Alcohols and Phenols

Alcohols (R–OH) and phenols (Ar–OH) both carry a hydroxyl group, yet their reactivity differs sharply because of the aromatic ring in phenol. The PMDC MDCAT 2026 syllabus expects you to compare the two classes, name and draw their structures, and master high-yield reactions such as the Lucas test, Williamson synthesis, and the acidity comparison. Expect 2–3 MCQs from this chapter.

PMC Table of Specifications. This chapter covers three PMDC subtopics — the difference between alcohols and phenols, nomenclature/structure/reactivity of alcohols, and nomenclature/structure/reactivity of phenols.

Difference between Alcohol and Phenol

Both classes contain a hydroxyl group, but in alcohols the –OH is attached to an sp³ carbon of an aliphatic chain, whereas in phenol the –OH is bonded directly to an sp² carbon of a benzene ring. This single structural difference governs every chemical contrast between them.

Structural definitions

Alcohol: A compound in which –OH is bonded to a saturated sp³ carbon. General formula R–OH. Example: CH₃CH₂OH (ethanol).
Phenol: A compound in which –OH is bonded directly to a benzene ring. General formula Ar–OH. Example: C₆H₅OH (carbolic acid).

Side-by-side comparison

Alcohol vs Phenol — structural & chemical contrast
Property	Alcohol (R–OH)	Phenol (Ar–OH)
C–OH carbon	sp³ (aliphatic)	sp² (aromatic ring)
Example	CH₃CH₂OH (ethanol)	C₆H₅OH (carbolic acid)
Acidity (pK_a)	~16–18 (weaker than water)	~10 (stronger than water, weaker than COOH)
Reaction with NaOH	No reaction	Forms sodium phenoxide (salt)
Reaction with NaHCO₃	No reaction	No reaction (too weak to liberate CO₂)
Reaction with Na metal	Yes — gives RONa + H₂↑	Yes — gives ArONa + H₂↑
FeCl₃ test	No colour	Violet / purple complex
Lucas test (ZnCl₂/HCl)	Distinguishes 1° / 2° / 3° alcohols	Not applicable
Esterification with RCOOH	Easy	Slower (less nucleophilic O)
Conjugate base stability	Alkoxide RO⁻ — no resonance	Phenoxide — stabilised by resonance over o, p ring carbons
Aromatic ring substitution	N/A	Highly reactive at o, p positions (ring activator)

Common trap. Phenol is more acidic than ethanol because the phenoxide ion is stabilised by resonance with the benzene ring — the negative charge is spread over three ring carbons (ortho and para). The ethoxide ion has no such stabilisation, so the ionisation equilibrium of ethanol lies far to the left.

Nomenclature, Structure and Reactivity of Alcohols

Alcohols are classified by the number of carbons attached to the carbon bearing the –OH group. The chemistry of an alcohol is dominated by two reactive sites: the polar O–H bond and the C–O bond.

IUPAC nomenclature

Choose the longest carbon chain that contains the –OH group.
Replace the parent alkane suffix –e with –ol (e.g. methane → methanol).
Number the chain to give the carbon bearing the –OH the lowest locant.
Examples: CH₃OH = methanol, CH₃CH₂OH = ethanol, (CH₃)₂CHOH = propan-2-ol, (CH₃)₃COH = 2-methylpropan-2-ol.

Classification

Primary (1°): –OH on a carbon bonded to one other carbon. Example: ethanol CH₃CH₂OH.
Secondary (2°): –OH on a carbon bonded to two other carbons. Example: propan-2-ol (CH₃)₂CHOH.
Tertiary (3°): –OH on a carbon bonded to three other carbons. Example: 2-methylpropan-2-ol (CH₃)₃COH.

Physical properties

Alcohols form intermolecular hydrogen bonds, so their boiling points are markedly higher than those of comparable alkanes or ethers. Lower alcohols (C₁–C₃) are completely miscible with water; solubility falls off as the hydrocarbon tail lengthens.

