JEE Chemistry Alcohols Phenols and Ethers Notes Set 10

Classification and Names

Alcohols have an OH group on a saturated carbon; phenols have OH on an aromatic ring; ethers have an oxygen linking two carbon groups.
By the number of OH groups: monohydric (ethanol), dihydric (ethylene glycol, catechol, resorcinol) and trihydric (glycerol, pyrogallol). By the carbon holding OH: primary (\( \ce{RCH2OH} \)), secondary (\( \ce{R2CHOH} \)) and tertiary (\( \ce{R3COH} \)). Benzyl and allyl alcohols have OH next to a ring or a double bond.
Common names to know: carbinol is methanol; wood spirit is methanol; carbolic acid is phenol; picric acid is 2,4,6-trinitrophenol; cresols are methylphenols; methylated spirit is ethanol with about 5 percent methanol; rectified spirit is 95.6 percent ethanol; absolute alcohol is 100 percent ethanol; amyl alcohol is pentanol (\( \ce{C5H12O} \)). Ethers: simple (same groups) and mixed; epoxides (oxirane) are cyclic ethers; acetals have two OR groups on one carbon. Ethers and alcohols of the same formula (for example \( \ce{C2H6O} \)) are functional isomers; ethers with different groups on oxygen (\( \ce{C4H10O} \)) are metamers (three of them).

Preparation of Alcohols

Preparation of Phenol and Ethers

Phenol: cumene (isopropylbenzene) is oxidised by air to hydroperoxide, which with dilute acid gives phenol and acetone; benzenediazonium chloride warmed with water; fusion of sodium benzenesulphonate with NaOH; Dow process (chlorobenzene with NaOH at high T and P); distillation of coal tar.
Ethers: dehydration of excess alcohol with conc. \( \ce{H2SO4} \) at 413 K (ethene at 443 K); Williamson synthesis \( \ce{RONa + R'X} \) (primary halide, SN2; with a tertiary halide the alkene forms, so methyl tert-butyl ether is made from sodium tert-butoxide and methyl iodide); alkyl halide with dry \( \ce{Ag2O} \); alcohol and alkene with acid; sodium phenoxide with alkyl halide gives alkyl aryl ethers.

Physical Properties

Hydrogen bonding makes alcohols boil much higher than alkanes, ethers or thiols of comparable mass. Boiling point increases with molecular mass and falls with branching: 1° > 2° > 3°. Lower alcohols are miscible with water; solubility falls with longer alkyl chains, and a branched isomer (tert-butyl alcohol) is more soluble than its n-isomer. Ethanol-water forms an azeotrope (95.6 percent), and mixing is exothermic with contraction in volume. Ortho-nitrophenol has intramolecular hydrogen bonding, so it is steam volatile and has a low melting and boiling point, while para-nitrophenol is associated. Glycerol is viscous because of extensive hydrogen bonding. Ethers have low boiling points and are only slightly soluble in water.

Acidity and Reactions of the OH Group

Acid strength: carboxylic acid > carbonic acid > phenol > water > primary alcohol > secondary > tertiary. Alkoxide stability falls with alkyl donation, so 1° alcohols react fastest with sodium. Phenoxide is stabilised by resonance, so phenol dissolves in NaOH but not in NaHCO3. Electron-withdrawing groups (nitro, especially at ortho and para) raise phenol acidity; donating groups (methyl, methoxy) lower it. Order: picric acid > 2,4-dinitrophenol > p-nitrophenol > m-nitrophenol > phenol > p-cresol.
Reactions of alcohols: with sodium gives alkoxide and H2; with HX (Lucas reagent, \( \ce{ZnCl2} \)/HCl), \( \ce{PCl5} \), \( \ce{PCl3} \), \( \ce{PBr3} \) or \( \ce{SOCl2} \) gives alkyl halides; with carboxylic acids and conc. \( \ce{H2SO4} \) gives esters (esterification: 1° > 2° > 3°); heated with conc. \( \ce{H2SO4} \) gives ether (413 K) or alkene (443 K); dehydration reactivity 3° > 2° > 1°; alumina at 623 K gives alkene.
Oxidation: primary alcohol gives aldehyde then acid; secondary gives ketone; tertiary resists mild oxidation. PCC stops at the aldehyde. Copper at 573 K dehydrogenates 1° to aldehyde and 2° to ketone; 3° alcohol is dehydrated to alkene.

Tests for Alcohols

Reactions of Phenol

Glycols, Glycerol and Ethers

Ethylene glycol: oxidised by periodic acid to two HCHO (all 1,2-diols cleave; 1,3-diols do not); with excess \( \ce{PCl5} \) gives 1,2-dichloroethane; with \( \ce{PI3} \) gives ethene; with anhydrous \( \ce{ZnCl2} \) gives ethanal; with fuming sulphuric acid gives dioxane; with dicarboxylic acids gives polyesters; used as antifreeze.
Glycerol: viscous, sweet, hygroscopic, boils at 563 K with decomposition so it is purified under reduced pressure. With conc. \( \ce{HNO3} \)/\( \ce{H2SO4} \) gives nitroglycerine (dynamite); with oxalic acid gives formic acid at 383 K and allyl alcohol at 533 K; with \( \ce{KHSO4} \) or dehydrating agents gives acrolein; with HI gives allyl iodide (then isopropyl iodide); with \( \ce{P + I2} \) gives allyl iodide.
Ethers: oxygen is sp3 hybridised (C-O-C about 110°) and is a Lewis base, forming etherates with \( \ce{BF3} \) or \( \ce{AlCl3} \). Cleavage with HI: dimethyl ether gives 2 \( \ce{CH3I} \); alkyl aryl ethers give phenol and alkyl halide; tertiary alkyl ethers cleave by SN1 at the tertiary carbon. Exposure to air gives explosive peroxides, which are tested with ferrous or KI solutions. Anisole undergoes electrophilic substitution at ortho and para positions.