Alcohols, Phenols & Ethers – Comprehensive Bullet-Point Notes

Objectives / Learning Outcomes

• After completing this unit, you should be able to:
  • Name alcohols, phenols and ethers using IUPAC rules.
  • Describe and write mechanisms for the preparation of alcohols from alkenes, carbonyl compounds and carboxylic acids.
  • Explain laboratory & industrial syntheses of phenols from haloarenes, benzene-sulphonic acids, diazonium salts and cumene.
  • Draft synthetic routes to ethers from alcohols or from alkyl / aryl halides via Williamson synthesis.
  • Correlate physical properties (b.p., m.p., solubility, acidity/basicity) with structure and H-bonding.
  • Predict and explain all major chemical reactions (cleavage of $\text{C–O}$ and $\text{O–H}$ bonds, electrophilic aromatic substitution, oxidation, dehydration, etc.) of these three families.
  • Recognise safety / health issues (e.g.
    methanol poisoning, denatured alcohol) and industrial relevance (detergents, antiseptics, perfumes, solvents).

1 Classification

• By no. of $\text{–OH}$ groups
  • Alcohols & phenols → monohydric, dihydric, trihydric, polyhydric.
• By hybridisation of C–OH
  • $sp^3$ (alkyl) alcohols
  • Primary, secondary, tertiary
  • Allylic (adjacent to $C=C$) and benzylic (next to aromatic ring)
  • $sp^2$ alcohols (vinylic, aryl) → phenols & vinylic alcohols.
• Ethers
  • Simple / symmetrical: both alkyl (or aryl) groups identical e.g. $C<em>2H</em>5OC<em>2H</em>5$.
  • Mixed / unsymmetrical: groups different e.g. $C<em>2H</em>5OCH_3$.

2 IUPAC Nomenclature Rules (Quick Guide)

• Alcohols
  • Parent = longest chain containing $–OH$.
  • Replace –e of alkane with –ol; retain –e for polyols: $HOCH<em>2CH</em>2OH \rightarrow\text{ ethane-1,2-diol}$.
  • Number from nearer end to $–OH$.
  • Cyclic: prefix cyclo, $–OH$ at C-1.
• Phenols
  • Parent = phenol (accepted IUPAC & common).
  • Ortho (1,2), meta (1,3), para (1,4) prefixes common in trivial naming.
  • Dihydroxy benzenes: catechol (1,2-), resorcinol (1,3-), hydroquinone (1,4-).
• Ethers (IUPAC)
  • Larger alkyl/aryl chain = parent; smaller + O = alkoxy substituent.
  • $CH<em>3OCH</em>2CH_3 \rightarrow \text{1-methoxypropane}$.
  • Phenyl derivatives: $C<em>6H</em>5OCH_3 \rightarrow \text{methoxybenzene (anisole)}$.

3 Structural Highlights

• Alcohols: $\sigma$ bond between $sp^3$–C and $sp^3$–O; bond angle slightly < $109.5^{\circ}$ due to lone-pair repulsion.
• Phenols: $O$ attached to $sp^2$–C; $C–O$ length $\approx136\,\text{pm}$ (shorter than methanol) because of partial $\pi$ character via resonance.
• Ethers: tetrahedral $sp^3$ O with 2 lone pairs; $C–O–C$ angle >109.5^{\circ} (repulsion of bulky R groups); $C–O$ length $\approx141\,\text{pm}$.

