Alcohols, Phenols and Ethers studies three oxygen-bearing families, how to name and classify them, how they are prepared (hydration, reduction, Grignard, cumene process, Williamson synthesis), and the reactions that define each. The chapter also explains hydrogen bonding behind their physical properties and the acidity order phenol greater than water greater than alcohol. It is a high-yield Class 12 Organic chapter for NEET, with reliable questions on Lucas test, the cumene process, phenol reactions like Kolbe and Reimer-Tiemann, and ether cleavage by HX.
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Three Oxygen Families
Alcohols, phenols and ethers all hold an oxygen, but the oxygen sits in a different neighbourhood in each.
Phenol = –OH directly on a benzene ring; benzyl/allyl –OH on a side chain is still an alcohol.
- Alcohol: –OH on an sp3 carbon (e.g. ethanol)
- Phenol: –OH bolted onto the aromatic ring carbon
- Ether: O bridges two carbons, no O–H at all
1° / 2° / 3° and IUPAC Names
Count the carbons on the –OH carbon to classify; that one number predicts most reactions.
Ethers: larger group = parent alkane, smaller –OR = alkoxy; CH3–O–C2H5 = methoxyethane.
- 0–1 other C → 1°, two → 2°, three → 3° (count carbons, not H)
- Number the chain so –OH gets the lowest locant
- Glycols are dihydric (two –OH groups)
Preparing Alcohols
Build alcohols from alkenes or carbonyls, choosing the route by the class you need.
LiAlH4 reduces –COOH to 1° alcohol; mild NaBH4 will not touch a carboxylic acid.
- Acid hydration of alkene → Markovnikov (OH on more substituted C)
- Hydroboration–oxidation → anti-Markovnikov 1° alcohol
- Aldehyde→1°, ketone→2° on reduction
Cumene Process & Williamson
Phenol comes from cumene industrially; ethers from an alkoxide plus a halide.
Cumene process gives phenol + acetone (not methanol). Diethyl ether: ethanol + conc. H2SO4 at 413 K.
- Williamson is Sₙ2 → use a 1° halide; 3° halides eliminate to alkene
- Unsymmetrical ether: hindered group as alkoxide, simple as 1° halide
- Cumene → cumene hydroperoxide → H⁺ → phenol + acetone
Boiling Points & H-Bonding
Hydrogen bonding decides who boils high and who boils low.
Ethers have no O–H → cannot self H-bond; b.p. rises with chain length, falls with branching.
- Alcohols/phenols self H-bond → high boiling points
- Diethyl ether b.p. ~35°C vs butan-1-ol ~118°C (similar mass)
- Branching = compact shape = less contact = lower b.p.
Acidic Character
Acidity is set by how stable the leftover –O⁻ ion is after losing H⁺.
Among alcohols 1° > 2° > 3° (alkyl +I destabilises alkoxide). This is opposite to the carbocation order.
- Phenol strongest: phenoxide resonance-stabilised into the ring
- EWG (–NO2, o/p) raise phenol acidity; EDG (–CH3, –OCH3) lower it
- Both react with Na→H2; only phenol reacts with NaOH
What Alcohols Do
Sort every reaction by which bond breaks: the O–H or the C–O.
Lucas (conc. HCl + ZnCl2): 3° turbid at once, 2° in ~5 min, 1° no reaction at RT.
- O–H cleavage: esterification (reversible) and Na → alkoxide + H2
- Oxidation: 1°→aldehyde→acid, 2°→ketone, 3° resists; PCC stops 1° at aldehyde
- Dehydration (conc. H2SO4) follows Saytzeff → more substituted alkene
What Phenols Do
–OH activates the ring strongly and directs incoming groups to ortho/para.
Phenol’s strong C–O bond means no Lucas/HX substitution like alcohols. FeCl3 gives a violet colour.
- Kolbe: phenoxide + CO2 then H⁺ → salicylic acid (2-OH-benzoic acid)
- Reimer-Tiemann: phenol + CHCl3 + NaOH → salicylaldehyde
- Conc. HNO3 → picric acid (2,4,6-trinitrophenol); oxidation → benzoquinone
What Ethers Do
Ethers are inert solvents; only HX cleavage and aromatic substitution matter for NEET.
Sₙ2 default: iodide hits the less-hindered carbon. If a group is 3°/allyl/benzyl → Sₙ1, stable cation takes the halide.
- CH3–O–C2H5 + HI → CH3I + C2H5OH (methyl less hindered)
- Anisole + HI → phenol + CH3I (aryl C–O never cleaves)
- –OR is activating, o/p-directing in aromatic ethers
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Related Chapters in Class 12 Chemistry
- Aldehydes, Ketones and Carboxylic Acids Class 12 Notes
- Haloalkanes and Haloarenes Class 12 Notes
- Amines Class 12 Notes
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Frequently Asked Questions
An alcohol has an -OH group bonded to an sp3 carbon, a phenol has an -OH bonded directly to a carbon of an aromatic ring, and an ether has an oxygen bridging two carbon groups (R-O-R’) with no O-H bond at all. So a benzene ring with -OH on a side-chain carbon is still an alcohol, not a phenol.
The Lucas reagent is concentrated HCl plus anhydrous ZnCl2, used to distinguish primary, secondary and tertiary alcohols. A tertiary alcohol turns turbid immediately, a secondary alcohol in about 5 minutes, and a primary alcohol shows no reaction at room temperature, because reactivity toward HX follows 3 degree greater than 2 degree greater than 1 degree (SN1 carbocation stability).
Phenol is made on a large scale by the cumene process: cumene (isopropylbenzene) is oxidised by air to cumene hydroperoxide, which on treatment with dilute acid gives phenol and acetone as the byproduct. Acetone, not methanol, is the coproduct, which is a common exam trap.
Acidity depends on the stability of the conjugate base left after losing H+. The phenoxide ion is resonance-stabilised because its negative charge is delocalised into the aromatic ring, while the alkoxide from an alcohol is not, so the order is phenol greater than water greater than alcohol. Electron-withdrawing groups like -NO2 increase phenol acidity and electron-donating groups decrease it.
Yes, it is a scoring Class 12 Organic Chemistry chapter that regularly contributes NEET questions. High-frequency areas are the Lucas test, the cumene process, distinguishing reactions of phenol such as Kolbe and Reimer-Tiemann and bromination, the acidity order, and cleavage of ethers by HX (HI greater than HBr greater than HCl).