Class 12 Chemistry · Chapter 7 NotesAlcohols, Phenols and Ethers

Alcohols and phenols are classified by the number of hydroxyl groups as mono-, di-, tri- or polyhydric. Alcohols are further classified by the hybridisation of the carbon bearing –OH: compounds with –OH on an sp3 carbon include primary, secondary and tertiary alcohols, allylic alcohols (where –OH is on a carbon next to a C=C bond) and benzylic alcohols (where –OH is on a carbon next to an aromatic ring). When –OH is bonded to an sp2 carbon of a double bond, the compound is a vinylic alcohol. Ethers are classified as simple or symmetrical if the two groups attached to oxygen are identical, and mixed or unsymmetrical if they differ. IUPAC names of alcohols are obtained by replacing the final 'e' of the parent alkane with 'ol' and numbering the chain from the end nearer the –OH group. For polyhydric alcohols, the 'e' is retained and multiplicative prefixes di, tri, etc. are added before 'ol'. Phenol is both the common and accepted IUPAC name; substituted phenols use ortho, meta and para for 1,2-, 1,3- and 1,4-disubstitution. Ethers are named as alkoxyalkanes, choosing the larger group as the parent chain.

• Monohydricone –OH; dihydric: two –OH; trihydric: three –OH
• Primary, secondary and tertiary alcohols have –OH on primary, secondary and tertiary carbons respectively
• Allylic alcohol–OH on an sp3 carbon adjacent to a C=C bond
• Benzylic alcohol–OH on an sp3 carbon next to an aromatic ring
• Vinylic alcohol–OH attached to an sp2 carbon of a C=C bond
• Symmetrical etherR–O–R; unsymmetrical ether: R–O–R′

In alcohols, oxygen is attached to an sp3 carbon by a sigma bond formed from the overlap of an sp3 orbital of carbon with an sp3 orbital of oxygen. The C–O–H bond angle is slightly less than the tetrahedral angle because of repulsion between the lone pairs on oxygen. In phenol, the –OH group is attached to an sp2 carbon of the aromatic ring; the C–O bond length is slightly shorter than in methanol because of partial double bond character arising from conjugation of the oxygen lone pair with the ring. In ethers, the two bond pairs and two lone pairs on oxygen are arranged roughly tetrahedrally, and the C–O–C angle is slightly larger than tetrahedral because of repulsion between the bulky alkyl groups. Alcohols and phenols have much higher boiling points than hydrocarbons, ethers and haloalkanes of comparable molecular mass because of intermolecular hydrogen bonding through the –OH group. Boiling point increases with the number of carbon atoms and decreases with branching. Solubility in water is due to hydrogen bonding between the –OH group and water; it decreases as the size of the alkyl or aryl group increases. Lower alcohols are miscible with water in all proportions. Ethers have a net dipole moment but no hydrogen bonding among themselves, so their boiling points resemble those of alkanes of similar mass, though they can form hydrogen bonds with water and are therefore somewhat soluble in it.

• C–O bond length in phenolabout 136 pm; in methanol: slightly longer
• C–O bond length in ethersabout 141 pm
• Intermolecular hydrogen bonding raises boiling points of alcohols and phenols
• Boiling point orderalcohol > ether ≈ alkane of similar mass
• Solubility in water decreases as the hydrocarbon part becomes larger

Alcohols can be prepared from alkenes by acid-catalysed hydration, where the alkene reacts with water in the presence of an acid; for unsymmetrical alkenes the addition follows Markovnikov's rule. The mechanism involves protonation of the alkene to form a carbocation, nucleophilic attack by water and finally deprotonation. Hydroboration–oxidation is another route: diborane adds to the alkene to give a trialkylborane, which on oxidation with hydrogen peroxide in aqueous sodium hydroxide gives an alcohol with anti-Markovnikov orientation. Aldehydes and ketones can be reduced to alcohols by catalytic hydrogenation using platinum, palladium or nickel, or by sodium borohydride or lithium aluminium hydride; aldehydes give primary alcohols and ketones give secondary alcohols. Carboxylic acids and esters are reduced to primary alcohols by lithium aluminium hydride, and commercially by converting the acid to an ester followed by catalytic hydrogenation. Grignard reagents react with aldehydes and ketones to give alcohols: methanal gives a primary alcohol, other aldehydes give secondary alcohols and ketones give tertiary alcohols.

• Acid-catalysed hydration of alkenes follows Markovnikov's rule
• Hydroboration–oxidation gives anti-Markovnikov addition of water
• Aldehyde → primary alcohol; ketone → secondary alcohol
• Carboxylic acid → primary alcohol (LiAlH4)
• Grignard reagent + methanal → primary alcohol; + other aldehyde → secondary alcohol; + ketone → tertiary alcohol

Phenol is prepared from benzene derivatives by several routes. Chlorobenzene fused with sodium hydroxide at 623 K and high pressure gives sodium phenoxide, which on acidification gives phenol. Benzene is sulphonated with oleum to benzenesulphonic acid, which is heated with molten sodium hydroxide to form sodium phenoxide; acidification then gives phenol. Aromatic primary amines treated with nitrous acid (NaNO2 + HCl) at 273–278 K form diazonium salts, which are hydrolysed by warming with water or dilute acid to give phenols. Industrially, most phenol is made from cumene (isopropylbenzene): cumene is oxidised with air to cumene hydroperoxide, which is treated with dilute acid to give phenol and acetone as a by-product. Acetone is obtained in large quantities by this method.