4 Preparation of Alcohols

• (a) From Alkenes
  • Acid-catalysed hydration: $RCH=CH<em>2 + H</em>2O \xrightarrow{H^+} RCH(OH)CH_3$ (Markovnikov).
  • Mechanism: protonation → carbocation → nucleophilic water attack → deprotonation.
  • Hydroboration–oxidation: $\text{BH}<em>3\,(\text{or } B</em>2H<em>6)$, then $H</em>2O_2/NaOH$ → anti-Markovnikov syn-addition.
  • $CH<em>3CH=CH</em>2 \xrightarrow[{NaOH}]{BH<em>3;H</em>2O<em>2} CH</em>3CH<em>2CH</em>2OH$.
• (b) From Carbonyl Compounds
  • Catalytic hydrogenation $\bigl[H_2/Pt, Pd, Ni\bigr]$.
  • Hydride reagents: $NaBH<em>4$, $LiAlH</em>4$ → aldehyde → 1° alcohol; ketone → 2° alcohol.
• (c) From Carboxylic Acids / Esters
  • $LiAlH<em>4$ reduction of $RCOOH$ → $RCH</em>2OH$.
  • Industrial: esterification → hydrogenolysis (catalytic H_2).
• (d) Grignard Synthesis $RMgX + H<em>2C=O \xrightarrow{\text{ether}} RCH</em>2OMgX \xrightarrow{H<em>3O^+} RCH</em>2OH$.
  • > Aldehyde → 2° alcohol; ketone → 3° alcohol.

5 Preparation of Phenols

• (i) From Haloarenes (Dow process)
  • $C<em>6H</em>5Cl + 6\,\text{NaOH}\,(623\,\text{K},\,320\,\text{atm}) \rightarrow C<em>6H</em>5ONa \xrightarrow{H^+} C<em>6H</em>5OH$.
• (ii) From Benzenesulphonic Acid
  • $C<em>6H</em>5SO<em>3H \xrightarrow[NaOH]{\Delta} C</em>6H<em>5ONa \xrightarrow{H^+} C</em>6H_5OH$.
• (iii) From Diazonium Salts $C<em>6H</em>5N<em>2^+Cl^- + H</em>2O \xrightarrow{warm} C<em>6H</em>5OH + N_2 + HCl$.
• (iv) Cumene Process (Industrial, major)
  • $\text{Cumene} (\text{isopropylbenzene}) + O<em>2 \rightarrow \text{cumene hydroperoxide} \xrightarrow{H^+} C</em>6H<em>5OH + (CH</em>3)_2C=O$ (acetone co-product).

6 Physical Properties

• Boiling Point (b.p.) trend \text{alcohol} > \text{phenol} > \text{ether} \approx \text{alkane} (same M_r) due to $\text{H-bonding}$.
  • e.g. $\text{ethanol (351\,K)} \gg \text{propane (231\,K)}$.
• Solubility in water
  • Small alcohols & phenols miscible via H-bonding with water.
  • Solubility ↓ with longer hydrophobic R / Ar.
• Acidity (pK_a values)
  • C6H5OH\,(10.0) < o\text{-cresol}\,(10.2) < C2H5OH\,(15.9) → lower pK_a = stronger acid.
  • EWG (e.g.$\,NO<em>2$) ↑ acidity; ERG (e.g.$\,CH</em>3$) ↓ acidity.

7 Chemical Reactions of Alcohols & Phenols

7.1 Reactions via $\text{O–H}$ Bond

• Metal reaction $2ROH + 2Na \rightarrow 2RONa + H_2$.
• Acidic behaviour $ROH + B^- \rightleftharpoons RO^- + HB$ (Bronsted acid).
• Esterification
  • $ROH + R'COOH \xrightarrow{H^+} R'COOR + H<em>2O$ (reversible; remove $H</em>2O$).
  • Acid chloride / anhydride faster; acetylation = intro of $CH_3CO–$.

