Alcohols, Phenols and Ethers — NEET Notes

Classification & nomenclature

An alcohol carries the hydroxyl group on an sp³ (saturated) carbon. A phenol carries the same group on an sp² ring carbon of an aromatic system. An ether places an oxygen between two carbons with no hydrogen on oxygen at all. The structural difference looks small on paper; the chemical consequences fill an entire NEET unit.

Alcohols are further graded by the carbon that bears the hydroxyl. If that carbon is attached to one other carbon, the alcohol is primary (1°); to two other carbons, secondary (2°); to three other carbons, tertiary (3°). This single label predicts Lucas test behaviour, oxidation product, and dehydration ease — three NEET-favourite themes. Alcohols are also classified by hybridisation: allylic alcohols sit next to a C=C, benzylic alcohols next to an aromatic ring, and vinylic alcohols (with OH directly on a sp² carbon) are usually unstable, tautomerising to carbonyls.

Under the IUPAC system, alcohols carry the suffix -ol. The principal chain is the longest carbon chain bearing the hydroxyl; numbering starts from whichever end gives the OH the lower locant. So CH₃-CH(OH)-CH₂-CH₃ is butan-2-ol — not butan-3-ol. Phenols are named as derivatives of the parent compound phenol itself, with substituents prefixed (e.g. 2-nitrophenol, 4-methylphenol). For ethers, IUPAC names treat the smaller alkyl-O group as an alkoxy substituent on the larger alkane: CH₃-O-C₂H₅ becomes methoxyethane, not ethyl methyl ether. The common name "anisole" persists for methoxybenzene because NCERT and NEET retain it.

Preparation of alcohols

Three NCERT routes dominate this section, and every one has appeared in a NEET PYQ. The first two start from alkenes and exploit the polarity of the C=C bond. The third starts from a carbonyl group and uses an organometallic nucleophile. Together they cover almost every laboratory and industrial synthesis a NEET candidate will need.

1. Acid-catalysed hydration (Markovnikov)

An alkene treated with dilute H₂SO₄ at moderate temperature adds water across the double bond. The proton goes to the carbon with more hydrogens already attached, generating the more stable carbocation, which is then trapped by water. The result is Markovnikov addition. Propene gives propan-2-ol, not propan-1-ol. The mechanism is three steps: protonation of the alkene → carbocation formation → nucleophilic trap by water → deprotonation. Watch for rearrangement when a more stable carbocation lies one shift away.

2. Hydroboration-oxidation (anti-Markovnikov)

To defy Markovnikov and make the primary alcohol, the alkene is first treated with diborane (B₂H₆) in tetrahydrofuran. The boron — being electron-poor — bonds to the less hindered, less substituted carbon; hydrogen attaches to the more substituted one. Oxidation of the resulting trialkylborane with alkaline H₂O₂ replaces boron with OH, retaining its position. Propene gives propan-1-ol. The reaction is syn, single-step in mechanism, and shows no rearrangement.

3. Grignard addition to carbonyl compounds

A Grignard reagent (R-MgX) is a carbon nucleophile: the C-Mg bond is so polarised that the R group attacks as a carbanion. Add it to an aldehyde and you get a secondary alcohol after workup; add it to a ketone and you get a tertiary alcohol; add it to formaldehyde (HCHO) and you get a primary alcohol; add it to CO₂ and you get a carboxylic acid. The ether solvent (dry diethyl ether or THF) is essential to stabilise the reagent. Water destroys it instantly — that's why workup is done with aqueous acid only after the addition step.

Properties of alcohols

The O-H bond changes everything. Oxygen is electronegative, so the hydroxyl group is strongly polar. Hydrogen on oxygen is acidic enough to leave with the right reagent, and lone pairs on oxygen accept hydrogen bonds from other molecules. Two macroscopic consequences follow, and NEET tests both.

