanol, pyridine and dyes to make it undrinkable and non-excisable for drinking.
• Law and duty: potable alcohol is a State List subject - State Excise Acts and State Prohibition Acts govern it (Gujarat and Bihar prohibit it); industrial alcohol and denaturation are regulated under central rules administered through State excise machinery. Under GST (slabs 5%, 18%, 40% since 22 September 2025) alcoholic liquor for human consumption stays outside GST, taxed by State excise; ethanol supplied for blending has carried a concessional rate - verify the current rate notification before quoting it.
• Safety and standards: phenol is corrosive and absorbed through skin; methanol causes blindness; diethyl ether forms peroxides. These fall under Manufacture, Storage and Import of Hazardous Chemical Rules and the occupational safety provisions of the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025 with the other three Labour Codes (which replaced the Factories Act, 1948). Specifications are issued as Indian Standards by BIS for ethanol, methanol, glycerine, phenol and ether - quote the current IS number and edition from the BIS catalogue rather than from memory.
• Major Indian players: ethanol/glycerine from sugar co-operatives and distilleries (Praj Industries supplies plants), phenol from Hindustan Organic Chemicals and SI Group, glycols from Reliance and IOCL, surfactants (ethoxylates) from Galaxy Surfactants.

Key points

• Alcohol vs phenol - -OH on sp³ carbon vs on an aromatic ring; phenol is ~10⁶ times more acidic.
• Phenol acidity - resonance stabilisation of phenoxide plus the sp² carbon's electronegativity; pKa ≈ 10.
• NaHCO₃ test - carboxylic acid effervesces, phenol does not; both dissolve in NaOH, alcohols in neither.
• Boiling point anomaly - ethanol 78 °C versus dimethyl ether −24 °C, same formula C₂H₆O, because only ethanol self-associates by H-bonds.
• Lucas test - ZnCl₂/conc. HCl: 3° turbid immediately, 2° in minutes, 1° on heating; works through Sₙ1 carbocation stability.
• Victor Meyer - red (1°), blue (2°), colourless (3°) nitrolic colours.
• FeCl₃ violet colour - phenols and enols; alcohols give nothing.
• Hydroboration-oxidation - anti-Markovnikov, *syn*, no rearrangement; acid hydration is Markovnikov with rearrangement.
• Oxidation ladder - 1° → aldehyde (PCC) → acid; 2° → ketone; 3° resists.
• Dehydration ease - 3° > 2° > 1°, Saytzeff product, E1 via carbocation.
• Williamson synthesis - Sₙ2; pair a hindered alkoxide with an unhindered halide; 3° halide gives an alkene.
• Ether cleavage - only by hot HI/HBr; anisole gives phenol + CH₃I.
• Epoxide regiochemistry - base attacks the less substituted carbon, acid the more substituted one.
• Reimer-Tiemann - salicylaldehyde via dichlorocarbene; Kolbe-Schmitt - salicylic acid via CO₂.
• Claisen rearrangement - [3,3] shift of allyl aryl ether to *o*-allylphenol; Fries - acyl migration in aryl esters with AlCl₃.
• Pinacol-pinacolone - 1,2-diol to ketone with 1,2-methyl migration under acid.
• Cumene process - benzene + propene → phenol + acetone; the industrial source of both.
• Crown ethers - cavity-selective cation binders (18-crown-6 ↔ K⁺) used in phase-transfer catalysis.

Formulas and examples

Example 1 - pH of a phenol solution. Find the pH of 0.100 M phenol, Ka = 1.0 × 10⁻¹⁰.

• [H⁺] = √(Ka·c) = √(1.0 × 10⁻¹⁰ × 0.100) = √(1.0 × 10⁻¹¹) = 3.16 × 10⁻⁶ M.
• pH = −log(3.16 × 10⁻⁶) = 5.50.
• α = (3.16 × 10⁻⁶/0.100) × 100 = 0.0032% - phenol is a very weak acid, so it cannot liberate CO₂ from bicarbonate.

