Alcohols, Phenols and Ethers

2.1 Core concepts

• Classification of alcohols/phenols — Mono-, di-, tri- or polyhydric based on number of -OH groups; monohydric alcohols are further sub-classified by hybridisation of the C bearing -OH into Csp3-OH (primary, secondary, tertiary, allylic, benzylic) and Csp2-OH (vinylic, aryl). Phenols are similarly classified as mono-, di-, trihydric (NCERT §7.1.1–7.1.2, p. 194–195).
• Ethers are simple/symmetrical if the two R/Ar groups are identical (e.g., diethyl ether) and mixed/unsymmetrical otherwise (e.g., C2H5OC6H5) (NCERT §7.1.3, p. 195).
• IUPAC nomenclature — Alcohol = parent alkane with terminal "e" replaced by "ol", chain numbered to give -OH the lowest locant; polyhydric alcohols retain the "e" and add di/tri-ol (ethane-1,2-diol). Phenols use ortho/meta/para nomenclature in common names but locants in IUPAC (2-methylphenol). Ethers are named with the smaller group as alkoxy substituent on the larger parent (e.g., methoxybenzene = anisole) (NCERT §7.2, p. 195–198; Table 7.1, p. 196; Table 7.2, p. 197).
• Structures — In alcohols, C-O-H angle is slightly less than 109°28′ due to lone-pair repulsion on O; in phenols, C-O bond (136 pm) is shorter than in methanol because of partial double-bond character from conjugation of O lone pair with the ring and sp2 carbon; in ethers C-O-C angle is slightly greater than tetrahedral due to bulky R groups (NCERT §7.3, p. 198–199).
• Preparation of alcohols — (i) Acid-catalysed hydration of alkenes follows Markovnikov's rule via a carbocation intermediate; (ii) Hydroboration-oxidation with (BH3)2 then H2O2/OH gives anti-Markovnikov alcohol in excellent yield (boron adds to the less substituted carbon); (iii) Catalytic hydrogenation (Pt/Pd/Ni) or NaBH4/LiAlH4 reduction of aldehydes gives 1° and of ketones gives 2° alcohols; LiAlH4 reduces carboxylic acids/esters to 1° alcohols; (iv) Grignard reagents with HCHO give 1° alcohol, with other aldehydes give 2° alcohol, with ketones give 3° alcohol (NCERT §7.4.1, p. 199–201).
• Preparation of phenols — (1) Chlorobenzene + fused NaOH at 623 K/320 atm then acidification (Dow process); (2) benzene → benzenesulphonic acid (oleum) → fuse with NaOH → acidify; (3) Aniline → diazonium salt at 273–278 K → warm with water; (4) Cumene process — isopropylbenzene + air → cumene hydroperoxide → dilute acid → phenol + acetone (industrial, gives acetone as by-product) (NCERT §7.4.2, p. 201–202).
• Physical properties — Boiling points rise with carbon number and fall with branching; alcohols and phenols boil far higher than comparable hydrocarbons, ethers and haloalkanes because of intermolecular H-bonding through -OH; ethers lack H-bonding so b.p. ≈ alkanes of similar mass; lower alcohols are miscible with water in all proportions, solubility falls as the hydrophobic alkyl tail grows (NCERT §7.4.3, p. 203–204; §7.6.2, p. 217).
• Acidity of alcohols and phenols — Both are Brønsted acids (reaction with Na/K/Al gives alkoxide/phenoxide + H2); phenols also react with aqueous NaOH but alcohols do not. Electron-releasing alkyl groups raise electron density on O and weaken acidity, so the order is 1° > 2° > 3° alcohol. Alcohols are weaker acids than water (alkoxides are stronger bases than OH−). Phenol (pKa ≈ 10) is about a million times more acidic than ethanol (pKa ≈ 15.9) because the phenoxide ion stabilises negative charge by delocalisation over the ring (five resonance structures), while alkoxide localises the charge on O. Electron-withdrawing groups (especially -NO2 at ortho/para) further raise phenol's acidity (p-nitrophenol pKa 7.1; 2,4,6-trinitrophenol = picric acid is a strong acid); -CH3 (cresols) lowers acidity (NCERT §7.4.4, p. 204–207; Table 7.3, p. 207).
• Reactions involving O-H cleavage — (1) Reaction with metals; (2) Esterification with carboxylic acids (H2SO4 catalyst, reversible), with acid anhydrides (H2SO4), and with acid chlorides (in pyridine to neutralise HCl); acetylation of salicylic acid gives aspirin (NCERT §7.4.4, p. 205–208).
• Reactions involving C-O cleavage (alcohols only, except phenol + Zn) — (1) ROH + HX → RX (Lucas test: alcohol + conc. HCl/ZnCl2 — 3° gives immediate turbidity, 2° gives turbidity in 5 min, 1° gives no turbidity at room temperature); (2) PBr3 converts alcohol → alkyl bromide; (3) Dehydration with conc. H2SO4 / H3PO4 / Al2O3 — ethanol → ethene at 443 K; ease of dehydration 3° > 2° > 1° (more stable carbocation); mechanism = protonation → carbocation (slow, rate-determining) → loss of proton; (4) Oxidation — primary alcohol → aldehyde (with CrO3 anhydrous or PCC) → carboxylic acid (with acidified KMnO4); secondary alcohol → ketone (CrO3); tertiary alcohol resists oxidation but under harsh conditions undergoes C-C cleavage; Cu/573 K dehydrogenates 1°/2° alcohols to aldehydes/ketones while dehydrating 3° alcohols (NCERT §7.4.4, p. 208–210).
• Reactions of phenols only — (i) Nitration — dilute HNO3 / 298 K gives o- and p-nitrophenols (separable by steam distillation because o-isomer is steam-volatile due to intramolecular H-bonding while p-isomer associates by intermolecular H-bonding); conc. HNO3 gives 2,4,6-trinitrophenol (picric acid). (ii) Halogenation — Br2 in CHCl3/CS2 at low temperature gives monobromophenols (mainly p-); Br2 in water gives 2,4,6-tribromophenol as a white precipitate. (iii) Kolbe's reaction — phenoxide + CO2 gives ortho-hydroxybenzoic acid (salicylic acid). (iv) Reimer-Tiemann — phenol + CHCl3 + aq. NaOH introduces -CHO at the ortho position, giving salicylaldehyde via a benzal-chloride intermediate. (v) Phenol + Zn dust → benzene. (vi) Oxidation with chromic acid → benzoquinone (NCERT §7.4.4, p. 211–213).
• Commercial alcohols — Methanol ("wood spirit") made by CO + 2H2 over ZnO-Cr2O3 catalyst at high T, P; b.p. 337 K; toxic (causes blindness/death). Ethanol made by fermentation of sugars (invertase converts sucrose → glucose + fructose; zymase converts these to ethanol + CO2 anaerobically; zymase is inhibited above ~14% ethanol). Now also made by hydration of ethene. Denaturation = adding CuSO4 + pyridine to make it unfit for drinking (NCERT §7.5, p. 214).
• Preparation of ethers — (1) Acid dehydration of alcohols — ethanol with conc. H2SO4 at 413 K gives ethoxyethane (S_N2 attack of alcohol on protonated alcohol); works only for primary, unhindered alkyl groups at low T; fails for 2°/3° (elimination wins). (2) Williamson synthesis — RX + R′O−Na+ → R-O-R′ (S_N2); best with primary RX (with 2°/3° halide elimination dominates — e.g. CH3ONa + (CH3)3CBr gives 2-methylpropene exclusively); phenoxides are also used to give aryl ethers (NCERT §7.6.1, p. 215–217).
• Reactions of ethers — (1) HX cleavage — dialkyl ether + excess HX (HI > HBr > HCl) gives two alkyl halides; mixed ethers cleave by S_N2 with I− attacking the less substituted C, so the smaller alkyl group becomes the alkyl iodide; if one group is tertiary the reaction switches to S_N1 (stable 3° carbocation) and the 3° group ends up as the halide; alkyl aryl ethers (e.g., anisole + HI) cleave at the alkyl-O bond giving phenol + alkyl halide because aryl-O is stronger (partial double-bond, sp2 C cannot undergo S_N) and phenols do not react further with HI. (2) Electrophilic substitution in aryl alkyl ethers — alkoxy group is activating and ortho/para directing: anisole + Br2/CH3COOH gives p-bromoanisole (90%) without FeBr3; Friedel-Crafts alkylation/acylation with AlCl3 gives o-/p- products; nitration with conc. HNO3/H2SO4 gives o-/p-nitroanisole (NCERT §7.6.3, p. 217–220).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Fig. 7.1 — Structures of methanol, phenol and methoxymethane showing bond angles and C-O bond lengths (136 pm in phenol, 141 pm in ether) (p. 198).
• Resonance structures (I–V) of phenoxide ion showing delocalisation of negative charge onto ortho and para carbons of the ring — basis of phenol's acidity (p. 206).
• Three-step mechanism of acid-catalysed hydration of alkene — protonation → nucleophilic water attack → deprotonation (p. 199).
• Three-step mechanism of ethanol dehydration — protonation → carbocation (slow, RDS) → loss of H+ to give ethene (p. 209).
• HI cleavage mechanism of an ether — protonation to oxonium, then S_N2 attack of I− on the less substituted C (for primary/secondary), but S_N1 path with retention of the tertiary group as the halide when one alkyl is 3° (p. 218).
• Industrial cumene → cumene hydroperoxide → phenol + acetone scheme (p. 202).
• Intramolecular H-bonding in o-nitrophenol vs. intermolecular in p-nitrophenol — explains steam volatility (p. 211).
• Table 7.3 pKa data: phenol 10.0; o/p-nitrophenol 7.2/7.1; m-nitrophenol 8.3; cresols ≈ 10.2; ethanol 15.9 (p. 207).

