Alcohols, Phenols and Ethers: NEET notes

1. Classification

NCERT §7.1

• An alcohol has an –OH group on an aliphatic carbon; a phenol has –OH directly on a carbon of an aromatic ring. Replacing the H of –OH by an alkyl or aryl group gives an ether, R–O–R′.
• Counting –OH groups sorts alcohols and phenols into monohydric (one), dihydric (two), trihydric (three) and polyhydric (many). Ethane-1,2-diol is dihydric and propane-1,2,3-triol is trihydric.
• Monohydric alcohols with the –OH on an sp³ carbon are primary (1°), secondary (2°) or tertiary (3°), matching the kind of carbon that holds the –OH.
• Allylic alcohols carry –OH on the sp³ carbon beside a C=C; benzylic alcohols carry it on the sp³ carbon joined to an aromatic ring. Each of these can itself be 1°, 2° or 3°.
• When –OH sits on an sp² carbon of a C=C the compound is a vinylic alcohol, such as CH₂=CH–OH. Phenols are the aromatic case of –OH on an sp² carbon.
• Ethers are symmetrical when both groups on oxygen are the same (C₂H₅OC₂H₅) and unsymmetrical, or mixed, when they differ (C₂H₅OCH₃, C₂H₅OC₆H₅).
• These classes are the starting points for detergents (alcohols), antiseptics (phenols) and fragrances (ethers). Furniture-polishing spirit is mostly ethanol, and sugar, cotton and paper are all built from –OH compounds.

2. Nomenclature and structure

NCERT §7.2–7.3

• Common names of alcohols add 'alcohol' to the alkyl name (methyl alcohol). IUPAC names replace the final 'e' of the parent alkane by 'ol' and give the –OH carbon the lowest possible number: CH₃CH(OH)CH₃ is propan-2-ol.
• IUPAC names with their common names: butan-1-ol (n-butyl alcohol); butan-2-ol (sec-butyl alcohol); 2-methylpropan-1-ol (isobutyl alcohol); 2-methylpropan-2-ol (tert-butyl alcohol); ethane-1,2-diol (ethylene glycol); propane-1,2,3-triol (glycerol).
• For polyhydric alcohols the 'e' of the alkane is kept and di, tri and so on are placed before 'ol'. Cyclic alcohols take the prefix cyclo, as in cyclohexanol and 2-methylcyclopentanol, with the –OH carbon numbered 1.
• Hydroxybenzene is simply called phenol, and this is also its accepted IUPAC name. The methylphenols are the cresols (2-, 3- and 4-methylphenol); benzene-1,2-diol, 1,3-diol and 1,4-diol are catechol, resorcinol and hydroquinone (quinol).
• Common names of ethers list the two groups in alphabetical order followed by 'ether' (ethylmethyl ether); with two identical groups the prefix di is used (diethyl ether). IUPAC names treat the ether as a hydrocarbon with an alkoxy or aryloxy substituent, the larger group being the parent.
• Ether name pairs: dimethyl ether = methoxymethane, diethyl ether = ethoxyethane, anisole = methoxybenzene, phenetole = ethoxybenzene, phenyl isopentyl ether = 3-methylbutoxybenzene; CH₃OCH₂CH₂OCH₃ is 1,2-dimethoxyethane.
• In alcohols the oxygen is sp³ hybridised with two lone pairs. Lone-pair repulsion squeezes the C–O–H angle to slightly below the tetrahedral 109°28′.
• In phenols the C–O bond is 136 pm, shorter than in an alcohol, because the oxygen lone pair is shared with the ring (partial double-bond character) and the carbon is sp² hybridised.
• In ethers the two bulky groups push apart, so the C–O–C angle is slightly larger than tetrahedral; the C–O bond length (141 pm) is close to that in alcohols.

