Alcohols, Phenols, Ethers, Carbonyl Compounds and Carboxylic Acids

1. Alcohols: Acidity, Tests, Dehydration and Oxidation

• Acidity order of the O–H family: RCOOH > H₂CO₃ > phenol > H₂O > ROH, and within the alcohols CH₃OH > 1° > 2° > 3°. Alkyl groups push electrons (+I) onto oxygen and also block solvation of the alkoxide, so 3° alcohols are the weakest acids. JEE keys treat water as more acidic than every alcohol (methanol is in fact nearly equal to water).
• What dissolves in what: carboxylic acids dissolve in NaHCO₃ (CO₂ fizz) and NaOH; ordinary phenols dissolve in NaOH only; alcohols dissolve in neither but react with sodium metal (H₂ gas). This three-step ladder is the standard way a question separates an acid, a phenol and an alcohol in one mixture.

• Lucas test (anhydrous ZnCl₂ + conc. HCl) is an SN1 test: turbidity is the insoluble alkyl chloride separating out. Allylic and benzylic 1° alcohols give cloudiness at once, like 3°, because their carbocations are resonance-stabilised. Methanol and lower 1° alcohols need heating and can fail it altogether.
• Oxidation: PCC (or Cu at 573 K) stops a 1° alcohol at the aldehyde; K₂Cr₂O₇/H⁺, KMnO₄ or Jones reagent runs it on to the acid. A 2° alcohol gives a ketone with any of these; a 3° alcohol resists, which is why oxidation separates 3° from 1°/2° in a mixture.

• Ether vs alkene: ethanol with conc. H₂SO₄ gives diethyl ether at 413 K (excess alcohol, SN2) and ethene at 443 K (excess acid, elimination). Because a carbocation is involved above that temperature, rearrangement is possible: 3,3-dimethylbutan-2-ol gives 2,3-dimethylbut-2-ene by a 1,2-methyl shift, not the unrearranged alkene.
• Diols and triols: ethylene glycol is cleaved by HIO₄ or Pb(OAc)₄ between the two carbinol carbons to give 2 HCHO (this cleavage works on any vicinal diol); glycerol dehydrates with KHSO₄ to pungent acrolein (CH₂=CH–CHO) and nitrates to nitroglycerine (dynamite when absorbed on kieselguhr).

2. Phenols: Acidity and Ring Reactions

• Why phenol beats an alcohol by ~6 pKa units: the phenoxide ion delocalises its charge into the ring (resonance), while an alkoxide is localised on oxygen and destabilised by +I. Substituents then act exactly as in benzoic acid: −M/−I groups at ortho/para (NO₂) strengthen it enormously, +I/+M groups (CH₃, OCH₃) weaken it slightly, and meta-NO₂ helps only through −I (a smaller effect). Order: p-nitrophenol > o-nitrophenol > m-nitrophenol > phenol > p-cresol; picric acid sits far above all of them.
• Reactivity of the ring: −OH is a strong o/p director and activator, so phenol brominates in water without a catalyst to the tribromo compound. Friedel–Crafts is poor on phenol because AlCl₃ coordinates to oxygen.

• Preparation of phenol: cumene hydroperoxide rearrangement (phenol + acetone, the industrial route); chlorobenzene with NaOH at 623 K and 300 atm (Dow); diazonium salt warmed with water; sodium benzenesulphonate fused with NaOH.
• Identification: neutral FeCl₃ gives a violet colour; Br₂/water gives a white precipitate; NaHCO₃ gives no fizz (except picric acid).

3. Ethers: Williamson Design and HI Cleavage

• Williamson = SN2. Choose the pair so the halide is methyl or 1° and the alkoxide carries any bulky group. A 3° halide with an alkoxide gives an alkene (E2); an aryl or vinyl halide gives nothing. Intramolecular Williamson on a halohydrin (X–CH₂CH₂–OH + base) closes an epoxide.
• HI cleavage decides by mechanism: primary/secondary alkyl groups go by SN2, so iodide attacks the smaller alkyl; a tertiary group goes by SN1, so iodide ends up on the tertiary carbon. An alkyl aryl ether always gives phenol + alkyl iodide. With excess HI both alkyl groups end up as RI.
• Physical and hazard facts: ethers have b.p. close to alkanes of equal mass (no H-bond donor) but dissolve in cold conc. H₂SO₄ (oxonium salt formation), which separates them from alkanes. On standing in air they form explosive hydroperoxides at the α-carbon; FeSO₄/KSCN turns blood-red (Fe³⁺) to detect them, and shaking with FeSO₄ or storing over Cu₂O/Na removes them.