7.2 Reactions via $\text{C–O}$ Bond (mainly alcohols)

• (a) Halogenation
  • $ROH + HX \xrightarrow{ZnCl<em>2} RX + H</em>2O$ (Lucas test distinguishes 1°,2°,3°).
  • $3ROH + PBr<em>3 \rightarrow 3RBr + H</em>3PO_3$.
• (b) Dehydration
  • $RCH<em>2CH</em>2OH \xrightarrow[443\,K]{conc\,H<em>2SO</em>4} RCH=CH<em>2 + H</em>2O$.
  • Order: 3° > 2° > 1° (via more stable carbocation).
• (c) Oxidation / Dehydrogenation
  • Mild: $\text{PCC}$, $CrO_3$ (anhyd.) → 1° → aldehyde.
  • Strong: $KMnO_4/H^+$ → 1° → acid; 2° → ketone; 3° resistant.
  • $ROH \xrightarrow[573\,K]{Cu} RCHO$ (1°) or $RCOR'$ (2°).

7.3 Phenol-specific Reactions

• Electrophilic Substitution (activating, o/p-director)
  • Nitration: C6H5OH + HNO3\,(dil,298K) \rightarrow o",&" p\text{-nitrophenol}; conc $HNO</em>3$ → picric acid $\bigl(2,4,6\text{-trinitrophenol}\bigr)$.
  • Bromination: $C<em>6H</em>5OH + Br<em>2\,(H</em>2O) \rightarrow 2,4,6\text{-tribromophenol}$ (white ppt).
• Kolbe CO₂ carboxylation $C<em>6H</em>5ONa + CO_2\,(373K,6\,atm) \rightarrow o\text{-hydroxybenzoic acid (salicylic acid)}$.
• Reimer–Tiemann $C<em>6H</em>5OH + CHCl<em>3 + 3NaOH \rightarrow o\text{-formyl phenol (salicylaldehyde)} + 3NaCl + 2H</em>2O$.
• Zn dust: $C<em>6H</em>5OH \xrightarrow{Zn,\Delta} C<em>6H</em>6$.
• Oxidation: $C<em>6H</em>5OH \xrightarrow{CrO_3}$ p-benzoquinone (conjugated diketone).

8 Commercially Important Alcohols

• Methanol ($CH_3OH$)
  • Prepared by $CO + 2H<em>2 \xrightarrow[ZnO/Cr</em>2O<em>3]{50–100\,atm,\,523–573\,K} CH</em>3OH$.
  • Toxic: 15 mL causes blindness; 30 mL fatal; treated with IV ethanol (competitive substrate for alcohol dehydrogenase).
  • Solvent; feedstock for formaldehyde, MTBE fuel additive.
• Ethanol ($C<em>2H</em>5OH$)
  • Fermentation: $C<em>6H</em>{12}O<em>6 \xrightarrow[zymase]{298–303\,K} 2C</em>2H<em>5OH + 2CO</em>2$.
  • Hydration of ethene: $CH<em>2=CH</em>2 + H<em>2O \xrightarrow{H</em>3PO<em>4,300\,K,60\,atm} C</em>2H_5OH$.
  • Denatured with $CuSO_4$ (colour) & pyridine (odour) to prevent misuse.

9 Ethers

9.1 Laboratory & Industrial Preparation

• (a) Acidic dehydration of alcohols (good for symmetrical 1° ethers)
  • $2RCH<em>2OH \xrightarrow[413\,K]{conc\,H</em>2SO<em>4} RCH</em>2OCH<em>2R + H</em>2O$.
  • Competes with alkene formation at higher $T$.
• (b) Williamson Synthesis (SN2)
  • $R'ONa + R–X \rightarrow R'OR + NaX$.
  • Best when $R$ = 1° haloalkane; bulky halide leads to elimination.
  • Phenoxide + 1° alkyl halide → aryl-alkyl ether e.g. $\text{PhO}^-Na^+ + CH_3I \rightarrow anisole$.

9.2 Physical Features

• $C–O$ bonds polar → net dipole; however lack $\text{H-bond (intermolecular)}$ so b.p. ≈ alkanes.
  • $C<em>2H</em>5OC<em>2H</em>5 (307.6\,K) \approx n\text{-pentane (309.1 K)} \ll CH<em>3(CH</em>2)_3OH (390 K).$
• Miscible with water to some extent (H-bonding through O lone pair).