3. Preparing alcohols

NCERT §7.4.1

• Acid-catalysed hydration: an alkene reacts with water in the presence of acid to give an alcohol. With an unsymmetrical alkene the addition follows Markovnikov's rule.
• Hydration runs in three steps: the alkene takes a proton from H₃O⁺ to form a carbocation; water attacks the carbocation; the protonated alcohol then loses H⁺ to give the alcohol.
• Hydroboration–oxidation: diborane, (BH₃)₂, adds across the C=C to give a trialkylborane, which aqueous sodium hydroxide and hydrogen peroxide oxidise to the alcohol. The yield is excellent.
• In hydroboration, boron bonds to the double-bond carbon carrying more hydrogens, so the net result resembles anti-Markovnikov addition of water: propene gives propan-1-ol. H.C. Brown reported the method in 1959; he and G. Wittig were jointly awarded the 1979 Nobel Prize in Chemistry.
• Aldehydes and ketones are reduced to alcohols by hydrogen over finely divided Pt, Pd or Ni, or by sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄). Aldehydes give primary alcohols; ketones give secondary alcohols.
• Carboxylic acids are reduced to primary alcohols by LiAlH₄, a strong reducing agent. LiAlH₄ is costly, so industry first turns the acid into an ester and then reduces the ester with hydrogen over a catalyst.
• Grignard reagents add to the C=O of aldehydes and ketones to give an adduct, which is hydrolysed to the alcohol.
• With a Grignard reagent, methanal gives a primary alcohol, any other aldehyde gives a secondary alcohol and a ketone gives a tertiary alcohol.

4. Preparing phenols

NCERT §7.4.2

• Phenol (carbolic acid) was originally obtained from coal tar, early in the nineteenth century. Today most of it is made synthetically.
• From haloarenes: chlorobenzene is fused with NaOH at 623 K and 320 atm to give sodium phenoxide, and acidifying the phenoxide gives phenol.
• From benzenesulphonic acid: benzene is sulphonated with oleum, the sulphonic acid is heated with molten sodium hydroxide to give sodium phenoxide, and acid then liberates phenol.
• From diazonium salts: an aromatic primary amine treated with nitrous acid (NaNO₂ + HCl) at 273–278 K forms a diazonium salt, which is hydrolysed to phenol by warming with water or dilute acid.
• From cumene: cumene (isopropylbenzene) is oxidised by air to cumene hydroperoxide, and dilute acid converts this into phenol and acetone.
• The cumene route supplies most of the world's phenol, and its second product, acetone, is also sold in large amounts.

5. Physical properties

NCERT §7.4.3

• Alcohols and phenols are an alkyl or aryl group joined to –OH. The –OH part is polar and forms hydrogen bonds; the hydrocarbon part is not.
• Boiling points rise as the number of carbons grows, because van der Waals forces grow. Within isomers, branching lowers the boiling point because the surface area of contact shrinks.
• Alcohols and phenols boil much higher than hydrocarbons, ethers and haloarenes or haloalkanes of similar mass, because their molecules are linked by intermolecular hydrogen bonds: ethanol boils far above propane, with methoxymethane in between.
• Worked order: n-butane < ethoxyethane < pentanal < pentan-1-ol. The alcohol has hydrogen bonding; the aldehyde has dipole–dipole forces; the ether only weak dipoles; the alkane only dispersion forces.
• Worked order, highest boiling point first: pentan-1-ol > butan-1-ol > butan-2-ol > propan-1-ol > ethanol > methanol. Longer chains boil higher, and the branched butan-2-ol sits below its straight-chain isomer.
• Water solubility comes from hydrogen bonds between –OH and water. It falls as the hydrophobic alkyl or aryl part grows. The lower alcohols dissolve in water in all proportions.