4. Making Aldehydes and Ketones

• Shared routes: oxidation or dehydrogenation of 1°/2° alcohols; ozonolysis with Zn/H₂O (the Zn stops over-oxidation by H₂O₂); hydration of alkynes with Hg²⁺/H₂SO₄ (ethyne gives CH₃CHO, every other alkyne a ketone); alkaline hydrolysis of gem-dihalides (terminal gives aldehyde, non-terminal gives ketone).
• Ketone-specific routes: Friedel–Crafts acylation (ArH + RCOCl/AlCl₃); R₂Cd + RCOCl (stops at the ketone, unlike RMgX); nitrile + RMgX then H₃O⁺; dry distillation of calcium salts; decarboxylation of β-keto acids; Wacker oxidation of higher 1-alkenes.
• Counting from ozonolysis: one mole of a compound giving two carbonyls in one molecule means a ring or diene was present; the number of C=O groups produced tells you how many C=C were cleaved.

5. Nucleophilic Addition to C=O

• Mechanism: the nucleophile attacks the planar sp² carbon perpendicular to the plane (slow step) to give a tetrahedral alkoxide, then protonation. Reactivity falls as steric crowding and electron donation rise, and is raised by −I groups (chloral hydrate, ninhydrin, glyoxal hydrates are stable gem-diols).
• HCN gives a cyanohydrin (base-catalysed, because CN^⊖ is the real nucleophile); hydrolysis gives an α-hydroxy acid. NaHSO₃ gives a crystalline adduct with aldehydes, methyl ketones and cyclic ketones, but not bulky ketones (acetophenone, benzophenone), and the carbonyl is regenerated with acid or alkali, so it is used for purification.
• Alcohols: aldehyde + 2 ROH/HCl(g) gives an acetal; ketone + ethylene glycol gives a cyclic ketal. Acetals are stable to base, Grignard reagents and LiAlH₄, but hydrolysed by dilute aqueous acid, which makes them protecting groups (protect the aldehyde, reduce the ester elsewhere, deprotect).

6. α-Hydrogen Chemistry: Aldol, Cannizzaro, Haloform

• α-H acidity (pKa ~19–20) comes from the −I effect of C=O and resonance in the enolate. It drives aldol, haloform, HVZ and every condensation below.
• Aldol: enolate formation, attack on another carbonyl (rate-determining), protonation, then dehydration by E1cB on heating. Intramolecular aldol of 1,4- and 1,5-diketones closes 5- and 6-membered rings. Count the products: two different carbonyls both with α-H give four products; one without α-H gives two; neither gives none.
• Cannizzaro: no α-H, conc. alkali, hydride transfer is rate-determining. HCHO, ArCHO and R₃C–CHO qualify; glyoxal gives glycolate by the intramolecular version. Remember that an aldehyde WITH α-H under the same conditions does aldol instead.
• Haloform (X₂ + NaOH, i.e. NaOX): substrate must contain CH₃CO– or CH₃CH(OH)– (the latter is first oxidised by NaOX). Products: CHX₃ + carboxylate with one carbon fewer. Iodoform is a yellow solid (m.p. 119 °C).
• Related condensations: Claisen–Schmidt (ArCHO + methyl ketone, base), Perkin (ArCHO + anhydride + sodium salt of the acid, cinnamic acid), Knoevenagel (active methylene + carbonyl, pyridine/piperidine), benzoin (2 PhCHO, CN^⊖ catalyst).