Example 2 - ethanol yield from molasses. A distillery processes 100 tonnes/day of molasses containing 45% fermentable sugars; fermentation efficiency is 90% of theory and distillation recovery 96%.

• Fermentable sugar = 100 × 0.45 = 45 t.
• Theoretical ethanol = 45 × 0.5114 = 23.01 t.
• After fermentation efficiency: 23.01 × 0.90 = 20.71 t; after recovery: 20.71 × 0.96 = 19.88 t/day.
• Volume at density 0.789 kg/L: 19 880/0.789 = 25 197 L/day ≈ 25.2 kL/day.
• At an indicative ethanol procurement price of ₹60/L the daily revenue is about 25 197 × 60 ≈ ₹15.1 lakh (procurement prices are notified periodically by the central government and change each supply year).

Example 3 - Zeisel methoxyl determination. 0.2500 g of a methylated sugar gave 0.4700 g of AgI.

• Moles AgI = 0.4700/234.77 = 2.002 × 10⁻³ mol → same moles of OCH₃.
• Mass of OCH₃ = 2.002 × 10⁻³ × 31.03 = 0.06212 g.
• % OCH₃ = (0.06212/0.2500) × 100 = 24.85%.
• One OCH₃ per molecule would imply M ≈ 31.03/0.2485 ≈ 125 g/mol; so a 250 g/mol sugar carries two methoxy groups.

Example 4 - structure from formula and tests. A compound C₄H₁₀O is optically active, gives H₂ with Na, turns turbid with Lucas reagent in about five minutes and gives a positive iodoform test.

• DoU = (2×4 + 2 − 10)/2 = 0 - saturated and acyclic.
• H₂ with Na → it is an alcohol, not an ether.
• Lucas in ~5 minutes → secondary; iodoform positive → it has a CH₃-CH(OH)- unit.
• Optical activity needs a stereocentre: the compound is butan-2-ol, CH₃CH(OH)CH₂CH₃; oxidation with PCC gives butan-2-one.

Example 5 - Williamson planning. To make *tert*-butyl methyl ether (MTBE):

• Wrong route: CH₃O⁻ + (CH₃)₃C-Br → the 3° halide undergoes E2, giving isobutene.
• Correct route: (CH₃)₃CO⁻K⁺ + CH₃I → MTBE by Sₙ2 at the unhindered methyl carbon.
• Yield check: 0.100 mol of potassium *tert*-butoxide at 85% yield gives 0.085 mol × 88.15 g/mol = 7.49 g of MTBE.

Where it is used

• Sugar and distillery sector: molasses- and grain-based ethanol for the EBP programme, extra-neutral alcohol for potable liquor, and CO₂ recovery; process and QC chemist roles at Balrampur Chini, Shree Renuka, Triveni and co-operative distilleries in Maharashtra and UP.
• Pharmaceuticals and APIs: salicylic acid → aspirin, paracetamol (a *p*-aminophenol derivative), guaifenesin and countless intermediates; ethanol and isopropanol as solvents and sanitisers - jobs in R&D, process development and QC at Cipla, Sun Pharma, Divi's and Aurobindo.
• Petrochemicals and polymers: cumene-phenol-acetone chains, bisphenol-A for epoxy resins and polycarbonate, ethylene oxide/glycol for PET and antifreeze, polyether polyols for polyurethane foam - Reliance, IOCL, HOCL, GAIL.
• Surfactants, personal care and agrochemicals: fatty alcohol ethoxylates, glycerine in cosmetics and pharma, phenoxy herbicides (2,4-D); Galaxy Surfactants, Godrej Industries, UPL.
• Dyes, resins and adhesives: phenol-formaldehyde (bakelite, plywood resins), azo dyes from diazonium-phenol coupling - the Gujarat and Maharashtra dye clusters.
• Analytical and academic roles: FeCl₃, iodoform and Lucas tests still appear in B.Sc practicals and in QC protocols; GC with FID for alcohol assay, Karl Fischer titration for water in alcohols.