6. Acidity of alcohols and phenols

NCERT §7.4.4

• Alcohols and phenols release hydrogen gas with reactive metals such as sodium, potassium and aluminium, forming alkoxides and phenoxides. Phenols also react with aqueous sodium hydroxide; alcohols do not.
• The polar O–H bond makes alcohols weak Brönsted acids. An alkyl group pushes electron density towards oxygen (+I), which strengthens the O–H bond and destabilises the alkoxide, so acidity runs primary > secondary > tertiary.
• Alcohols are weaker acids than water: water displaces the alcohol from an alkoxide, and the alkoxide is the stronger base. Sodium ethoxide is a stronger base than sodium hydroxide.
• Through the lone pairs on oxygen, alcohols also act as Brönsted bases and accept a proton from strong acids.
• Phenol is more acidic than an alcohol. The –OH is on an electron-withdrawing sp² carbon, and the negative charge of phenoxide is spread over the ring by resonance, which makes the ion more stable than phenol itself.
• Table values of pKa: o-nitrophenol 7.2, m-nitrophenol 8.3, p-nitrophenol 7.1, phenol 10.0, o-cresol 10.2, m-cresol 10.1, p-cresol 10.2, ethanol 15.9. A larger pKa means a weaker acid.
• Phenol (pKa 10.0) is roughly a million times as acidic as ethanol (pKa 15.9): the pKa gap of 5.9 is a factor of about 7.9 × 10⁵ in Ka.
• An electron-withdrawing group such as –NO₂ raises the acidity of phenol, most strongly from the ortho and para positions where it delocalises the charge. An electron-releasing alkyl group, as in the cresols, lowers it.
• Worked order, most acidic first: 2,4,6-trinitrophenol > 3,5-dinitrophenol > 3-nitrophenol > phenol > 4-methylphenol > propan-1-ol.

7. Esterification, HX and dehydration

NCERT §7.4.4

• Esterification: an alcohol or a phenol gives an ester with a carboxylic acid, an acid anhydride or an acid chloride. With acids or anhydrides a little concentrated sulphuric acid is added, and water is removed as it forms because the reaction is reversible.
• With an acid chloride the reaction is run in the presence of pyridine, a base that neutralises the HCl formed and shifts the equilibrium forward. Acetylating salicylic acid gives aspirin, an analgesic, anti-inflammatory and antipyretic.
• Reactions that break the C–O bond are shown by alcohols; phenols undergo this only with zinc. Alcohols react with hydrogen halides to give alkyl halides.
• Lucas test: the reagent is concentrated HCl with ZnCl₂. Alcohols dissolve in it, but the alkyl chlorides formed do not, so the mixture turns cloudy. A tertiary alcohol gives turbidity at once; a primary alcohol gives none at room temperature.
• Phosphorus trihalides convert alcohols into alkyl halides; PBr₃ gives alkyl bromides.
• Dehydration: alcohols lose water to form alkenes when heated with a protic acid (concentrated H₂SO₄ or H₃PO₄) or with catalysts such as anhydrous zinc chloride or alumina. Ethanol with concentrated H₂SO₄ at 443 K gives ethene.
• Ease of dehydration is tertiary > secondary > primary, because tertiary carbocations form most easily. Milder conditions suffice for the higher classes: propan-2-ol gives propene with 85% H₃PO₄ at 440 K, while 2-methylpropan-2-ol gives 2-methylpropene with only 20% H₃PO₄ at 358 K.
• Mechanism for ethanol: the alcohol is protonated; the protonated alcohol loses water to give a carbocation, the slow, rate-determining step; the carbocation loses H⁺ to give the alkene.
• The acid used in the first step is given back in the last step, and ethene is removed as it forms so the equilibrium keeps moving to the right.

