Chapter 7: Alcohols, Phenols and Ethers | Summary

In IUPAC nomenclature, alcohols are named by replacing the terminal 'e' of the parent alkane with the suffix 'ol', numbering the chain from the end nearest the –OH group. Polyhydric alcohols retain the 'e' and add 'diol', 'triol', etc., with appropriate locants. Cyclic alcohols use the prefix 'cyclo' with –OH at C-1. Common names are derived by adding 'alcohol' after the alkyl group name.

Phenols are named as derivatives of the parent 'phenol'. Dihydroxy benzenes are called catechol (1,2-), resorcinol (1,3-) and hydroquinone (1,4-benzenediol). For ethers, IUPAC names treat the compound as an alkane in which H is replaced by an –OR/–OAr group; the larger group is the parent chain. Common ether names are formed by listing the alkyl/aryl groups alphabetically followed by 'ether'.

In alcohols, the oxygen of –OH is sp3 hybridised; the C–O bond is formed by overlap of sp3 orbitals of carbon and oxygen. The bond angle is slightly less than the tetrahedral angle (109°28') due to repulsion between lone pairs on oxygen. In phenol, –OH is on sp2 carbon of the ring; partial double bond character due to resonance shortens the C–O bond to 136 pm compared to 142 pm in methanol.

In ethers, oxygen uses sp3 hybridised orbitals forming two C–O bonds with a bond angle slightly greater than tetrahedral due to steric repulsion between the bulky alkyl groups. The C–O bond length in ethers (141 pm) is similar to that in alcohols.

Alcohols are prepared by: (1) Hydration of alkenes — acid-catalysed hydration follows Markovnikov's rule, while hydroboration-oxidation (anti-Markovnikov) uses diborane followed by H2O2/NaOH to give the alcohol in excellent yield. (2) From carbonyl compounds — aldehydes give primary alcohols and ketones give secondary alcohols on catalytic hydrogenation or reduction with NaBH4/LiAlH4. Carboxylic acids are reduced to primary alcohols by LiAlH4 or by catalytic hydrogenation of their esters.

(3) From Grignard reagents — nucleophilic addition of RMgX to methanal gives primary alcohols, to other aldehydes gives secondary alcohols, and to ketones gives tertiary alcohols, followed by hydrolysis of the adduct.

Phenols are prepared by: (1) From haloarenes — chlorobenzene is fused with NaOH at 623 K and 320 atm, and the resulting sodium phenoxide is acidified to give phenol. (2) From benzenesulphonic acid — sulphonation with oleum followed by fusion with molten NaOH and acidification. (3) From diazonium salts — aromatic primary amines are diazotised (NaNO2/HCl, 273–278 K) and the diazonium salt is hydrolysed by warming with water or dilute acid.

(4) From cumene — industrially, cumene (isopropylbenzene) is oxidised by air to cumene hydroperoxide, which on treatment with dilute acid gives phenol and acetone. Most worldwide production of phenol uses this cumene process.

Boiling points of alcohols increase with increasing carbon chain length (greater van der Waals forces) and decrease with branching (reduced surface area). The boiling points of alcohols and phenols are significantly higher than those of hydrocarbons, ethers and haloalkanes of comparable molecular mass due to extensive intermolecular hydrogen bonding through the –OH group. Ethers cannot form hydrogen bonds among themselves, so their boiling points are much lower than those of alcohols of similar mass.

Solubility of alcohols and phenols in water arises from hydrogen bond formation with water molecules. Solubility decreases as the size of the hydrophobic alkyl or aryl group increases. Lower molecular mass alcohols are miscible with water in all proportions.

Reactions involving O–H bond cleavage: Alcohols and phenols react with active metals (Na, K, Al) to liberate H2 and form alkoxides/phenoxides. Phenols also react with aqueous NaOH, showing they are stronger acids than alcohols. Acidity of alcohols decreases in order: primary > secondary > tertiary, because electron-donating alkyl groups destabilise the alkoxide ion. Phenols are far more acidic than alcohols (pKa ~10 vs ~16) because the phenoxide ion is stabilised by resonance delocalisation of negative charge over the ring. Electron-withdrawing substituents (especially at ortho/para) increase phenol acidity; electron-releasing groups decrease it. Esterification occurs with carboxylic acids, acid chlorides and acid anhydrides to give esters; acetylation of salicylic acid gives aspirin.

