Alcohols, Phenols and Ethers

An alcohol has –OH on an sp³ carbon of an aliphatic chain; a phenol has –OH bonded directly to an sp² carbon of a benzene ring; an ether has an oxygen bonded to two carbon groups, R–O–R′. Everything about reactivity in this chapter follows from which carbon the oxygen is attached to.

Alcohols and phenols are classified first by the number of –OH groups: monohydric (ethanol, phenol), dihydric (ethane-1,2-diol, catechol) and trihydric (propane-1,2,3-triol, glycerol). Monohydric alcohols are then divided by the carbon carrying the –OH. If that carbon is sp³, the alcohol is primary (1°), secondary (2°) or tertiary (3°) according to whether the carbon is joined to one, two or three other carbons. Allylic alcohols have –OH on a carbon next to a C=C (CH₂=CH–CH₂OH) and benzylic alcohols have –OH on a carbon attached to a benzene ring (C₆H₅CH₂OH); both may be 1°, 2° or 3°. If the –OH is on an sp² carbon the compound is a vinylic alcohol (CH₂=CH–OH, unstable) or a phenol.

Ethers are simple or symmetrical when both groups on oxygen are the same (C₂H₅OC₂H₅) and mixed or unsymmetrical when they differ (CH₃OC₂H₅, anisole C₆H₅OCH₃).

Alcohols. Replace the final -e of the alkane by -ol and number the chain so the –OH carbon gets the lowest locant: CH₃CH(OH)CH₃ is propan-2-ol, (CH₃)₃COH is 2-methylpropan-2-ol. With two or three –OH groups keep the -e: ethane-1,2-diol, propane-1,2,3-triol. A ring carrying –OH is a cycloalkanol; when the –OH is on a side chain the ring becomes a prefix, as in cyclohexylmethanol. Double bonds go before the -ol: prop-2-en-1-ol.

Phenols. The parent is phenol, numbered from the –OH carbon, or use ortho/meta/para: 2-methylphenol is o-cresol, 4-methylphenol is p-cresol. Benzene-1,2-diol is catechol, benzene-1,3-diol is resorcinol, benzene-1,4-diol is hydroquinone. 2-Hydroxybenzoic acid is salicylic acid and 2-hydroxybenzaldehyde is salicylaldehyde; both names recur in Kolbe and Reimer-Tiemann questions.

Ethers. Common names list both alkyl groups alphabetically: ethyl methyl ether. In IUPAC the ether is an alkoxyalkane: the larger group is the parent alkane and the smaller group with the oxygen is the alkoxy prefix. CH₃OC₂H₅ is methoxyethane, CH₃OCH(CH₃)₂ is 2-methoxypropane, C₆H₅OCH₃ is methoxybenzene (anisole), C₆H₅OC₂H₅ is ethoxybenzene (phenetole), and C₂H₅OCH₂CH(CH₃)₂ is 1-ethoxy-2-methylpropane.

In alcohols the oxygen is sp³ hybridised with two lone pairs, so C–O–H is bent. In methanol the C–O–H angle is 108.9°, slightly less than 109.5°, because the two lone pairs repel the bond pairs more strongly. The C–O bond length is 142 pm.

In phenol the C–O bond is shorter, 136 pm, for two reasons you must both state: the oxygen lone pair is conjugated with the ring, giving the bond partial double-bond character, and the sp² carbon is more electronegative than an sp³ carbon so it holds the bond closer.

In ethers the oxygen is again sp³, but the C–O–C angle is larger than tetrahedral: 111.7° in dimethyl ether, with C–O bonds of 141 pm. The angle opens because the two bulky alkyl groups repel each other. Since the molecule is bent, the C–O dipoles do not cancel and ethers have a net dipole moment.

Two standard 1-mark questions come from here: why the C–O bond of phenol is shorter than in methanol, and why the C–O–C angle of an ether exceeds 109.5°.

From alkenes. (i) Acid-catalysed hydration: an alkene adds water with dilute H₂SO₄ following Markovnikov's rule, so propene gives propan-2-ol. Mechanism, three steps: (1) the alkene is protonated by H₃O⁺ to give a carbocation (slow, rate determining); (2) water attacks the carbocation to give a protonated alcohol; (3) the protonated alcohol loses a proton to water, giving the alcohol and regenerating H₃O⁺. (ii) Hydroboration-oxidation: diborane (BH₃)₂ adds to the alkene to give a trialkylborane, which is oxidised by H₂O₂ in aqueous NaOH to the alcohol. The result is anti-Markovnikov addition of water: propene gives propan-1-ol.