8. Oxidation of alcohols

NCERT §7.4.4

• Oxidising an alcohol forms a C=O bond and removes an O–H and a C–H hydrogen together, so it is also called dehydrogenation.
• A primary alcohol is oxidised first to an aldehyde and then to a carboxylic acid. Strong oxidising agents such as acidified potassium permanganate take it straight through to the acid.
• To stop at the aldehyde, anhydrous CrO₃ is used. Pyridinium chlorochromate (PCC), a complex of CrO₃ with pyridine and HCl, gives the aldehyde in better yield.
• A secondary alcohol is oxidised by chromium trioxide (CrO₃, chromic anhydride) to a ketone, which resists further mild oxidation.
• A tertiary alcohol has no hydrogen on the –OH carbon and is not oxidised under these conditions. Under strong conditions such as KMnO₄ with heat, C–C bonds break and a mixture of acids with fewer carbons forms.
• Passing alcohol vapour over heated copper at 573 K dehydrogenates primary and secondary alcohols to aldehydes and ketones, while tertiary alcohols are dehydrated to alkenes.
• In the body methanol is oxidised to methanal and then to methanoic acid, which can cause blindness and death. Treatment is an intravenous infusion of diluted ethanol, which keeps the enzyme busy so the kidneys can excrete the methanol.

9. Reactions of phenols

NCERT §7.4.4

• –OH makes the ring more reactive to electrophiles and sends the new group to the ortho and para positions, the carbons whose electron density resonance raises.
• With dilute nitric acid at 298 K phenol gives a mixture of ortho- and para-nitrophenols. Steam distillation separates them: the ortho isomer is volatile because of intramolecular hydrogen bonding, while the para isomer is held back by intermolecular hydrogen bonds.
• Concentrated nitric acid converts phenol to 2,4,6-trinitrophenol (picric acid), but the yield is poor. It is now made by sulphonating phenol to phenol-2,4-disulphonic acid first and then treating that with concentrated nitric acid. Picric acid is a strong acid.
• Bromination needs no Lewis acid because the –OH makes the ring so reactive. Bromine in a solvent of low polarity (CHCl₃ or CS₂) at low temperature gives monobromophenols.
• With bromine water, phenol gives a white precipitate of 2,4,6-tribromophenol.
• Kolbe's reaction: phenoxide (from phenol and NaOH) is even more reactive than phenol, so it is substituted even by the weak electrophile CO₂; ortho-hydroxybenzoic acid is the main product.
• Reimer–Tiemann reaction: phenol with chloroform and sodium hydroxide gains a –CHO group at the ortho position. The intermediate, a substituted benzal chloride, is hydrolysed by alkali to salicylaldehyde.
• Heating phenol with zinc dust converts it to benzene. Chromic acid oxidises phenol to benzoquinone, and phenols left in air slowly darken as quinones form.

10. Methanol and ethanol

NCERT §7.5

• Methanol, CH₃OH, was once obtained by destructive distillation of wood, hence 'wood spirit'. Today most of it comes from carbon monoxide and hydrogen combined over a ZnO–Cr₂O₃ catalyst at high temperature and pressure.
• Methanol is a colourless liquid boiling at 337 K. It is highly poisonous: small amounts can cause blindness and larger amounts death. It is a solvent for paints and varnishes and is used chiefly to make formaldehyde.
• Ethanol, C₂H₅OH, is made commercially by fermentation of sugars. The enzyme invertase turns the sugar of molasses, sugarcane or grapes into glucose and fructose, both C₆H₁₂O₆, and zymase from yeast ferments these to ethanol.
• Fermentation is anaerobic (no air), and carbon dioxide is given off. Zymase stops working once the alcohol content passes 14 percent.
• If air enters the fermenting mixture, oxygen oxidises ethanol to ethanoic acid, which spoils the taste of the drink.
• Ethanol is a colourless liquid boiling at 351 K, used as a solvent in the paint industry and to make many carbon compounds. Large amounts are now also made by hydration of ethene.
• Denaturation: commercial alcohol is made unfit to drink by adding a little copper sulphate (for colour) and pyridine (for a foul smell).
• Taken in, ethanol acts on the central nervous system: moderate amounts impair judgement, larger amounts cause nausea and loss of consciousness, and still more can stop spontaneous breathing.