Reactions involving C–O bond cleavage: Alcohols react with hydrogen halides (HX) to give alkyl halides; reactivity order is 3° > 2° > 1° (Lucas test). Phosphorus trihalides convert alcohols to alkyl bromides. Dehydration with conc. H2SO4 or H3PO4 gives alkenes; ease of dehydration follows 3° > 2° > 1°. Oxidation of primary alcohols gives aldehydes (with PCC or CrO3) or carboxylic acids (with KMnO4); secondary alcohols give ketones (CrO3); tertiary alcohols resist oxidation. Reactions specific to phenols include electrophilic aromatic substitution (nitration, halogenation directed to ortho/para positions), Kolbe's reaction (carboxylation with CO2 in alkaline medium to give salicylic acid), Reimer-Tiemann reaction (introduction of –CHO at ortho position using CHCl3/NaOH to give salicylaldehyde), reduction with zinc dust to benzene, and oxidation with chromic acid to benzoquinone.

Methanol (wood spirit, CH3OH) is industrially produced by catalytic hydrogenation of CO over ZnO–Cr2O3 catalyst at high pressure and temperature. It boils at 337 K, is highly toxic (causes blindness and death even in small amounts), and is used as a solvent and in making formaldehyde.

Ethanol (C2H5OH) is produced commercially by fermentation of sugars using invertase (to convert sucrose to glucose and fructose) and zymase (to convert them to ethanol and CO2). It boils at 351 K, is used as a solvent and in synthesis. Commercial alcohol is denatured by adding copper sulphate and pyridine to make it unfit for consumption.

Ethers are prepared by: (1) Acid-catalysed dehydration of primary alcohols at lower temperature (e.g., ethanol at 413 K with H2SO4 gives diethyl ether via SN2); at higher temperature alkene is the major product. Secondary and tertiary alcohols undergo elimination preferentially. (2) Williamson synthesis — an alkyl halide reacts with sodium alkoxide (SN2); best with primary alkyl halides. Secondary or tertiary halides give elimination products. Phenols can also be used as phenoxide to prepare aryl ethers.

Ethers have boiling points similar to alkanes of comparable mass (much lower than alcohols) due to lack of O–H bonds for H-bonding. They are soluble in water to a similar extent as analogous alcohols because the ether oxygen can form H-bonds with water. Chemically, ethers are least reactive. C–O bond cleavage by conc. HI or HBr (order: HI > HBr > HCl) gives alkyl halides; alkyl aryl ethers cleave at the alkyl-oxygen bond. In electrophilic aromatic substitution, the –OR group activates the ring and directs incoming groups to ortho and para positions; anisole undergoes halogenation, nitration and Friedel-Crafts reactions accordingly.

Chapter 7 establishes that alcohols, phenols and ethers, though all containing the C–O bond, differ markedly in properties owing to differences in the nature of the carbon to which oxygen is bonded. The acidic strength increases dramatically from alcohols to phenols due to resonance stabilisation of the phenoxide ion. Chemical reactivity of alcohols is governed by cleavage of either the O–H or C–O bond, while phenols uniquely undergo electrophilic aromatic substitution (Kolbe's, Reimer-Tiemann, nitration, halogenation) because the –OH group activates the benzene ring. Ethers are relatively inert but can be cleaved by strong hydrohalic acids.

For CBSE board examinations, this chapter is consistently high-yield. Students must master IUPAC nomenclature of all three classes, the mechanisms of acid-catalysed hydration and dehydration, Williamson synthesis, distinction between acidity of alcohols and phenols with reasoning, and named reactions such as Kolbe's and Reimer-Tiemann reactions. Reaction sequences involving Grignard reagents, oxidation states of alcohols and the preparation of phenol from cumene are frequently examined as multi-step problems carrying 3 to 5 marks.