From carbonyl compounds. Aldehydes give primary and ketones give secondary alcohols with H₂/Pd, NaBH₄ or LiAlH₄. Carboxylic acids are reduced to primary alcohols by LiAlH₄; commercially the acid is first converted to an ester which is hydrogenated.

From Grignard reagents. RMgX adds to a C=O to give an adduct, which is hydrolysed to the alcohol. Methanal gives a primary alcohol, any other aldehyde a secondary alcohol, a ketone a tertiary alcohol. So CH₃MgBr with propanone gives 2-methylpropan-2-ol; C₂H₅MgCl with methanal gives propan-1-ol.

NCERT gives four routes, each of which has been asked as a 1- or 2-mark equation.

From haloarenes. Chlorobenzene is fused with NaOH at 623 K and 300 atm to give sodium phenoxide; acidification gives phenol. The harsh conditions are needed because the aryl C–Cl bond resists nucleophilic substitution.

From benzenesulphonic acid. Benzene is sulphonated with oleum; the acid is fused with molten NaOH to give sodium phenoxide, then acidified. This answers the NCERT question that gives you benzene, conc. H₂SO₄ and NaOH.

From diazonium salts. Aniline is diazotised with NaNO₂/HCl at 273–278 K to benzenediazonium chloride; warming with water hydrolyses it to phenol with loss of N₂.

From cumene (industrial). Cumene (isopropylbenzene, from benzene and propene by Friedel-Crafts alkylation) is oxidised by air to cumene hydroperoxide, C₆H₅C(CH₃)₂OOH, which with dilute acid gives phenol and propanone. Both products are valuable, which is why the process is preferred.

Boiling points. Alcohols and phenols boil much higher than hydrocarbons, ethers and haloalkanes of comparable molecular mass because of intermolecular hydrogen bonding through the polar O–H group. Propan-1-ol (molar mass 60, b.p. 370 K) boils far above butane (58, b.p. 273 K), and ethanol (351 K) above its isomer methoxymethane (249 K), which has no O–H. Within alcohols the boiling point rises with the number of carbons (larger van der Waals forces) and falls with branching (smaller surface area), so the order is propan-1-ol < butan-2-ol < butan-1-ol < pentan-1-ol.

Solubility. Alcohols and phenols dissolve in water by hydrogen bonding with water. Solubility falls as the hydrophobic alkyl or aryl part grows: methanol, ethanol and propanol are miscible, higher alcohols are not, phenol is only sparingly soluble.

o- and p-Nitrophenol. In o-nitrophenol the –OH and –NO₂ are close enough to form an intramolecular hydrogen bond, so the molecules are not associated with each other; it has a lower boiling point, lower water solubility and is steam volatile. p-Nitrophenol forms intermolecular hydrogen bonds and is not steam volatile. A mixture of the two is therefore separated by steam distillation, the ortho isomer distilling over.

Alcohols and phenols react with Na, K and Al to give alkoxides or phenoxides and hydrogen: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. So they are Bronsted acids; they also act as Bronsted bases through the lone pairs on oxygen (protonated alcohols appear in many mechanisms). Phenol additionally reacts with aqueous NaOH to give sodium phenoxide; alcohols do not.

Acidity of alcohols. Order 1° > 2° > 3°, and all alcohols are weaker acids than water. An alkyl group is electron releasing (+I): it raises electron density on oxygen and destabilises the alkoxide ion by intensifying its negative charge. Water (pK_a about 15.7) is a slightly stronger acid than ethanol (15.9), so an alkoxide is a stronger base than hydroxide.

Acidity of phenol. Phenol (pK_a 10.0) is about a million times more acidic than ethanol. The oxygen is bonded to an electron-withdrawing sp² carbon, which polarises the O–H bond, and the phenoxide ion is stabilised by resonance, the negative charge being delocalised over the ortho and para carbons; the resonance structures of phenol itself carry charge separation and are less stabilising. Ethoxide has no delocalisation. Order: phenol > water > ethanol.

Substituents. Electron-withdrawing groups at ortho or para stabilise the phenoxide and raise acidity: p-nitrophenol pK_a 7.1, o-nitrophenol 7.2, m-nitrophenol 8.3 (weaker, no direct resonance with the O⁻). Electron-releasing groups (–CH₃, –OCH₃) lower acidity: cresols 10.1–10.2. Hence o-nitrophenol is more acidic than o-methoxyphenol.

Esterification. With carboxylic acids (conc. H₂SO₄, reversible, water removed), acid chlorides (in pyridine) or anhydrides, alcohols and phenols give esters. Acetylation of salicylic acid with ethanoic anhydride gives aspirin.

These reactions remove –OH as water. Only alcohols do this; phenols do not, because the C–O bond of phenol has partial double-bond character and the sp² carbon cannot be attacked by a nucleophile.