11. Preparing ethers

NCERT §7.6.1

• Protic acids (H₂SO₄, H₃PO₄) dehydrate alcohols, and the conditions decide the product: ethanol with sulphuric acid gives ethene at 443 K but mainly ethoxyethane at 413 K.
• Ether formation from an alcohol is an SN2 reaction: one alcohol molecule attacks a protonated alcohol, water leaves, and the protonated ether loses H⁺.
• This route suits only primary alcohols with unhindered alkyl groups at low temperature. With secondary and tertiary alcohols, elimination to the alkene wins over substitution.
• Williamson synthesis: an alkyl halide reacts with a sodium alkoxide to give an ether, R–X + R′–O⁻Na⁺ → R–O–R′ + NaX. It makes both symmetrical and unsymmetrical ethers.
• The alkoxide attacks the alkyl halide by SN2, so primary alkyl halides work well. With secondary and tertiary halides elimination competes, and a tertiary halide gives only the alkene.
• Example: sodium methoxide with (CH₃)₃C–Br gives only 2-methylpropene, so tert-butyl methyl ether must be made from the tertiary alkoxide and a methyl halide instead.
• Phenols are converted to ethers the same way: the phenoxide ion is the nucleophile that attacks the alkyl halide.
• Diethyl ether was once widely used as an inhalation anaesthetic, but its slow action and unpleasant recovery led to its replacement by other compounds.

12. Ethers: properties and reactions

NCERT §7.6.2–7.6.3

• The C–O bonds are polar, so ethers have a net dipole moment, but this weak polarity barely raises the boiling point: it stays close to that of an alkane of similar mass and far below that of an alcohol.
• Boiling points: n-pentane 309.1 K, ethoxyethane 307.6 K, butan-1-ol 390 K. Alcohols are linked by hydrogen bonds; ether molecules are not.
• Ethers of low mass dissolve in water about as well as alcohols of similar mass, because ether oxygen can accept hydrogen bonds from water. Ethoxyethane and butan-1-ol dissolve to 7.5 g and 9 g per 100 mL of water; pentane hardly dissolves.
• Ethers are among the least reactive functional groups. The C–O bond is cleaved only under drastic conditions with an excess of a hydrogen halide; the reactivity order is HI > HBr > HCl.
• A dialkyl ether with HX gives two alkyl halides. The ether oxygen is first protonated; the halide ion then attacks by SN2 at the less substituted carbon, so the smaller alkyl group ends up as the halide.
• When one group is tertiary the cleavage goes by SN1 through the stable tertiary carbocation, and the tertiary group becomes the halide.
• Anisole with HI gives phenol and methyl iodide: the O–C₆H₅ bond has partial double-bond character and does not break, and phenyl halides never form this way.
• The alkoxy group activates the ring and directs to ortho and para. Anisole brominates in ethanoic acid even without an iron(III) bromide catalyst, the para isomer forming in 90% yield.
• Anisole undergoes Friedel–Crafts alkylation and acylation with AlCl₃, and a mixture of concentrated sulphuric and nitric acids gives ortho- and para-nitroanisole.