7. Reduction, Oxidation and Distinguishing Tests

• Pick Clemmensen or Wolff–Kishner by the other groups present: acid-sensitive (acetal, tertiary OH) means Wolff–Kishner; base-sensitive (ester, alkyl halide) means Clemmensen.
• Mild oxidants: Tollens' reagent (ammoniacal AgNO₃, silver mirror) oxidises ALL aldehydes, including benzaldehyde, plus α-hydroxy ketones (fructose) and formic acid. Fehling's and Benedict's (Cu²⁺ to red Cu₂O) oxidise aliphatic aldehydes only, not benzaldehyde. Schiff's test restores magenta colour with aldehydes, not ketones.
• Baeyer's test (cold dilute KMnO₄, purple to brown) detects unsaturation (C=C, C≡C) and some oxidisable groups; carbonyl compounds with no C=C do not respond, which separates them from unsaturated alcohols.

• Formaldehyde and acetaldehyde polymers: HCHO gives paraformaldehyde (linear) and trioxane (cyclic trimer), and urotropine with NH₃ (urinary antiseptic); CH₃CHO gives paraldehyde (trimer, conc. H₂SO₄, sedative) and metaldehyde (tetramer, dry HCl at 0 °C). Acetone with conc. H₂SO₄ gives mesitylene (1,3,5-trimethylbenzene).

8. Named Rearrangements and the Wittig Reaction

• Pinacol: protonate one OH, lose water to the more stable cation, then a group from the neighbouring carbon shifts and a carbonyl forms. The exact aryl-versus-hydrogen ranking varies with the substrate, so JEE questions are built where the cation or the aryl group clearly decides.
• Beckmann stereospecificity: the group anti to the OH migrates, so the E or Z oxime geometry decides which amide forms. Acetophenone oxime (phenyl anti to OH) gives acetanilide, which hydrolyses to aniline and acetic acid.
• Wittig: Ph₃P=CR₂ + C=O gives C=C plus Ph₃P=O through an oxaphosphetane; the strong P=O bond is the driving force, and the double bond forms exactly where the carbonyl was.

9. Grignard Reagent as a Synthetic Tool

• Preparation rules: R–X + Mg in dry ether (the ether lone pairs solvate Mg); reactivity of halide I > Br > Cl >> F. The substrate must not contain acidic H (–OH, –COOH, –NH₂, terminal alkyne) or a group the reagent would attack (–CHO, –CN, –NO₂).
• Reaction priority in a polyfunctional molecule: acidic H > aldehyde > ketone > acid chloride/ester > alkyl halide. A Grignard reagent plus terminal alkyne gives the alkyne's Grignard and an alkane (CH₄ from CH₃MgBr): this is how methane volumes get measured in numericals.

10. Carboxylic Acids: Acidity and Reactions

• Dimer and physical trends: acids exist as cyclic hydrogen-bonded dimers even in the vapour, giving b.p. above alcohols of similar mass; even-carbon acids melt higher than their odd neighbours (better packing).
• Dicarboxylic acids: oxalic pKa₁ 1.27 is far stronger than a monoacid because the second COOH is adjacent and electron-withdrawing; maleic (cis) is a stronger first acid than fumaric (trans) because the mono-anion is stabilised by an intramolecular H-bond, but its second dissociation is weaker for the same reason.
• Reactions of the acid: Fischer esterification (acid-catalysed, reversible; use excess alcohol or remove water; the oxygen of the alcohol ends up in the ester, so acyl–oxygen bond breaks); HVZ (X₂/red P then H₂O, needs an α-H, so HCOOH and benzoic acid cannot); LiAlH₄ or B₂H₆ reduce COOH to CH₂OH (NaBH₄ cannot, and B₂H₆ leaves esters and NO₂ untouched); Arndt–Eistert adds one CH₂ (SOCl₂, CH₂N₂, Ag₂O/H₂O).
• Decarboxylations: soda-lime on RCOONa gives R–H (one carbon fewer); Kolbe electrolysis of RCOO^⊖ gives R–R at the anode (2(n−1) carbons); Hunsdiecker (RCOOAg + Br₂, CCl₄) gives R–Br with one carbon fewer by free-radical chain; β-keto acids and malonic acids lose CO₂ through a six-membered cyclic transition state at mild temperature.
• Ring substitution: −COOH is a meta director and deactivator, so nitration or chlorination of benzoic acid gives the 3-substituted product.