2.4 Common confusions / NTA trap points

• Acidity order — Students often write tertiary > secondary > primary alcohol; the correct order (per NCERT) is primary > secondary > tertiary, because electron-releasing alkyls destabilise the alkoxide. The full chain is phenol > water > 1° > 2° > 3° alcohol.
• Markovnikov vs anti-Markovnikov — Acid-catalysed hydration gives Markovnikov product (OH on more substituted C); hydroboration-oxidation gives the anti-Markovnikov product. NTA often pairs these in the same MCQ.
• Lucas test timings — 3° = immediate; 2° = within ~5 minutes; 1° = no turbidity at RT. Mixing these up is a frequent trap.
• Br2 in CS2/CHCl3 vs Br2 in water — Low-polarity solvent at low T gives monobromophenol (mainly p-); aqueous bromine gives the white precipitate 2,4,6-tribromophenol — used as a test for phenol.
• HI cleavage of anisole — Always gives phenol + CH3I (never methanol + iodobenzene), because the aryl-O bond is stronger and phenols can't undergo nucleophilic substitution at sp2 C.
• Williamson with tertiary halide — Gives the alkene (elimination), not the ether; the correct combination for t-butyl methyl ether is t-butoxide + methyl halide, not methoxide + t-butyl halide.
• Steam-volatile isomer — o-nitrophenol (intramolecular H-bonding) is steam-volatile; p-nitrophenol (intermolecular H-bonding) is not.