• Chlorobenzene + NaOH (623 K, high pressure) → sodium phenoxide → phenol
• Benzenesulphonic acid + molten NaOH → sodium phenoxide → phenol
• Diazonium salt + warm water → phenol + N2 + HCl
• Cumene → cumene hydroperoxide → phenol + acetone

Alcohols and phenols can react by cleavage of the O–H bond or the C–O bond. The O–H bond is broken when they act as acids: both react with active metals such as sodium, potassium and aluminium to give alkoxides or phenoxides and hydrogen. Phenol also reacts with aqueous sodium hydroxide to form sodium phenoxide, showing that phenols are stronger acids than alcohols and water. Alcohol acidity decreases with electron-releasing groups; phenol is about a million times more acidic than ethanol. Electron-withdrawing groups such as –NO2 on the phenol ring increase acidity, especially at ortho and para positions, while electron-releasing groups such as alkyl groups decrease it. Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters; the reaction with acid chlorides is carried out in the presence of pyridine. Acetylation of salicylic acid gives aspirin. Reactions involving C–O bond cleavage occur mainly in alcohols: with hydrogen halides they form alkyl halides, and with phosphorus tribromide they form alkyl bromides. Alcohols undergo dehydration to alkenes with concentrated H2SO4 or H3PO4, and the ease of dehydration follows tertiary > secondary > primary. Oxidation of primary alcohols gives aldehydes with mild oxidising agents such as CrO3 or PCC and carboxylic acids with strong oxidising agents such as acidified KMnO4; secondary alcohols give ketones; tertiary alcohols resist oxidation. Phenols undergo electrophilic aromatic substitution because the –OH group activates the ring and directs incoming groups to ortho and para positions. Nitration with dilute nitric acid at 298 K gives ortho and para nitrophenols, while concentrated nitric acid gives 2,4,6-trinitrophenol (picric acid). Bromination in low-polarity solvents gives monobromophenols, and with bromine water gives 2,4,6-tribromophenol as a white precipitate. Phenoxide ion, formed by treating phenol with sodium hydroxide, undergoes Kolbe's reaction with carbon dioxide to give ortho hydroxybenzoic acid, and Reimer–Tiemann reaction with chloroform in alkali to give salicylaldehyde. Phenol is converted to benzene on heating with zinc dust, and oxidation with chromic acid gives benzoquinone.

• Alcohols and phenols react with Na, K or Al to give alkoxides/phenoxides and H2
• Phenol + NaOH → sodium phenoxide + water
• Acid strengthphenol > water > alcohols
• Esterificationalcohol/phenol + acid chloride → ester + HCl (pyridine)
• Dehydration easetertiary > secondary > primary
• Primary alcohol → aldehyde (mild oxidant) or carboxylic acid (strong oxidant); secondary alcohol → ketone
• Phenol + dilute HNO3 → o- and p-nitrophenol; + conc. HNO3 → picric acid
• Phenol + Br2 water → 2,4,6-tribromophenol (white precipitate)

Ethers can be prepared by dehydration of alcohols in the presence of protic acids such as H2SO4 or H3PO4. Ethanol gives ethoxyethane at 413 K, while at 443 K ethene is the main product. This method works well only for primary alcohols; secondary and tertiary alcohols tend to give alkenes by elimination. Williamson synthesis is a versatile laboratory method for both symmetrical and unsymmetrical ethers: an alkyl halide reacts with a sodium alkoxide to give an ether and sodium halide. The reaction follows an SN2 pathway and gives good results with primary alkyl halides; with secondary and tertiary halides, elimination competes and alkenes are formed. Phenols can also be converted to ethers by this method using phenoxide. Ethers have polar C–O bonds and a net dipole moment, but their boiling points are close to those of alkanes of similar molecular mass and much lower than alcohols because they cannot form hydrogen bonds among themselves. They are, however, miscible with water to an extent similar to alcohols of comparable mass because the oxygen atom can form hydrogen bonds with water. Ethers are the least reactive of the three classes. The C–O bond is cleaved by concentrated HI or HBr at high temperature: dialkyl ethers give two alkyl halide molecules, and alkyl aryl ethers cleave at the alkyl–oxygen bond to give phenol and an alkyl halide. In mixed ethers with a primary or secondary alkyl group, the lower alkyl group forms the iodide by an SN2 pathway; if a tertiary group is present, the tertiary halide is formed by an SN1 pathway. Aryl alkyl ethers such as anisole undergo electrophilic substitution: halogenation with bromine in ethanoic acid gives mainly the para isomer, Friedel–Crafts alkylation and acylation occur at ortho and para positions, and nitration with a mixture of concentrated sulphuric and nitric acids gives ortho and para nitroanisole.

• Ethanol at 413 K → ethoxyethane; at 443 K → ethene
• Williamson synthesisR–X + R′–ONa → R–O–R′ + NaX
• Williamson synthesis works best with primary alkyl halides
• Ethers have lower boiling points than alcohols of similar mass
• Ethers are cleaved by concentrated HI or HBr at high temperature
• Anisole undergoes bromination, Friedel–Crafts reactions and nitration at ortho and para positions