Reactions of alcohols

Reaction with sodium — acidic O–H

2 R–OH + 2 Na → 2 R–ONa + H₂↑. Sodium displaces the hydroxylic hydrogen to give a sodium alkoxide and hydrogen gas. The reaction is slower than that of water.

Reaction with HX (HBr, HCl, HI)

R–OH + HX → R–X + H₂O. Reactivity order of HX: HI > HBr > HCl. Reactivity order of alcohols: 3° > 2° > 1° (because tertiary carbocations are more stable in the S_N1 pathway).

Lucas test (distinguishes 1°, 2°, 3° alcohols)

Reagent: anhydrous ZnCl₂ in concentrated HCl.
3° alcohol → turbidity immediately.
2° alcohol → turbidity in 5–10 minutes.
1° alcohol → no turbidity at room temperature.

Dehydration to alkenes

R–CH₂–CH₂–OH ⟶{conc. H₂SO₄, 170°C} R–CH=CH₂ + H₂O. Ease of dehydration: 3° > 2° > 1°. Mechanism: E1 (3°, 2°) or E2 (1°). Follows Saytzeff's rule — the more substituted alkene is the major product.

Oxidation

1° alcohol ⟶{[O], KMnO₄ or K₂Cr₂O₇} aldehyde ⟶{[O]} carboxylic acid.
2° alcohol ⟶{[O]} ketone (no further oxidation under mild conditions).
3° alcohol — not oxidised under normal conditions (no α-H attached to C–OH).

Esterification (Fischer)

R–OH + R′COOH ⟶{conc. H₂SO₄} R′COOR + H₂O. Reversible; driven forward by removing water.

Williamson ether synthesis

R–ONa + R′–X → R–O–R′ + NaX. The sodium alkoxide attacks the alkyl halide via S_N2; works best with 1° halides — tertiary halides give elimination instead.

Mnemonic for Lucas times. "Three–immediate, two–wait, one–won't." Tertiary clouds the tube on contact, secondary takes a few minutes, primary needs heat to react at all.

Nomenclature, Structure and Reactivity of Phenols

Phenol, C₆H₅OH, was the first commercial antiseptic (Joseph Lister, 1865). Modern uses include resin manufacture (Bakelite) and aspirin synthesis. The aromatic ring activates strongly toward electrophilic substitution and stabilises the conjugate base.

IUPAC nomenclature

The parent is "phenol" (retained IUPAC name) or "benzenol".
Substituents are numbered such that –OH is C-1.
Common examples: 2-methylphenol (o-cresol), 4-nitrophenol, 2,4,6-trinitrophenol (picric acid), 1,2-dihydroxybenzene (catechol), 1,3-dihydroxybenzene (resorcinol), 1,4-dihydroxybenzene (hydroquinone).

Structure and bonding

The oxygen is sp²-hybridised and donates a lone pair into the π system, increasing electron density at the ortho and para positions. This makes phenol an activating, ortho/para-directing substrate in electrophilic aromatic substitution and explains its higher acidity relative to alcohols.

Acidity of phenol

Phenol pK_a ≈ 10 vs ethanol pK_a ≈ 16. Reasons: (i) the phenoxide negative charge is delocalised over the ring; (ii) the C–O bond has partial double-bond character. Electron-withdrawing groups (–NO₂, –CN, –X) at ortho/para positions further increase acidity — picric acid (2,4,6-trinitrophenol) has pK_a ~ 0.4 and is as strong as a mineral acid.

Reactions of phenol

Reaction with NaOH

C₆H₅OH + NaOH → C₆H₅ONa + H₂O. Phenol is acidic enough to dissolve in dilute NaOH — alcohols are not. (Distinguishing test.)

FeCl₃ colour test

Phenols give a deep violet/purple colour with neutral aqueous FeCl₃. Alcohols give no colour. Diagnostic test for phenolic –OH.