With hydrogen halides. R–OH + HX → R–X + H₂O. Tertiary alcohols react with conc. HCl at room temperature; primary and secondary need anhydrous ZnCl₂. The Lucas test (conc. HCl + anhydrous ZnCl₂) distinguishes the classes by the time taken for turbidity (the insoluble alkyl chloride): tertiary immediately, secondary within about five minutes, primary none at room temperature. Reactivity 3° > 2° > 1°, following carbocation stability. Bromides are made with NaBr/H₂SO₄ or PBr₃.

With PCl₅ and SOCl₂. R–OH + PCl₅ → R–Cl + POCl₃ + HCl; R–OH + SOCl₂ → R–Cl + SO₂ + HCl. Thionyl chloride is preferred because both by-products are gases.

Dehydration. With conc. H₂SO₄, H₃PO₄ or hot alumina, alcohols lose water to give alkenes. Ethanol gives ethene with conc. H₂SO₄ at 443 K; propan-2-ol needs 85% H₃PO₄ at 440 K and 2-methylpropan-2-ol only 20% H₃PO₄ at 358 K, so ease is 3° > 2° > 1°. Mechanism: (1) protonation of ethanol to the protonated alcohol; (2) loss of water to give the ethyl carbocation (slow, rate determining); (3) loss of a proton from the adjacent carbon to give ethene and regenerate the acid. Where two alkenes are possible the more substituted one is major (Saytzeff rule).

Oxidation removes hydrogen from the carbon carrying –OH and from the –OH itself, so it is also called dehydrogenation. The product depends on the class of alcohol and the strength of the oxidising agent, and the examiner tests whether you pick the right reagent.

Primary alcohols. Acidified KMnO₄ or acidified K₂Cr₂O₇ takes a primary alcohol through the aldehyde to the carboxylic acid. To stop at the aldehyde use CrO₃ in anhydrous medium or, better, PCC (pyridinium chlorochromate, a complex of chromium trioxide with pyridine and HCl). Propan-1-ol gives propanoic acid with alkaline KMnO₄ and propanal with PCC.

Secondary alcohols give ketones with CrO₃ or acidified dichromate: propan-2-ol → propanone.

Tertiary alcohols do not oxidise under normal conditions because the carbinol carbon has no hydrogen. Under vigorous conditions C–C bonds break, giving a mixture of carboxylic acids with fewer carbons.

Copper at 573 K. Alcohol vapour over heated copper gives an aldehyde from a primary alcohol, a ketone from a secondary alcohol, and an alkene (dehydration) from a tertiary alcohol; a good test to distinguish the three.

The –OH group is strongly activating and ortho/para directing: the oxygen lone pair is delocalised into the ring and the resonance structures place negative charge at the ortho and para carbons, so electrophiles attack there under mild conditions.

Nitration. Dilute HNO₃ at 298 K gives o- and p-nitrophenol, separated by steam distillation. Conc. HNO₃ gives 2,4,6-trinitrophenol, picric acid; because the yield is poor, picric acid is made by sulphonating phenol to phenol-2,4-disulphonic acid and then treating with conc. HNO₃.

Halogenation. In CS₂ at 273 K, where phenol is not ionised, bromine gives o- and p-bromophenol (para major). In water phenol ionises to the strongly activated phenoxide, and bromine water gives a white precipitate of 2,4,6-tribromophenol at once.

Kolbe's reaction. Sodium phenoxide (a stronger nucleophile than phenol) with CO₂ under pressure, then acidification, gives 2-hydroxybenzoic acid, salicylic acid.

Reimer-Tiemann reaction. Phenol heated with chloroform and aqueous NaOH (about 340 K): dichlorocarbene (:CCl₂) from CHCl₃ attacks the ortho position, the –CHCl₂ group is hydrolysed by alkali, and acidification gives 2-hydroxybenzaldehyde, salicylaldehyde. With CCl₄ the product is salicylic acid.

Zinc dust. Phenol distilled with zinc dust is reduced to benzene: C₆H₅OH + Zn → C₆H₆ + ZnO. Oxidation. Chromic acid oxidises phenol to benzoquinone; in air phenol slowly darkens.

Methanol, CH₃OH, is called wood spirit because it was once made by destructive distillation of wood. Today it is made from water gas over a ZnO–Cr₂O₃ catalyst at 573–673 K and 200–300 atm: CO + 2H₂ → CH₃OH. It is a colourless liquid, b.p. 337 K, and highly poisonous: small quantities cause blindness and large quantities death. It is used as a solvent in paints and varnishes and to make methanal.

