JEE Chemistry Alcohols Phenols and Ethers Notes Set 09

Phenols: Preparation and Acidity

1. Phenol is made industrially from cumene (isopropylbenzene) by air oxidation to cumene hydroperoxide, followed by acid cleavage to phenol and acetone.
2. Laboratory methods: fusion of sodium benzenesulphonate with NaOH, hydrolysis of benzenediazonium chloride by warm water, heating chlorobenzene with NaOH at about 630 K and 300 atm (Dow's process), and decarboxylation of sodium salicylate with soda lime.
3. Phenol is much more acidic than alcohols (pKa about 10) because the phenoxide ion is stabilised by delocalisation of the negative charge into the ring. Electron-withdrawing groups such as –NO₂ increase acidity (2,4,6-trinitrophenol, pKa about 0.4, dissolves in NaHCO₃) while electron-donating groups decrease it.
4. Phenols do not dissolve in NaHCO₃ (except picric acid) because they are weaker acids than carbonic acid, but they dissolve in NaOH. Carboxylic acids dissolve in NaHCO₃ and liberate CO₂.
5. o-Nitrophenol has intramolecular hydrogen bonding and is steam volatile; p-nitrophenol has intermolecular hydrogen bonding and is not.

Reactions of Phenols

1. Electrophilic substitution is easy because the –OH group is strongly activating and directs ortho and para. Nitration with dilute HNO₃ gives o- and p-nitrophenol, and with conc. HNO₃ gives 2,4,6-trinitrophenol (picric acid).
2. Bromine water gives a white precipitate of 2,4,6-tribromophenol. Bromination in CS₂ at low temperature gives a mixture of o- and p-bromophenol. A mixture of KBr and KBrO₃ in acid also generates bromine.
3. Reimer–Tiemann reaction: phenol, CHCl₃ and aqueous NaOH give salicylaldehyde mainly (through dichlorocarbene).
4. Kolbe–Schmidt reaction: sodium phenoxide with CO₂ at about 400 K and 5 atm, then H⁺, gives salicylic acid. Acetylation of salicylic acid gives aspirin, and its methyl ester is oil of wintergreen.
5. NaNO₂ and conc. H₂SO₄ (Liebermann's reaction) give p-nitrosophenol. Zinc dust distillation converts phenol to benzene.
6. Phenol with neutral FeCl₃ gives a violet colour. Phenol reacts with acid chlorides or anhydrides to give esters (phenyl acetate with CH₃COCl), and sodium phenoxide with alkyl halides gives ethers (Williamson synthesis).

Ethers

1. Williamson synthesis: R–ONa + R′–X → R–O–R′ + NaX. It works best with primary alkyl halides; tertiary halides give alkenes. Dehydration of ethanol at 413 K gives diethyl ether.
2. Ethers have lower boiling points than alcohols of similar mass because they cannot form hydrogen bonds. They are weak Lewis bases and dissolve in cold concentrated acids by forming oxonium salts.
3. Cleavage by HI: excess hot HI converts R–O–R to two molecules of RI. For mixed ethers the halide attacks the smaller alkyl group (SN₂). With aryl alkyl ethers, the products are always phenol and alkyl iodide, because the aryl C–O bond is strong: C₆H₅OCH₃ + HI → C₆H₅OH + CH₃I.
4. Ethers are unreactive to bases, reducing agents and most oxidants, so they make useful solvents. On standing in air they form explosive peroxides.
5. The aromatic ring of anisole is activated by the –OCH₃ group, so anisole undergoes halogenation, nitration and Friedel–Crafts reactions at the ortho and para positions.