First, boiling points are higher than those of hydrocarbons and even ethers of comparable molecular mass. Ethanol (Mr 46) boils at 78°C; propane (Mr 44) boils at −42°C. The difference — about 120°C — is the cost of breaking intermolecular hydrogen bonds. Dimethyl ether, an isomer of ethanol with the same molecular formula, boils at −24°C, because ethers cannot donate hydrogen bonds (no O-H), only accept them. Second, lower alcohols are miscible with water for the same hydrogen-bonding reason; solubility falls as the hydrocarbon chain lengthens because the non-polar tail eventually overwhelms the polar head.

The acidic character of an alcohol — the willingness of its O-H to release a proton — is modest. Alcohols are more acidic than water in some comparisons but actually weaker than water in aqueous solution because the alkyl group is electron-donating and destabilises the alkoxide. The pKa of ethanol is about 16; water is 15.7. In Lewis terms, alcohols are amphoteric: they accept a proton on oxygen (to form ROH₂⁺) and donate one to a strong base (to form RO⁻). The corresponding alkoxide RO⁻ is a strong base — stronger than OH⁻.

Reactions of alcohols

Reactions of alcohols cleave one of three bonds: the O-H bond (acid-base, esterification), the C-O bond (substitution by halide, dehydration), or the C-H bond on the carbon bearing OH (oxidation). Knowing which bond a reagent attacks tells you the product class instantly.

Reaction with hydrogen halides — the Lucas test

Alcohols react with HX (X = Cl, Br, I) to give alkyl halides and water. The mechanism depends on the alcohol class. Tertiary alcohols proceed by SN1 through a stable 3° carbocation, secondary alcohols through SN1 or SN2 depending on conditions, and primary alcohols through SN2. The relative rates are 3° > 2° > 1° — the basis of the Lucas test. Lucas reagent (concentrated HCl + anhydrous ZnCl₂) accelerates the reaction by complexing the OH (making it a better leaving group). The alkyl chloride formed is oil-like and insoluble in the aqueous reagent, producing visible turbidity.

Reaction with PCl₃, PCl₅ and SOCl₂

To convert an alcohol cleanly to an alkyl halide without rearrangement, NCERT lists three phosphorus and sulphur reagents. Phosphorus trichloride (PCl₃) gives alkyl chloride plus H₃PO₃; phosphorus pentachloride (PCl₅) gives alkyl chloride plus HCl and POCl₃; thionyl chloride (SOCl₂) is the preferred reagent because the by-products (SO₂ and HCl) are gases that escape, leaving a pure alkyl chloride. NEET 2018 Q.72 tested the C₂H₅OH + PCl₅ → C₂H₅Cl conversion as a stepping stone to diethyl ether.

Esterification with carboxylic acids (Fischer)

Heated with a carboxylic acid and a trace of mineral acid (H₂SO₄ or HCl) as catalyst, an alcohol forms an ester. The reaction is reversible — Fischer esterification — and an excess of either alcohol or acid drives the equilibrium toward ester. The OH that leaves comes from the carboxylic acid, not the alcohol; isotopic labelling experiments using ¹⁸O confirmed this. With acid anhydrides or acid chlorides the reaction is irreversible and faster.

Oxidation — class decides the product

Oxidation breaks C-H bonds on the carbon bearing OH. A primary alcohol has two such hydrogens, so it can lose both: first to an aldehyde, then to a carboxylic acid. Pyridinium chlorochromate (PCC) in dichloromethane stops at the aldehyde; KMnO₄ or K₂Cr₂O₇ goes all the way to the acid. A secondary alcohol has one C-H on the carbinol carbon and oxidises to a ketone — which is terminal because ketones lack a C-H on the carbonyl carbon. A tertiary alcohol has no such hydrogen at all and resists oxidation under normal conditions; only harsh cleavage with hot acidic KMnO₄ breaks C-C bonds.

Dehydration to alkenes

An alcohol heated with concentrated H₂SO₄ or H₃PO₄ loses water and forms an alkene. The mechanism is E1: protonation of OH → loss of water to give a carbocation → loss of β-hydrogen to form C=C. The rate order is therefore 3° > 2° > 1° — the same as for HX substitution, because both depend on carbocation stability. With multiple β-hydrogens, the more substituted alkene predominates (Saytzeff's rule). NEET 2023 Q.94 directly tested which alcohol dehydrates fastest — answer: the one giving the most stable carbocation.

