This chapter covers three classes of oxygen-containing organic compounds: alcohols, phenols, and ethers. The lecture systematically covers their preparation, chemical reactions, and key properties, with emphasis on mechanisms and board-relevant reactions.
Introduction and Classification ⏱ 0:00
•Alcohols, phenols, and ethers all contain oxygen as a heteroatom.•Alcohols: hydrocarbon compounds with an -OH group.•Ethers: oxygen bonded to two alkyl or aryl groups.•Phenols: benzene ring with an -OH group.•Alcohols classified by hybridization of carbon bearing OH: sp3 (saturated), sp2 (vinyl alcohol), sp (alkynol).•Vinyl alcohol undergoes tautomerism to form ketones or aldehydes; keto form is more stable.•Classification by number of -OH groups: monohydric, dihydric (e.g., ethylene glycol), trihydric (e.g., glycerol), polyhydric.•Vicinal diols have -OH on adjacent carbons; geminal diols have two -OH on same carbon (unstable, dehydrates to carbonyl).Preparation of Alcohols from Alkenes ⏱ 5:40
•Dilute H2SO4 (hydration): H+ adds to alkene forming carbocation; rearrangement may occur. Water attacks carbocation, then deprotonation yields alcohol. Follows Markovnikov's rule but consideration of carbocation stability is essential.•Osmium tetroxide (OsO4) with H2O2: Syn addition of two -OH groups across double bond, giving vicinal diol (syn diol).•Lead tetraacetate: Similar syn dihydroxylation.•KMnO4 (cold, dilute) – Baeyer's reagent: Syn addition of -OH groups, producing vicinal diol.•Hydroboration-oxidation: BH3 (from B2H6 in THF) adds to alkene; the boron atom attaches to less substituted carbon. Then H2O2/OH- oxidizes the C-B bond to alcohol. Anti-Markovnikov addition, no carbocation rearrangement.•Oxymercuration-demercuration: Hg(OAc)2 in water followed by NaBH4. Markovnikov addition, no rearrangement.•Epoxidation then ring opening: Peracid (RCO3H) forms epoxide; H+ opens epoxide via Sn2 attack by water, giving anti vicinal diol.Preparation of Alcohols via Grignard Reagents and Other Methods ⏱ 45:50
•From alkyl halides: Moist Ag2O (source of OH-) gives alcohol via Sn2.•Grignard reagent (RMgX): Prepared from alkyl halide and Mg in dry ether. Reacts with: - Epoxides: forms primary alcohol (after workup), with attack at less hindered carbon.
- Esters: yields tertiary alcohol (after two equivalents) or can stop at ketone.
- Formaldehyde: primary alcohol.
- Other aldehydes: secondary alcohol.
- Ketones: tertiary alcohol.
- Acid chlorides: tertiary alcohol.
•Reaction with O2: Grignard reagent with oxygen then hydrolysis gives alcohol.Conversion of Alcohols to Alkyl Halides ⏱ 55:40
•SOCl2 (thionyl chloride): Converts alcohol to RCl; byproducts SO2 and HCl gases escape, driving reaction forward. Follows SN1 mechanism (racemization).•SOCl2 + pyridine: Inversion of stereochemistry; SN2 mechanism.•HX: Primary alcohols SN2, secondary/tertiary SN1.•Lucas test (ZnCl2 + conc. HCl): Turbidity appears immediately for tertiary alcohols, within 5-15 min for secondary, and longer (with heating) for primary.•PCl5: Produces RCl, with POCl3 and HCl as byproducts.•PCl3: All three chlorines used, forming RCl; byproduct H3PO3.•PBr3, PI3: Similar reactions for bromo and iodo compounds.•Most atom-economical: PCl3 uses all halogens.Dehydration and Oxidation of Alcohols ⏱ 63:20
•Dehydration: - Conc. H2SO4 or H3PO4: E1 mechanism, carbocation rearrangement, Zaitsev product (more substituted alkene).
- P2O5 or Al2O3 (heat): Dehydration, gives alkene (Zaitsev).
- ThO2 (heat above 400°C): Dehydration gives alkene via E2, Hofmann product (less substituted).
- Al2O3 (high temp): Also Hofmann product.
•Oxidation: - Strong oxidizing agents (KMnO4 alcoholic, acidic K2Cr2O7): Primary alcohol → carboxylic acid; secondary → ketone; tertiary → no reaction.
- Mild oxidizing agents (PCC, PDC, Collins reagent, Cu/300-400°C): Primary alcohol → aldehyde only; secondary → ketone.