Interview questions

• Q: Why is phenol more acidic than cyclohexanol? A: The phenoxide ion delocalises its negative charge into the ring by resonance and sits on a more electronegative sp² carbon, while the cyclohexanoxide charge stays localised on oxygen.
• Q: How would you separate phenol from benzoic acid in a mixture? A: Shake with saturated NaHCO₃: benzoic acid dissolves with effervescence as its sodium salt while phenol stays in the organic layer, since phenol is weaker than carbonic acid.
• Q: Ethanol and dimethyl ether have the same formula; why do their boiling points differ by about 100 °C? A: Ethanol has an O-H and self-associates through intermolecular hydrogen bonds; the ether has no O-H so only weak dipole and dispersion forces hold it.
• Q: Why is *o*-nitrophenol more volatile than *p*-nitrophenol? A: The *ortho* isomer forms an intramolecular hydrogen bond (chelation), so it cannot associate intermolecularly or hydrate well.
• Q: How does the Lucas test tell 1°, 2° and 3° alcohols apart? A: It forms the alkyl chloride by Sₙ1, so turbidity appears at a rate set by carbocation stability - immediate for 3°, a few minutes for 2° and only on heating for 1°.
• Q: Which alkyl halide and alkoxide would you pick for an unsymmetrical ether? A: The less hindered halide (ideally methyl or primary) with the more hindered alkoxide, because Williamson synthesis is Sₙ2 and a hindered halide would eliminate instead.
• Q: Where does the nucleophile attack an epoxide? A: Under basic conditions at the less substituted carbon (Sₙ2, steric control, anti opening); under acidic conditions at the more substituted carbon, since the protonated ring opens with carbocation-like character.
• Q: What is the industrial route to phenol and what is its co-product? A: The cumene (Hock) process - benzene alkylated with propene, air-oxidised to cumene hydroperoxide, then cleaved with acid to phenol and acetone.
• Q: Why must old bottles of diethyl ether never be distilled to dryness? A: Ether autoxidises in air to peroxides that concentrate in the residue and can detonate; stocks are tested and stabilised or discarded.
• Q: Why does the Reimer-Tiemann reaction give the *ortho* aldehyde? A: Dichlorocarbene attacks the phenoxide, and the *ortho* position is favoured through a hydrogen-bonded/chelated transition state with the alkoxide oxygen.
• Q: 3-Methylbutan-1-ol dehydrated with acid gives 2-methylbut-2-ene rather than the expected alkene. Why? A: The primary carbocation formed rearranges by a 1,2-hydride or methyl shift to a more stable tertiary cation before losing a proton, and the Saytzeff product dominates.
• Q: What is denatured spirit and why is it made? A: Ethanol deliberately adulterated with methanol, pyridine and dyes so it cannot be drunk; it keeps industrial alcohol out of the potable excise net, which is a State subject in India.

Common mistakes

• Calling benzyl alcohol or cyclohexanol a phenol - the -OH must be on the aromatic sp² carbon.
• Claiming phenol liberates CO₂ from NaHCO₃; it does not, and this is the standard separation question.
• Reversing alcohol acidity and dehydration orders - acidity is 1° > 2° > 3° but ease of dehydration and Sₙ1 reactivity are 3° > 2° > 1°.
• Writing the Williamson synthesis with a tertiary halide, or with an aryl halide plus alkoxide, and expecting an ether.
• Esterifying a phenol with a carboxylic acid; phenols need an acid chloride or anhydride.
• Mixing up epoxide regiochemistry between acidic and basic conditions, and forgetting that opening is *anti* (*trans*-diol from hydrolysis).
• Forgetting carbocation rearrangement in acid-catalysed hydration and dehydration, and so predicting the wrong alkene or alcohol.

Related topics

Organic reaction mechanisms · Aldehydes and ketones · Carboxylic acids and derivatives · Hydrocarbons · Stereochemistry · Organic nomenclature and isomerism