Lesson 13 of 13 · 16 min

Chapter review

Watch a class

The whole chapter on YouTube

Whole chapter revision with NCERT questions
NCERT Wallah · Hinglish · Whole chapter · Open on YouTube

Full chapter run-through for boards
Next Toppers - 12th Science · Hinglish · Whole chapter · Open on YouTube

Must-know facts

23 facts

1Alcohol: –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°.
2Catechol, resorcinol, hydroquinone = benzene-1,2-, 1,3- and 1,4-diol; cresols = methylphenols; anisole = methoxybenzene; phenetole = ethoxybenzene.
3C–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.
4Acid-catalysed hydration follows Markovnikov; hydroboration–oxidation gives the anti-Markovnikov-looking alcohol (propene → propan-1-ol).
5Grignard + methanal → 1° alcohol; + other aldehyde → 2°; + ketone → 3°.
6Aldehyde → 1° alcohol, ketone → 2° alcohol with H₂/Pt, Pd, Ni, NaBH₄ or LiAlH₄; acids need LiAlH₄ (industry: ester + H₂).
7Phenol routes: chlorobenzene + NaOH at 623 K, 320 atm; benzenesulphonic acid + molten NaOH; diazonium salt (273–278 K) + warm water; cumene → hydroperoxide → phenol + acetone.
8Boiling point rises with carbon number, falls with branching; hydrogen bonding puts alcohols far above ethers and alkanes of similar mass.
9pKa: 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).
10Alcohol acidity 1° > 2° > 3°; alcohols are weaker acids than water; alkoxide is a stronger base than hydroxide.
11Esterification with acid chlorides uses pyridine to remove HCl; salicylic acid acetylated → aspirin.
12Lucas reagent = conc. HCl + ZnCl₂: 3° alcohol turbid at once, 1° not turbid at room temperature.
13Dehydration ease 3° > 2° > 1°; carbocation formation is the slow step; ethanol + conc. H₂SO₄ → ethene at 443 K, ethoxyethane at 413 K.
14Oxidation: 1° → aldehyde (CrO₃ anhydrous or PCC) → acid (acidified KMnO₄); 2° → ketone (CrO₃); 3° resists. Cu at 573 K: 1° → aldehyde, 2° → ketone, 3° → alkene.
15Dilute HNO₃ (298 K) → o- + p-nitrophenol, separated by steam distillation (ortho is volatile, intramolecular H-bond).
16Bromine water + phenol → white 2,4,6-tribromophenol; Br₂ in CHCl₃ or CS₂ at low temperature → monobromophenols.
17Kolbe: sodium phenoxide + CO₂ → ortho-hydroxybenzoic acid. Reimer–Tiemann: CHCl₃ + NaOH → salicylaldehyde.
18Phenol + Zn dust → benzene; phenol + chromic acid → benzoquinone.
19Methanol: 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.
20Williamson: R–X + NaOR′ by SN2; use a primary halide. CH₃ONa + (CH₃)₃CBr gives only 2-methylpropene.
21b.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.
22Ether cleavage by HX: HI > HBr > HCl; halide attacks the smaller group (SN2) unless one group is tertiary (SN1). Anisole + HI → phenol + CH₃I.
23Anisole + Br₂ in ethanoic acid (no FeBr₃) → para-bromoanisole in 90% yield.

Common traps

Where marks are lost

Classing benzyl alcohol as a phenol because it contains a benzene ring.

In benzyl alcohol –OH is on the sp³ CH₂ carbon, so it is a benzylic (primary) alcohol. A phenol has –OH directly on a ring carbon.

Writing propan-2-ol as the product of hydroboration–oxidation of propene.

Boron goes to the terminal CH₂, so the –OH ends up there: propan-1-ol. Propan-2-ol is the Markovnikov product of acid-catalysed hydration.

Thinking a Grignard reagent gives a primary alcohol with any aldehyde.

Only methanal gives a primary alcohol. Every other aldehyde gives a secondary alcohol, and ketones give tertiary ones.

Ranking alcohols as more acidic than water because they react with sodium.

Both release H₂ with sodium, but water displaces alcohol from an alkoxide, so alcohols are the weaker acids and alkoxides the stronger bases.

Reading the higher pKa as the stronger acid.

pKa = −log Ka, so a larger pKa means a smaller Ka. Ethanol (15.9) is far weaker than phenol (10.0), and p-nitrophenol (7.1) is the strongest in the table.

Expecting m-nitrophenol to be as acidic as o- and p-nitrophenol.

–NO₂ delocalises the phenoxide charge by resonance only from ortho and para. From meta it acts only inductively: pKa 8.3 against 7.2 and 7.1.

Predicting that a tertiary alcohol is oxidised to a ketone.

The –OH carbon of a tertiary alcohol carries no H to lose, so mild oxidants leave it alone; only strong oxidation breaks C–C bonds to smaller acids.

Using acidified KMnO₄ to make an aldehyde from a primary alcohol.

KMnO₄ takes the alcohol through to the carboxylic acid. Use anhydrous CrO₃ or, better, PCC to stop at the aldehyde.

Making tert-butyl methyl ether from sodium methoxide and tert-butyl bromide.

The strong base eliminates HBr from the tertiary halide, giving only 2-methylpropene. Use sodium tert-butoxide with a methyl halide.

Writing iodobenzene and methanol as the products of anisole with HI.

The aryl C–O bond has partial double-bond character and does not break. HI cuts the methyl–O bond: phenol + CH₃I.

Explaining the volatility of o-nitrophenol by its lower mass.

Both isomers have the same mass. The ortho isomer bonds to itself (intramolecular H-bond); the para isomer bonds to its neighbours, so it is less volatile.

Formulas

4 to know

pKa and Ka

pKa = −log Ka; Ka ratio = 10^(ΔpKa)

Phenol vs ethanol: ΔpKa = 15.9 − 10.0 = 5.9, ratio ≈ 7.9 × 10⁵.

Fermentation

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Zymase in yeast; anaerobic; zymase stops above 14% alcohol.

Williamson synthesis

R–X + R′–O⁻Na⁺ → R–O–R′ + NaX

SN2; primary halide gives the ether, tertiary halide gives the alkene.

Cumene process

C₆H₅CH(CH₃)₂ + O₂ → C₆H₅C(CH₃)₂OOH; then H⁺/H₂O → C₆H₅OH + CH₃COCH₃

Air oxidation, then dilute acid.

Key terms

12 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.

Test yourself: 10 questionsExam-style questions on Alcohols, Phenols and Ethers, with full solutions.Start