- KMnO4 (purple) → MnO2 (brown) when reduced.
- K2Cr2O7 (orange) → Cr3+ (green).
Ethers: Preparation ⏱ 71:40
•From alcohols: Ethanol + H2SO4 at ~140°C gives diethyl ether (not alkene).•Williamson ether synthesis: Alkoxide ion (R-O-) reacts with alkyl halide via SN2; best for primary halides.•Diazomethane (CH2N2): Reacts with alcohol to give methyl ether, with N2 as byproduct.•Alkoxymercuration: Alkene + Hg(OAc)2 + alcohol, then NaBH4 reduction gives ether (Markovnikov addition).•Reaction of alkyl halide with dry Ag2O: Forms symmetrical ether.•From alkyl halide + sodium alkoxide: Symmetrical or unsymmetrical ether.Ethers: Reactions ⏱ 78:50
•Halogenation: - In dark: X2 replaces one α-hydrogen on each side of oxygen (α-haloether).
- In light: complete halogenation of all hydrogens.
•With HCl: Cleaves ether to alkyl halide and alcohol (H+ attacks oxygen, Cl- attacks carbon).•With acyl chloride (RCOCl)/ZnCl2: Forms ester and alkyl halide.•Combustion: Produces CO2 and H2O (water in gaseous state).•With dilute H2SO4 (H+): Ether protonated, water attacks, producing two alcohols (or one if symmetrical).•With HI: - Cold: alkyl halide + alcohol.
- Hot/excess: two alkyl halides (cleavage at both C-O bonds).
•With PCl5: Forms two alkyl chlorides.Phenols: Preparation and Reactions ⏱ 83:30
•Preparation: - Sodium benzenesulfonate fusion with NaOH.
- Diazonium salt hydrolysis (warm water).
- Decarboxylation of salicylic acid (soda lime + CaO).
- Cumene process: cumene + O2 → cumene hydroperoxide, acid treatment gives phenol.
- Grignard reagent (PhMgBr) + O2 → phenol after hydrolysis.
- Dow process: chlorobenzene + NaOH (high T, P).
•Reactions: - Bromine water: gives 2,4,6-tribromophenol (white precipitate). In CS2 (non-polar): para-bromophenol (93%) + ortho (7%).
- Conc. H2SO4 at 100°C: gives ortho- and para-phenolsulfonic acids; desulfonation possible.
- Nitration: with HNO3/H2SO4 gives trinitrophenol (picric acid) directly or via sulfonation intermediate.
- With NaNO2/H2SO4: Liebermann nitroso test (blue-green → red with H2O → blue with NaOH).
- Reimer–Tiemann reaction: chloroform + NaOH gives carbene (CCl2), then formyl group (ortho major).
- Gattermann reaction: HCN + HCl gives para-formyl phenol major.
- With acetic anhydride: forms phenyl acetate (pink color test with FeCl3).
- Kolbe–Schmitt reaction: CO2 + NaOH at 125°C gives salicylic acid (ortho major); at higher temp, para major.
- Coupling with diazonium salt: gives colored azo dyes.
- Condensation with acetone (conc. HCl): gives bisphenol A.
- With formaldehyde + NaOH: forms Bakelite (thermosetting polymer).
Key Takeaways
•Alcohols are classified by carbon hybridization (sp3, sp2, sp) and number of -OH groups, with vinyl alcohols undergoing keto-enol tautomerism.•Hydration of alkenes with dilute H2SO4 follows Markovnikov's rule via carbocation intermediates, which may rearrange before water attack.•Hydroboration-oxidation provides anti-Markovnikov alcohols without rearrangement, using BH3 then H2O2/OH-.•Grignard reagents react with carbonyl compounds (aldehydes, ketones, esters, acid chlorides) and epoxides to produce alcohols of varying substitution.•The Lucas test distinguishes primary, secondary, and tertiary alcohols based on turbidity time with ZnCl2/concentrated HCl.•Mild oxidizing agents like PCC stop at aldehydes, while strong agents like KMnO4 or K2Cr2O7 oxidize primary alcohols to carboxylic acids.•Ethers are cleaved by HI to give alkyl halides; Williamson ether synthesis is a key nucleophilic substitution method for ether preparation.•Phenols undergo electrophilic substitution (bromination, nitration, sulfonation) and special reactions like Reimer–Tiemann and Kolbe–Schmitt, forming ortho/para substituted products.Conclusion
This lecture provides a complete summary of alcohols, phenols, and ethers, covering every reaction from NCERT and beyond, with emphasis on mechanistic understanding.
