Alcohols, Phenols and Ethers

In this chapter: classification and nomenclature, structure of the functional groups, preparation of alcohols and phenols, physical properties, acidity, esterification, reactions involving C-O cleavage (HX, Lucas test, dehydration, oxidation), electrophilic substitution in phenol, Kolbe's and Reimer-Tiemann reactions, commercially important alcohols, and preparation and reactions of ethers.

Classification, nomenclature and structure

Alcohols have -OH on an sp³ carbon: primary (1°), secondary (2°) or tertiary (3°) depending on how many carbons are attached to that carbon; allylic and benzylic alcohols are special cases. Phenols have -OH directly on a benzene ring. Ethers have R-O-R′; they are simple (symmetrical) if both groups are the same and mixed (unsymmetrical) if they differ.

In alcohols the oxygen is sp³ hybridised; the C-O-H angle in methanol (108.9°) is slightly less than tetrahedral because of lone pair repulsion. In phenol the C-O bond (136 pm) is shorter than in methanol (142 pm), partly because a lone pair of oxygen is delocalised into the ring and partly because the carbon is sp². In ethers the C-O-C angle (111.7°) is slightly more than tetrahedral, because of repulsion between the two bulky alkyl groups.

Preparation of alcohols

• Acid-catalysed hydration of alkenes: water adds by Markovnikov's rule, via a carbocation. Propene gives propan-2-ol.
• Hydroboration-oxidation: diborane adds to the alkene to give a trialkylborane, which H₂O₂ in aqueous NaOH oxidises to the alcohol. The net result is anti-Markovnikov addition of water: propene gives propan-1-ol, in excellent yield.
• Reduction of carbonyl compounds: aldehydes give 1° alcohols and ketones give 2° alcohols with H₂ over Pt, Pd or Ni, or with NaBH₄ or LiAlH₄. Carboxylic acids are reduced to 1° alcohols by LiAlH₄; since LiAlH₄ is expensive, industry first converts acids to esters and hydrogenates those.
• Grignard reagents: RMgX adds to the carbonyl group and the adduct is hydrolysed. Methanal gives a 1° alcohol, other aldehydes give 2° alcohols, and ketones give 3° alcohols.

Preparation of phenol

• From haloarenes: chlorobenzene fused with NaOH at 623 K and 300 atm, then acidified.
• From benzenesulphonic acid: benzene is sulphonated with oleum; the sulphonic acid is fused with molten NaOH to sodium phenoxide, which is acidified.
• From diazonium salts: aniline is diazotised with NaNO₂ and HCl at 273 to 278 K; warming the diazonium salt with water gives phenol and N₂.
• From cumene (industrial): cumene (isopropylbenzene) is oxidised by air to cumene hydroperoxide, which dilute acid converts to phenol and acetone, a valuable by-product.

Physical properties

Alcohols and phenols have much higher boiling points than hydrocarbons, ethers and haloalkanes of similar molecular mass, because of intermolecular hydrogen bonding. Boiling point rises with the number of carbon atoms and falls with branching. Lower alcohols mix with water in all proportions because they form hydrogen bonds with it; solubility falls as the hydrophobic alkyl part grows.

Reactions of alcohols and phenols

Acidity

Alcohols and phenols react with active metals such as sodium to give alkoxides or phenoxides and hydrogen. Phenol also reacts with aqueous NaOH; alcohols do not.

• Alcohol acidity: 1° > 2° > 3°. Alkyl groups push electrons (+I effect) towards oxygen, making the O-H bond harder to break.
• Alcohols are weaker acids than water. The alkoxide ion is a stronger base than hydroxide: RO⁻ + H₂O → ROH + OH⁻.
• Phenol (pKa 10.0) is far more acidic than ethanol (pKa 15.9). The phenoxide ion is stabilised by delocalisation of its negative charge over the ring, while the ethoxide ion has no such stabilisation. In phenol the -OH is attached to an sp² carbon, which is more electronegative, so the O-H bond is more polar.
• Electron-withdrawing groups such as -NO₂ increase the acidity of phenol, especially at ortho and para positions, where they help delocalise the negative charge. Electron-releasing groups such as -CH₃ decrease it.

Esterification

Alcohols and phenols react with carboxylic acids (in the presence of a little concentrated H₂SO₄), acid chlorides or acid anhydrides to form esters. With acids the reaction is reversible, so water is removed as it forms. With acid chlorides, pyridine is added to neutralise the HCl. Acetylation of salicylic acid with acetic anhydride gives aspirin (acetylsalicylic acid).

Reactions involving cleavage of the C-O bond in alcohols

• With HX: ROH + HX → RX + H₂O. The Lucas test uses concentrated HCl and ZnCl₂. Alkyl chlorides are insoluble and make the solution turbid: tertiary alcohols give turbidity at once, secondary alcohols within a few minutes, and primary alcohols do not give turbidity at room temperature.
• With phosphorus halides: PCl₃, PCl₅ or PBr₃ give alkyl halides; SOCl₂ is also used.
• Dehydration: ethanol with concentrated H₂SO₄ at 443 K gives ethene. Secondary alcohols need milder conditions (85% H₃PO₄, 440 K) and tertiary alcohols milder still (20% H₃PO₄, 358 K), so the ease is 3° > 2° > 1°. The mechanism is protonation of -OH, loss of water to form a carbocation (slow step), and loss of H⁺ from the neighbouring carbon.
• Oxidation: primary alcohols give aldehydes, which are easily oxidised further to carboxylic acids (acidified KMnO₄ or K₂Cr₂O₇). To stop at the aldehyde, use CrO₃ in an anhydrous medium or PCC (pyridinium chlorochromate). Secondary alcohols give ketones (CrO₃). Tertiary alcohols resist oxidation; under strong conditions they break into a mixture of acids with fewer carbon atoms.
• Dehydrogenation: vapours passed over heated copper at 573 K: 1° alcohols give aldehydes, 2° alcohols give ketones, and 3° alcohols lose water to give alkenes.

