Alcohols, Phenols and Ethers

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Alcohols, Phenols and Ethers — Key Facts for MDCAT

Alcohols (R–OH):

• Primary (1°): R–CH$_2$OH — e.g., ethanol CH$_3$CH$_2$OH
• Secondary (2°): R$_2$CHOH — e.g., propan-2-ol (CH$_3$)_2CHOH
• Tertiary (3°): R$_3$COH — e.g., 2-methylpropan-2-ol (CH$_3$)_3COH

Phenols (Ar–OH):

• Phenol itself: C$_6$H$_5$OH — the –OH is directly attached to benzene ring
• Differ from alcohols: the oxygen lone pairs are delocalised into the benzene ring
• Acidity order: Phenol (pK$_a$ ≈ 10) > water > alcohols (pK$_a$ ≈ 15–19)
• Phenol is weakly acidic and reacts with NaOH to form sodium phenoxide

Ethers (R–O–R’):

• Symmetrical: R–O–R (both R groups same, e.g., diethyl ether)
• Unsymmetrical: R–O–R’ (different groups, e.g., methyl ethyl ether)
• General formula: C$n$H${2n+2}$O (isomeric with alcohols of same formula)

Physical Properties:

• Boiling points: Alcohols > Ethers (alcohols form hydrogen bonds; ethers do not)
• Water solubility: Lower alcohols (methanol, ethanol) are miscible with water; solubility decreases with increasing carbon chain
• Phenol has limited water solubility (6.7 g/100 mL at 25°C) due to the hydrophobic benzene ring

⚡ Exam tip: In MDCAT, the most commonly confused distinction is the acidity of phenol vs alcohols. Phenol reacts with NaOH but NOT with Na metal. Primary/secondary alcohols react with Na metal (but NOT with NaOH) to give alkoxides + H$_2$. This is a key test to distinguish phenols from alcohols.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students who want genuine understanding.

Alcohols, Phenols and Ethers — Complete Study Guide

Preparation of Alcohols:

• Hydration of alkenes: $CH_2=CH2 + H2O \xrightarrow{H+} CH3CH2OH$ (acid-catalysed, Markovnikov)
• Hydroboration-oxidation: $R–CH=CH2 \xrightarrow{(i) BH3} \xrightarrow{(ii) H2O2/NaOH} R–CH2–CH2OH$ (anti-Markovnikov, gives primary alcohol)
• Grignard + carbonyl: $RMgX + R’CHO \rightarrow R–R’CH–OH$ (after hydrolysis)
• Reduction of carbonyls: $RCHO \xrightarrow{NaBH4} R–CH2OH$; $RCOR’ \xrightarrow{NaBH4} R–CH(OH)–R’$

Reactions of Alcohols:

• With active metals (Na, K): $2R–OH + 2Na \rightarrow 2R–ONa + H_2$ (rate: methanol > 1° > 2° > 3°)
• With HCl/ZnCl$_2$ (Lucas test): Distinguishes 1°, 2°, 3° alcohols — 3° reacts immediately (cloudy), 2° in 5–10 min, 1° on heating
• Dehydration: Conc. H$_2$SO$_4$, 170°C → alkene; 140°C → ether
• Esterification: $R–OH + R’COOH \xrightarrow{H+} R’COOR + H_2O$
• Oxidation: 1° → aldehyde → carboxylic acid; 2° → ketone; 3° → no oxidation (no α-H)
  • PCC (mild oxidant) → aldehyde from 1°
  • K$_2$Cr$_2$O$_7$/H$_2$SO$_4$ → carboxylic acid from 1°

Reactions of Phenols:

• Acidity: $C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O$ (sodium phenoxide)
• With Br$_2$ water: Phenol + Br$_2$ → 2,4,6-tribromophenol (white precipitate, even in aqueous — very sensitive test)
• With FeCl$_3$: Phenol + FeCl$_3$ → violet complex (positive test for phenols/enols)
• Nitration: Dilute HNO$_3$ → mixture of o- and p-nitrophenol
• Kolbe’s reaction: Phenol + NaOH + CO$_2$ → salicylic acid (o-hydroxybenzoic acid)
• Reimer-Tiemann: Phenol + CHCl$_3$ + NaOH → o-hydroxybenzaldehyde (salicylaldehyde)