Must-know facts

1. Alcohol: –OH on sp³ carbon; phenol: –OH on an aromatic sp² carbon; vinylic alcohol: –OH on a C=C carbon. Allylic and benzylic alcohols are sp³ and can be 1°, 2° or 3°.
2. Catechol, resorcinol, hydroquinone = benzene-1,2-, 1,3- and 1,4-diol; cresols = methylphenols; anisole = methoxybenzene; phenetole = ethoxybenzene.
3. C–O–H angle slightly below 109°28′ (lone pairs); phenol C–O 136 pm (partial double bond); ether C–O–C angle slightly above tetrahedral, C–O 141 pm.
4. Acid-catalysed hydration follows Markovnikov; hydroboration–oxidation gives the anti-Markovnikov-looking alcohol (propene → propan-1-ol).
5. Grignard + methanal → 1° alcohol; + other aldehyde → 2°; + ketone → 3°.
6. Aldehyde → 1° alcohol, ketone → 2° alcohol with H₂/Pt, Pd, Ni, NaBH₄ or LiAlH₄; acids need LiAlH₄ (industry: ester + H₂).
7. Phenol routes: chlorobenzene + NaOH at 623 K, 320 atm; benzenesulphonic acid + molten NaOH; diazonium salt (273–278 K) + warm water; cumene → hydroperoxide → phenol + acetone.
8. Boiling point rises with carbon number, falls with branching; hydrogen bonding puts alcohols far above ethers and alkanes of similar mass.
9. pKa: p-nitrophenol 7.1 < o-nitrophenol 7.2 < m-nitrophenol 8.3 < phenol 10.0 < cresols 10.1–10.2 < ethanol 15.9 (acidity runs the other way).
10. Alcohol acidity 1° > 2° > 3°; alcohols are weaker acids than water; alkoxide is a stronger base than hydroxide.
11. Esterification with acid chlorides uses pyridine to remove HCl; salicylic acid acetylated → aspirin.
12. Lucas reagent = conc. HCl + ZnCl₂: 3° alcohol turbid at once, 1° not turbid at room temperature.
13. Dehydration ease 3° > 2° > 1°; carbocation formation is the slow step; ethanol + conc. H₂SO₄ → ethene at 443 K, ethoxyethane at 413 K.
14. Oxidation: 1° → aldehyde (CrO₃ anhydrous or PCC) → acid (acidified KMnO₄); 2° → ketone (CrO₃); 3° resists. Cu at 573 K: 1° → aldehyde, 2° → ketone, 3° → alkene.
15. Dilute HNO₃ (298 K) → o- + p-nitrophenol, separated by steam distillation (ortho is volatile, intramolecular H-bond).
16. Bromine water + phenol → white 2,4,6-tribromophenol; Br₂ in CHCl₃ or CS₂ at low temperature → monobromophenols.
17. Kolbe: sodium phenoxide + CO₂ → ortho-hydroxybenzoic acid. Reimer–Tiemann: CHCl₃ + NaOH → salicylaldehyde.
18. Phenol + Zn dust → benzene; phenol + chromic acid → benzoquinone.
19. Methanol: CO + H₂ over ZnO–Cr₂O₃, b.p. 337 K, poisonous. Ethanol: invertase then zymase, stops at 14%, b.p. 351 K; denatured with CuSO₄ + pyridine.
20. Williamson: R–X + NaOR′ by SN2; use a primary halide. CH₃ONa + (CH₃)₃CBr gives only 2-methylpropene.
21. b.p.: n-pentane 309.1 K, ethoxyethane 307.6 K, butan-1-ol 390 K; solubility 7.5 g vs 9 g per 100 mL water.
22. Ether cleavage by HX: HI > HBr > HCl; halide attacks the smaller group (SN2) unless one group is tertiary (SN1). Anisole + HI → phenol + CH₃I.
23. Anisole + Br₂ in ethanoic acid (no FeBr₃) → para-bromoanisole in 90% yield.

Common traps

Formulas

Key terms

Monohydric / polyhydric

Having one –OH group / many –OH groups.

Allylic alcohol

–OH on the sp³ carbon next to a C=C.

Benzylic alcohol

–OH on the sp³ carbon joined to an aromatic ring.

Vinylic alcohol

–OH on one of the sp² carbons of a C=C.

Symmetrical ether

An ether whose two groups on oxygen are the same.

Hydroboration–oxidation

Diborane addition to an alkene followed by H₂O₂/NaOH, giving the anti-Markovnikov-looking alcohol.

Phenoxide ion

C₆H₅O⁻, the conjugate base of phenol, stabilised by resonance with the ring.

Lucas reagent

Concentrated HCl with ZnCl₂, used to tell 1°, 2° and 3° alcohols apart.

PCC

Pyridinium chlorochromate, a CrO₃–pyridine–HCl complex that stops at the aldehyde.

Denatured alcohol

Ethanol made undrinkable with copper sulphate and pyridine.

Williamson synthesis

Ether formation from an alkyl halide and an alkoxide or phenoxide.

Picric acid

2,4,6-Trinitrophenol, a strong acid.