Reactions of phenol

The -OH group is strongly activating and ortho, para-directing, because its lone pair raises the electron density of the ring at those positions.

• Nitration: dilute HNO₃ at 298 K gives o- and p-nitrophenol. They are separated by steam distillation: o-nitrophenol is steam volatile because of intramolecular hydrogen bonding, while p-nitrophenol is held back by intermolecular hydrogen bonding. Concentrated HNO₃ gives 2,4,6-trinitrophenol (picric acid), though in poor yield; it is better made by sulphonating phenol first and then nitrating.
• Halogenation: with bromine in a solvent of low polarity (CHCl₃ or CS₂) at low temperature, monobromophenols form. With bromine water, a white precipitate of 2,4,6-tribromophenol forms at once.
• Kolbe's reaction: sodium phenoxide, being more reactive than phenol, reacts with CO₂ (a weak electrophile) to give, after acidification, salicylic acid (2-hydroxybenzoic acid).
• Reimer-Tiemann reaction: phenol with chloroform and aqueous NaOH forms a -CHCl₂ group at the ortho position, which is hydrolysed; acidification gives salicylaldehyde (2-hydroxybenzaldehyde).
• With zinc dust: phenol is reduced to benzene.
• Oxidation: chromic acid (Na₂Cr₂O₇ and H₂SO₄) gives benzoquinone. Phenols slowly turn dark in air for the same reason.

Commercially important alcohols

Methanol (wood spirit) was once made by destructive distillation of wood; today it is made by catalytic hydrogenation of carbon monoxide: CO + 2H₂ → CH₃OH (ZnO-Cr₂O₃ catalyst, 200 to 300 atm, 573 to 673 K). It is highly poisonous; small amounts cause blindness and large amounts cause death.

Ethanol is made by fermentation of sugars. The enzyme invertase converts sucrose to glucose and fructose, and zymase (both from yeast) converts these to ethanol and CO₂, in the absence of air. Commercial alcohol is made unfit for drinking (denatured) by adding copper sulphate, which gives it colour, and pyridine, which gives it a foul smell.

Ethers

Preparation

• Dehydration of alcohols: ethanol with concentrated H₂SO₄ at 413 K gives ethoxyethane (at 443 K, ethene is the main product). One alcohol molecule attacks a protonated one by SN2. The method suits only primary alcohols; secondary and tertiary alcohols give alkenes instead.
• Williamson synthesis: R-X + R′-O⁻Na⁺ → R-O-R′ + NaX. It is an SN2 reaction, so the halide should be primary. With a tertiary halide, the alkoxide acts as a base and elimination gives an alkene.

Worked example: Which pair of reagents gives 2-methoxy-2-methylpropane, (CH₃)₃C-O-CH₃, by Williamson synthesis: (CH₃)₃C-Br with CH₃ONa, or CH₃Br with (CH₃)₃C-ONa?
Solution: Williamson synthesis is SN2, which needs an unhindered halide. With (CH₃)₃C-Br, the methoxide ion removes a β-hydrogen and 2-methylpropene forms. The correct choice is CH₃Br + (CH₃)₃C-ONa: the tertiary part comes from the alkoxide and the halide is methyl.

Physical properties and reactions

Ethers have boiling points close to those of alkanes of similar mass and far below those of alcohols, since they cannot hydrogen bond with each other. Their solubility in water is similar to that of alcohols of the same mass, because the ether oxygen can accept hydrogen bonds from water.

• Cleavage by HX: ethers are unreactive, but concentrated HI or HBr at high temperature splits them: R-O-R + HX → RX + ROH (reactivity HI > HBr > HCl). In a methyl ether with a primary or secondary alkyl group, iodide attacks the less hindered methyl carbon by SN2, giving CH₃I and the alcohol. If one group is tertiary, the reaction goes by SN1 and the tertiary halide forms.
• Aryl alkyl ethers: anisole with HI gives phenol and methyl iodide, never iodobenzene. The O-aryl bond has partial double bond character, and SN2 attack on an sp² ring carbon is not possible.
• Electrophilic substitution in anisole: -OCH₃ activates the ring and directs ortho and para. Bromination in ethanoic acid needs no FeBr₃ and gives mainly 4-bromoanisole. Friedel-Crafts methylation and acetylation, and nitration, give the 2- and 4-substituted products, with the para isomer major.

Common mistakes: (1) Giving propan-2-ol as the product of hydroboration-oxidation of propene; it gives propan-1-ol. (2) Writing iodobenzene as a product of anisole with HI. (3) Choosing a tertiary halide in a Williamson synthesis. (4) Forgetting that phenol reacts with NaOH but alcohols do not. (5) Using KMnO₄ when the question wants an aldehyde from a primary alcohol; use PCC.

JEE and NEET focus

• Preparation of alcohols with regiochemistry: acid-catalysed hydration vs hydroboration-oxidation; Grignard routes to 1°, 2° and 3° alcohols.
• Acidity order of alcohols, water and substituted phenols, with reasons.
• Lucas test, dehydration conditions and ease, oxidation products and reagents (PCC, CrO₃, KMnO₄), Cu at 573 K.
• Phenol reactions: bromine water, nitration, Kolbe's and Reimer-Tiemann reactions, cumene process.
• Williamson synthesis reagent choice and products of ether cleavage with HI.

Practice questions