11. Acid Derivatives and Heating Effects

• Why the order: a weaker conjugate base leaves faster. Acyl chlorides also lack effective Cl→C=O resonance donation (poor 3p–2p overlap), while amides have strong N→C=O donation, so they are least electrophilic. Water-sensitive acyl chlorides and anhydrides cannot be handled in protic solvents.
• Acyl chloride toolbox: + R₂Cd gives a ketone (stops there); + RMgX (excess) gives a 3° alcohol; + LiAlH₄ gives RCH₂OH; + H₂/Pd–BaSO₄ gives RCHO (Rosenmund); + benzene/AlCl₃ gives a ketone.
• Esters: acid hydrolysis is reversible, saponification is irreversible (carboxylate is deprotonated and unreactive); LiAlH₄ or Na/EtOH (Bouveault–Blanc) gives two alcohols; DIBAL-H at −78 °C stops at the aldehyde; pyrolysis of an ester with a β-H on the alkyl side gives alkene + acid by syn elimination through a six-membered transition state.
• Amides: dehydration (P₂O₅) gives a nitrile; LiAlH₄ reduces to an amine with the SAME carbon count; Hofmann bromamide degradation (Br₂/KOH) gives an amine with ONE carbon fewer (details in Unit 18).

12. How JEE Frames These Questions

• Match-the-list on reagent → name: Rosenmund (H₂, Pd–BaSO₄), Stephen (SnCl₂, HCl), Etard (CrO₂Cl₂, CS₂), Gattermann–Koch (CO, HCl, AlCl₃), Wolff–Kishner (N₂H₄, KOH), Clemmensen (Zn–Hg, HCl), Tollens, Fehling. Learn each reagent→name pair as a flashcard, in both directions.
• Which pairs does a test distinguish? For each pair, ask whether exactly one member has the required group (CH₃CO–, free CHO, aliphatic CHO, acidic COOH). Pairs like ethanol/methanol and anisole/acetone work for the iodoform test; acetic/propanoic acid and diethyl ether/pentan-3-one do not.
• Statement-I / Statement-II on aldol: the "always gives four products" statement is false whenever one partner lacks α-H; read for "always", "only", "never".
• Hidden-aldehyde identification: a compound that is iodoform-positive but Tollens-negative, and after dilute acid is positive in both, is an acetal of a keto-aldehyde, e.g. CH₃COCH₂CH(OCH₃)₂.
• Rank the acids from reaction sequences: identify each product first (diazonium → CuCN → hydrolysis gives benzoic acid; oxidation of an aldehyde gives its own acid; RMgX + CO₂ gives RCOOH one carbon longer), then rank by pKa: HCOOH > PhCOOH > PhCH₂COOH > CH₃CH₂COOH.
• Isomer-set statements (same formula, different functional groups): test each statement against EVERY isomer separately — 2,4-DNP positive for all carbonyls, Baeyer's for only those with C=C, Tollens for only the aldehyde, iodoform for only the methyl ketone. All aromatic isomers give a sooty flame, so a statement naming several of them can still be true.

13. Quick Sheet and Checklist

Before the exam, check you can:

• Rank phenol, picric acid, acetic acid, carbonic acid, water and ethanol, and say which dissolve in NaHCO₃ and NaOH.
• Predict ether versus alkene from ethanol and conc. H₂SO₄ by temperature, and spot when a carbocation rearrangement intervenes.
• Design a Williamson synthesis for any unsymmetrical ether, and predict which bond HI cleaves in a given ether.
• Write the Kolbe and Reimer–Tiemann products and name the electrophile in each.
• Match every named aldehyde/ketone preparation to its reagent and starting material.
• Give BOTH reasons (electronic and steric) for aldehydes reacting faster than ketones.
• Decide aldol, Cannizzaro or crossed variants from the α-H count alone, and count the products.
• State exactly which compounds pass iodoform, Tollens, Fehling, Schiff and 2,4-DNP.
• Choose Clemmensen or Wolff–Kishner, NaBH₄ or LiAlH₄ from the other groups present.
• Say which group migrates in Pinacol, Beckmann and Baeyer–Villiger.
• Predict Grignard products from HCHO, RCHO, ketones, esters, nitriles, CO₂ and epoxides, and explain why acidic H wrecks it.
• Rank substituted benzoic and chloroacetic acids by pKa, including the meta/para methoxy and ortho effects.
• Predict the product of heating each dicarboxylic and hydroxy acid.