Preparation of phenols

Industrial phenol is made by the cumene process; classroom phenols come from haloarenes, benzene sulphonic acid, or diazonium salts. NEET tests the cumene route most because every intermediate is examinable.

Cumene process

Benzene is Friedel-Crafts alkylated with propene over anhydrous AlCl₃ to give isopropylbenzene (cumene). Cumene is air-oxidised to cumene hydroperoxide. Acid hydrolysis (dilute H₂SO₄) rearranges the peroxide and cleaves it to phenol + acetone. The economics work because acetone is a valuable solvent and feedstock; over 95% of world phenol now comes from this route. NEET 2018 Q.81 tested every intermediate — P (cumene), Q (phenol), R (acetone).

Dow process & classical routes

The Dow process treats chlorobenzene with aqueous NaOH at 350°C and 200 atm, giving sodium phenoxide; acidification releases phenol. NCERT also lists hydrolysis of benzene diazonium chloride (boiling its aqueous solution gives phenol + N₂) and alkali fusion of sodium benzene sulphonate. All four routes are NEET-relevant.

Acidity of phenols

Phenol is the most acidic monohydroxyl compound on the NEET syllabus that is not classified as an acid. Its pKa of about 10 sits between water (15.7) and a carboxylic acid (4–5). The cause is resonance: when phenol loses its proton, the resulting phenoxide ion can place the negative charge not only on oxygen but also on the ortho and para ring carbons through three additional resonance structures. The negative charge is therefore spread over four atoms instead of being localised on a single oxygen, as in an alkoxide. Spreading lowers energy; the lower the energy of the conjugate base, the stronger the acid.

Substituents tune phenol's acidity in two ways. Electron-withdrawing groups (-NO₂, -CN, halogens, -CHO) stabilise the phenoxide further and raise acidity. Electron-donating groups (-CH₃, -OMe, -NH₂) destabilise the phenoxide and lower acidity. Position also matters: a nitro group at the ortho or para position can withdraw electrons by both inductive (-I) and resonance (-R) effects, but at the meta position only -I operates — no resonance structure relays the charge onto a meta substituent. Hence ortho- and para-nitrophenol are stronger acids than meta-nitrophenol. The acidity ladder NEET 2017 Q.26 asked about ran cleanly upward as nitro groups were added: phenol < p-nitrophenol < 2,4-dinitrophenol < 2,4,6-trinitrophenol (picric acid, pKa 0.4 — strong enough to liberate CO₂ from sodium bicarbonate).

Reactions of phenols

Two reactions of the OH group, two of the ring, and one signature named-reaction round out the NCERT coverage. The OH oxygen donates its lone pair into the ring, so phenol is a powerful ortho/para director for electrophilic aromatic substitution — even more so than alkylbenzenes — and electrophiles attack almost exclusively at those positions.

Kolbe's reaction (phenol → salicylic acid)

Sodium phenoxide reacts with CO₂ under pressure (4–7 atm) at moderate temperature (125°C) to give sodium salicylate; acidification yields salicylic acid. The carboxylate attacks ortho. This is the industrial first step to aspirin (acetylsalicylic acid), which is salicylic acid acetylated with acetic anhydride.

Reimer-Tiemann reaction (phenol → salicylaldehyde)

Treat phenol with chloroform (CHCl₃) and aqueous NaOH at 60°C, then hydrolyse the intermediate. The product is 2-hydroxybenzaldehyde (salicylaldehyde) — a CHO group attached at the ortho position. The active electrophile is dichlorocarbene (:CCl₂), generated in situ from CHCl₃ and OH⁻. Reimer-Tiemann delivers a formyl group exactly where Kolbe delivers a carboxylate.

