Alcohols, Phenols & Ethers

Alcohols, phenols, and ethers are oxygen-containing organic compounds central to NEET organic chemistry. Their classification, reactivity differences, and named reactions form the core of this session.

Alcohols are classified as 1 degree (RCH₂OH, e.g., ethanol CCO), 2 degree (R₂CHOH, e.g., propan-2-ol CC(O)C), and 3 degree (R₃COH, e.g., 2-methylpropan-2-ol ). Preparation methods include acid-catalyzed hydration of alkenes (Markovnikov product), Grignard reaction (RMgX + carbonyl → alcohol after hydrolysis), and reduction of aldehydes/ketones (NaBH₄ or LiAlH₄) or carboxylic acids (LiAlH₄ only — NaBH₄ is too weak).

Dehydration to alkenes follows Saytzeff's rule: ROH + conc. H₂SO₄ at 443 K → more substituted alkene (major). Ease of dehydration: 3 degree > 2 degree > 1 degree (more stable carbocation intermediate). The oxidation ladder is crucial: 1 degree alcohol →(PCC, mild) aldehyde →(KMnO₄ or K₂Cr₂O₇, strong) carboxylic acid; 2 degree alcohol → ketone (with any oxidizing agent); 3 degree alcohol → resistant to oxidation (requires C-C bond cleavage). PCC (pyridinium chlorochromate) is the mild, selective reagent that STOPS at the aldehyde stage.

The Lucas test distinguishes alcohol types using ZnCl₂/conc. HCl: 3 degree → immediate turbidity (within 5 minutes, SN₁ — stable carbocation); 2 degree → turbidity in 5-20 minutes; 1 degree → no turbidity at room temperature (SN₂ too slow without heating).

Phenol (, pKa ~10) is significantly more acidic than alcohols (pKa ~16-18) because the phenoxide ion is stabilized by resonance — the negative charge delocalizes over the benzene ring through five resonance structures. Electron-withdrawing groups (-NO₂) increase acidity (p-nitrophenol is more acidic than phenol), while electron-donating groups (-CH₃) decrease it. Phenol undergoes electrophilic substitution more easily than benzene: -OH is a strong activating o/p director. Bromine water (Br₂/H₂O) directly gives 2,4,6-tribromophenol (white precipitate) WITHOUT any Lewis acid catalyst — the -OH group activates the ring sufficiently.

Named Reactions of Phenol: Kolbe's reaction (Kolbe-Schmitt): C₆H₅O-Na+ + CO₂ →(125 deg C, 4-7 atm) sodium salicylate →(H₃O+) salicylic acid (). Reimer-Tiemann reaction: C₆H₅OH + CHCl₃ + NaOH → salicylaldehyde (, ortho-hydroxybenzaldehyde); intermediate is dichlorocarbene (:CCl₂).

Ethers: Williamson synthesis is the key preparation: R-O-Na+ + R'-X → R-O-R' + NaX. This is an SN₂ reaction, so always use a 1 degree alkyl halide (2 degree/3 degree give elimination instead). The alkoxide provides the oxygen; the halide provides the carbon. Ether cleavage by excess HI: R-O-R' + HI → RI + R'OH, then R'OH + HI → R'I + H₂O. For unsymmetrical ethers, the smaller alkyl group preferentially forms the iodide (SN₂ path).

The key testable concept is that PCC selectively oxidizes 1 degree alcohol to aldehyde (stops there), while KMnO₄/K₂Cr₂O₇ oxidize all the way to carboxylic acid — and that Williamson synthesis requires a 1 degree halide to avoid elimination.