Ethanol, C₂H₅OH, is made by fermentation of the sugars in molasses (the liquor left after crystallising cane sugar) or from starch. Yeast supplies two enzymes: invertase hydrolyses sucrose to glucose and fructose, and zymase converts these to ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Fermentation is carried out in the absence of air, because air oxidises the ethanol to ethanoic acid and the wine turns sour; once the alcohol content reaches about 14 per cent the yeast is killed and fermentation stops.

Ethanol (b.p. 351 K) is a solvent in paints and medicine. Commercial alcohol is made unfit for drinking by adding copper sulphate (colour) and pyridine (foul smell), usually with methanol: this is denatured alcohol or methylated spirit. Ethanol is a mild intoxicant; drinking methylated spirit causes blindness and death.

Dehydration of alcohols. With conc. H₂SO₄ at 413 K ethanol loses water between two molecules to give ethoxyethane; at 443 K the same reagents give ethene. The reaction is S_N2: one ethanol is protonated, a second ethanol attacks its carbon and displaces water, and the protonated ether loses a proton. It works for primary alcohols only; secondary and tertiary alcohols eliminate instead, because the hindered carbon cannot be attacked from the back and the stable carbocation loses a proton to give an alkene. The ethers formed are symmetrical.

Williamson synthesis. An alkyl halide is heated with a sodium alkoxide: R–X + R′O⁻Na⁺ → R–O–R′ + NaX. The alkoxide displaces the halide by S_N2. Symmetrical and unsymmetrical ethers can be made, including aryl alkyl ethers from sodium phenoxide and CH₃Br (the halide must be the alkyl partner, since aryl halides do not undergo S_N2).

The limitation is the halide. It should be primary; a tertiary halide gives an alkene because the alkoxide acts as a base and eliminates HX. To make tert-butyl ethyl ether use sodium tert-butoxide and ethyl bromide; sodium ethoxide with tert-butyl bromide gives 2-methylpropene. Likewise 2-methoxy-2-methylpropane comes from sodium tert-butoxide and methyl bromide.

Ethers are colourless, pleasant-smelling liquids (dimethyl and ethyl methyl ether are gases). Because C–O–C is bent, the two C–O dipoles do not cancel and ethers have a small net dipole moment.

Boiling points. Ethers have no O–H, so they cannot hydrogen-bond with each other. Their boiling points are close to those of alkanes of similar molecular mass and far below the isomeric alcohols: methoxymethane (molar mass 46) boils at 249 K while ethanol boils at 351 K; ethoxyethane (74) boils at 308 K while butan-1-ol boils at 391 K. Diethyl ether is volatile and inflammable and is used as a low-boiling solvent.

Solubility. The ether oxygen can accept a hydrogen bond from water, so lower ethers are slightly soluble: the miscibility of ethoxyethane with water resembles that of butan-1-ol. Solubility falls as the alkyl groups grow. Ethers are good solvents for organic compounds and for Grignard reagents because they are unreactive and aprotic.

The two comparisons the examiner expects: alcohols boil higher than isomeric ethers because of intermolecular hydrogen bonding; ethers dissolve in water about as much as the isomeric alcohol because the ether oxygen still hydrogen-bonds with water.

Cleavage of the C–O bond. Ethers are the least reactive of the three classes; the C–O bond breaks only with excess HI or HBr under drastic conditions (HI most effective). R–O–R + HX → R–X + R–OH, and with excess HX the alcohol is also converted: R–O–R + 2HX → 2R–X + H₂O. Mechanism: (1) the ether oxygen is protonated to an oxonium ion; (2) the halide ion attacks the less hindered alkyl carbon by S_N2 and displaces the alcohol. In a mixed ether the halide goes on the smaller group: CH₃OC₂H₅ + HI → CH₃I + C₂H₅OH. Two exceptions carry marks. With a tertiary group the path is S_N1: the protonated ether loses the stable tertiary carbocation, so (CH₃)₃C–O–CH₃ + HI gives (CH₃)₃C–I and CH₃OH. With an aryl group the halide attacks the alkyl carbon only, because the aryl C–O bond has partial double-bond character and an sp² carbon can neither undergo S_N2 nor form a phenyl cation: anisole + HI → phenol + CH₃I, never iodobenzene.

Electrophilic substitution in anisole. The –OCH₃ group is activating and ortho/para directing. (i) Bromine in ethanoic acid gives mainly p-bromoanisole, with no Lewis acid needed. (ii) CH₃Cl with anhydrous AlCl₃ gives 2- and 4-methoxytoluene. (iii) CH₃COCl with anhydrous AlCl₃ gives 2- and 4-methoxyacetophenone (para major). (iv) Conc. HNO₃ with conc. H₂SO₄ gives 2- and 4-nitroanisole.