9.3 Chemical Behaviour

1. Cleavage by HX (HI > HBr ≫ HCl)
   • $R<em>2O + HX \xrightarrow{\Delta} RX + R'OH \xrightarrow{HX} R'X + H</em>2O$.
   • Mixed aryl–alkyl: only alkyl–O bond breaks ⇒ $\text{PhOR} + HI \rightarrow \text{PhOH} + RI$.
   • Mechanism: protonation → $S<em>N2$ (1°/2°) or $S</em>N1$ (3°) depending on stability.
2. Electrophilic Aromatic Substitution (alkoxy as o/p director)
   • Bromination: \text{Anisole} + Br2/CH3COOH \rightarrow ortho + para\text{-bromo anisoles (p > o)}.
   • Friedel–Crafts alkylation/acylation: AlCl₃ catalysed; yields o/p products.

10 Key Mechanisms to Master (exam favourites)

• Acid-catalysed hydration of ethene.
• Mechanism of dehydration of ethanol (carbocation pathway).
• Hydroboration–oxidation (syn-addition, anti-Markovnikov outcome).
• Williamson synthesis vs elimination (bulky base issue).
• Reimer–Tiemann formylation (dichlorocarbene intermediate).
• Kolbe carboxylation (electrophilic $CO_2$ addition to phenoxide).
• Ether cleavage by HI (protonation → $S<em>N2$ or $S</em>N1$).

11 Safety, Ethical & Industrial Notes

• Denatured alcohol prevents diversion of industrial ethanol for drinking; legal & ethical compliance for lab storage.
• Methanol poisoning incidents stress importance of proper labelling and student awareness.
• Cumene process couples phenol & acetone manufacture, illustrating green-chemistry co-product utilisation.

12 Quick-Reference Tables

• pK_a Values
  • Phenol 10.0; $o\text{-nitrophenol}$ 7.2; $o\text{-cresol}$ 10.2; Ethanol 15.9.
• Important Boiling Points (K)
  • $CH<em>3OH$ 337; $C</em>2H<em>5OH$ 351; $C</em>2H<em>5OC</em>2H_5$ 308; Propane 231.
• Order of Reactivity
  • Dehydration: 3° > 2° > 1°.
  • HX cleavage of ethers: HI > HBr > HCl.

13 Exam-Style Practice (with answers)

• Arrange acidity: C3H7OH < p\text{-CH}3C6H4OH < C6H5OH < m\text{-NO}2C6H4OH < 3,5\text{-dinitrophenol} < 2,4,6\text{-trinitrophenol}.
• Predict product: $2\text{-methylbutan-2-ol} + HBr \rightarrow (CH<em>3)</em>3CBr$ via carbocation rearrangement.
• Williamson set-up for $1\text{-methoxy-4-nitrobenzene}$: Use $p\text{-nitrophenoxide}$ + $CH<em>3I$; reverse combination fails (SN2 hindered by $p\text{-NO}</em>2C<em>6H</em>4 – CH_2I$ bulky aryl halide).

14 Summary Cheat Sheet

• Alcohols, phenols: H-bonding ⇒ ↑b.p., water solubility.
• Phenol acidity > aliphatic alcohols due to resonance & $sp^2$ C.
• Ethers = least reactive; main reaction = cleavage by strong HX.
• Synthesis map:
  • Alkene → alcohol (hydration / HB-oxy).
  • Alcohol → alkene (dehydration) or ether (dehydration at 413 K).
  • Alcohol → alkyl halide (HX/PBr₃) → Grignard → new alcohol.
  • Phenol derivations: Kolbe (acid), R–T (aldehyde).
• Industrial: methanol (CO + H₂), ethanol (fermentation or ethene hydration), phenol (cumene), diethyl ether (dehydration of EtOH).

Keep practicing structure drawing, mechanism arrows, and reagent selection—the exam often rewards clear logical synthesis routes and mechanism rationale.