Reactions of Ethers:

• Cleavage by HBr/HI: $R–O–R’ + HBr \rightarrow R–Br + R’–OH$ (HI is strongest, cleaves all ethers)
• Peroxide formation: Ethers form explosive peroxides on exposure to air/light — must be tested before distillation

⚡ Common mistakes: Confusing esterification of alcohols with acylation. For Lucas test, remembering the order: tertiary fastest, primary slowest. For Grignard reactions, forgetting that Grignard reagents CANNOT be used with acidic protons (alcohols, phenols, terminal alkynes, carboxylic acids) — they would react with the acidic H instead.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Alcohols, Phenols and Ethers — Advanced Notes

Mechanism of Dehydration of Alcohols (E1 reaction): Alcohols undergo dehydration in two steps:

1. Protonation of the –OH group → makes it a better leaving group (water, H$_2$O)
2. Loss of water to form a carbocation (rate-determining step)
3. Deprotonation to give alkene

Order of dehydration ease: 3° > 2° > 1° (follows carbocation stability) Under acidic conditions, rearrangement of carbocations is possible (methyl shift or hydride shift) → unexpected products.

Mechanism of Oxidation of Primary Alcohols: 1° alcohol → aldehyde (removal of 2H): $RCH_2OH \xrightarrow{[O]} RCHO$ Aldehyde → carboxylic acid (addition of O): $RCHO \xrightarrow{[O]} RCOOH$ To stop at aldehyde, use PCC (pyridinium chlorochromate) which is a mild oxidant that cannot oxidise aldehydes further.

Phenoxide Ion — Why Phenol is Acidic: In phenol, the oxygen lone pair is delocalised into the benzene ring via resonance. When phenol loses H$^+$, the phenoxide ion (C$_6$H$_5$O$^-$) has the negative charge delocalised over the ring, particularly at the ortho and para positions. This resonance stabilisation makes the phenoxide ion more stable than the alkoxide ion (R-O$^-$) from alcohols, which is why phenol is more acidic than alcohols.

The pK$_a$ values explain the relative acidity:

• Phenol: pK$_a$ = 10.0
• Ethanol: pK$_a$ = 15.9
• Why? Alkoxide ions have localised charge with no resonance stabilisation.

Electrophilic Aromatic Substitution in Phenols: The –OH group is an activating, ortho/para-directing group. This is because the lone pair on oxygen donates into the benzene ring by resonance, making the ortho and para positions electron-rich and thus more susceptible to electrophilic attack.

Ethers as Crown Ether Ligands: Crown ethers (e.g., 18-crown-6) are cyclic ethers that selectively complex with specific metal ions based on the size of their cavity. For example, 18-crown-6 (cavity radius ~2.6 Å) selectively complexes with K$^+$ (ionic radius ~2.66 Å). This property makes them useful in phase-transfer catalysis.

Distinguishing Tests:

Test	Alcohol (1°/2°)	Alcohol (3°)	Phenol	Ether
Na metal	Effervescence (H$_2$)	Effervescence	No reaction	No reaction
NaOH	No reaction	No reaction	Dissolves	No reaction
Lucas test	Slow/heating	Moderate	No reaction	No reaction
Br$_2$ water	No reaction	No reaction	White precipitate	No reaction
FeCl$_3$	No reaction	No reaction	Violet colour	No reaction

MDCAT Question Patterns: Common MDCAT Pakistan questions test: (1) distinguishing 1°, 2°, 3° alcohols by Lucas test, (2) oxidation products of alcohols, (3) acidic nature of phenol vs alcohols, (4) Reimer-Tiemann and Kolbe’s reactions, (5) dehydration conditions and products, (6) naming alcohols, phenols, and ethers by IUPAC. Expect 2–3 questions per paper on these topics.

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