Bromination

C₆H₅OH + 3 Br₂ (aq) → 2,4,6-tribromophenol (white precipitate) + 3 HBr. No catalyst needed — the ring is so activated that all three ortho/para positions are attacked at once.

Nitration

Dilute HNO₃ at low temperature gives a mixture of o- and p-nitrophenol. Concentrated HNO₃ with H₂SO₄ drives full substitution to picric acid (2,4,6-trinitrophenol).

Kolbe–Schmitt reaction

Sodium phenoxide + CO₂ at 125°C/4–7 atm, then acid, gives salicylic acid (2-hydroxybenzoic acid) — the precursor of aspirin.

Reimer–Tiemann reaction

Phenol + CHCl₃ + NaOH → salicylaldehyde (2-hydroxybenzaldehyde) on acidic workup. Introduces an –CHO group ortho to the –OH.

High-yield trio. Memorise the three diagnostic tests — Lucas (1°/2°/3° alcohols), FeCl₃ (phenol violet colour), and NaOH solubility (phenol dissolves, alcohol does not). At least one of them appears almost every year.

Worked MCQs

Five MCQs that capture the high-yield testing patterns for this chapter. Read the explanation even when you get the answer right — that's where the deeper concept lives.

Q1. The Lucas reagent (anhydrous ZnCl₂ in concentrated HCl) reacts immediately with which of the following?

Methanol
Ethanol
Propan-2-ol
2-methylpropan-2-ol

2-methylpropan-2-ol is a tertiary alcohol; it forms a stable 3° carbocation that captures Cl⁻ at once, producing an immediate cloudiness. Secondary alcohols take 5–10 minutes; primary alcohols give no turbidity at room temperature.

Q2. Phenol is more acidic than ethanol primarily because:

Phenol contains more carbon atoms
The O–H bond in phenol is shorter
The phenoxide ion is stabilised by resonance with the benzene ring
Phenol forms stronger hydrogen bonds with water

When phenol loses a proton, the resulting negative charge is delocalised over the ortho and para ring carbons. This resonance stabilisation lowers the energy of the phenoxide ion, shifting the ionisation equilibrium far to the right relative to ethanol.

Q3. Which alcohol cannot be oxidised by acidified K₂Cr₂O₇ under normal conditions?

Methanol
Propan-1-ol
Propan-2-ol
2-methylpropan-2-ol

A tertiary alcohol has no α-hydrogen on the carbon bearing the –OH, so the C–H bond required for oxidation is absent. Primary alcohols oxidise to acids and secondary alcohols to ketones; tertiary alcohols resist oxidation unless very harsh conditions break the C–C skeleton.

Q4. Phenol on reaction with excess bromine water gives:

Bromobenzene
2-bromophenol
4-bromophenol
2,4,6-tribromophenol

The ring in phenol is so strongly activated by the –OH that no Lewis-acid catalyst is required. All three ortho/para positions are brominated simultaneously, precipitating white 2,4,6-tribromophenol from solution.

Q5. The Williamson ether synthesis works best with:

A primary alkyl halide
A secondary alkyl halide
A tertiary alkyl halide
An aryl halide

Williamson synthesis proceeds via S_N2: the alkoxide is a strong nucleophile/base and 1° halides have the least steric hindrance. With 3° halides, the alkoxide acts as a base instead, giving alkene by E2.

Quick Recap

Alcohol: –OH on sp³ C; phenol: –OH on sp² C of an aromatic ring.
Acidity: carboxylic acid > phenol > water > alcohol.
Lucas test: 3° instant, 2° in minutes, 1° no reaction.
Oxidation: 1° → aldehyde → acid; 2° → ketone; 3° → no reaction.
Williamson ether synthesis prefers 1° halides (S_N2).
Phenol gives violet colour with FeCl₃; dissolves in NaOH (alcohols don't).
Phenol + 3 Br₂ (aq) → 2,4,6-tribromophenol; Kolbe–Schmitt → salicylic acid (aspirin precursor).

Test yourself. Take a timed practice test or browse topic-wise MCQs to lock these concepts in.