Electrophilic substitution — bromination, nitration, sulphonation

Bromine water converts phenol almost instantly to 2,4,6-tribromophenol (white precipitate) — no Lewis acid needed, because the ring is already highly activated. In non-aqueous solvents (CS₂ at low temperature), monobromination dominates and gives ortho- and para-bromophenol. Nitration with dilute HNO₃ gives o- and p-nitrophenol; the ortho isomer is steam-volatile (intramolecular H-bonding) and separable from the higher-boiling para isomer. Concentrated HNO₃ gives picric acid (2,4,6-trinitrophenol).

Esterification & ether formation

Phenols form esters with acid anhydrides or acid chlorides in the presence of base — this is how aspirin is made. Phenols form aryl alkyl ethers (e.g. anisole) by the Williamson route only as phenoxide + alkyl halide, never the reverse.

Commercially important alcohols — methanol and ethanol

Two alcohols carry the industrial weight: methanol and ethanol. Methanol (CH₃OH), called "wood spirit" because it was once made by destructive distillation of wood, is now manufactured by catalytic hydrogenation of carbon monoxide over a copper-zinc oxide catalyst at 200-300°C and high pressure (CO + 2 H₂ → CH₃OH). Methanol is fatally toxic — ingestion of even a few mL causes blindness because human alcohol dehydrogenase oxidises it to formaldehyde and formic acid. It is used as a solvent, antifreeze, and a feedstock for formaldehyde (and thus Bakelite).

Ethanol (C₂H₅OH), the alcohol of beverages, is made industrially by fermentation of sugars (glucose → 2 ethanol + 2 CO₂) catalysed by yeast enzymes (zymase). Fermentation tops out at about 14% ethanol; higher concentrations are reached by distillation up to the 95.6% azeotrope ("rectified spirit"). Absolute ethanol (100%) is obtained by drying with benzene or molecular sieves. Industrial ethanol is denatured with toxic adjuncts (methanol, pyridine) to prevent human consumption and avoid excise duty. Ethanol is also produced by direct hydration of ethylene from petrochemical streams.

Ethers — Williamson synthesis & cleavage

An ether is a quiet molecule: oxygen wears two carbon coats, has no acidic hydrogen, and the C-O-C linkage is inert to most reagents. Ethers are used as solvents (diethyl ether, THF) precisely because they coexist peacefully with strong nucleophiles, Grignard reagents, and metal hydrides. They have two NEET-tested reactions: how to make them (Williamson) and how to break them (HX cleavage).

Williamson ether synthesis

An alkoxide (or aryloxide) is treated with an alkyl halide in an SN2 displacement. The oxygen attacks the carbon bearing the leaving group; halide leaves. The reaction works best with primary alkyl halides, because SN2 demands an unhindered backside. With secondary halides, E2 elimination competes; with tertiary halides, elimination dominates and almost no ether forms. To make tert-butyl methyl ether, one must therefore use tert-butoxide + methyl iodide — never methoxide + tert-butyl bromide, which would simply give isobutylene. NEET 2016 Q.7 classified the reaction (ArO⁻Na⁺ + Me-I → ArOMe) as Williamson ether synthesis.

Cleavage of ethers by HI / HBr

Ethers resist most reagents but break under hot concentrated HI. The mechanism depends on the ether class. With a dialkyl ether (R-O-R'), iodide attacks the less hindered alkyl carbon by SN2, giving alkyl iodide + alcohol; with excess HI at high temperature, the alcohol then reacts further to give a second alkyl iodide. With an aryl alkyl ether (e.g. anisole, C₆H₅-OCH₃), the alkyl-oxygen bond breaks — never the aryl-oxygen bond, because the C(aryl)-O bond has partial double-bond character from ring resonance. Anisole + HI therefore gives phenol + methyl iodide, not iodobenzene + methanol. NEET 2017 Q.4 and 2020 Q.179 both tested this exact outcome.

Physical properties of ethers reflect the structural difference: a low-mass ether like dimethyl ether (Mr 46) boils at −24°C, much lower than ethanol (78°C, same Mr), because ethers cannot self-associate by H-bonding. Yet ethers can accept hydrogen bonds from water and alcohols, so they show moderate water solubility for low molecular masses (diethyl ether: 6 g/100 mL water). Ethers form explosive peroxides on standing in air; always test before distillation.