Alcohols, Phenols & Ethers – Comprehensive Bullet-Point Notes

7.0 Context & Learning Outcomes

• After mastering this unit you should be able to:
  • Name alcohols, phenols and ethers by IUPAC rules; recognise and use common names.
  • Outline and explain preparative reactions of alcohols (from alkenes, carbonyl compounds, carboxylic acids), phenols (from haloarenes, sulphonates, diazonium salts, cumene) and ethers (from alcohols or Williamson-type reactions).
  • Correlate physical properties (b.p., solubility, acidity) with structure & hydrogen-bonding capacity.
  • Predict and explain chemical reactivity on the basis of bond cleavage (O–H vs. C–O), resonance and carbocation stability.
  • Work mechanistic steps for electrophilic substitution, dehydration, oxidation, Kolbe, Reimer–Tiemann and Williamson syntheses.

7.1 Classification

• By number of $\text{–OH}$ groups
  • Monohydric, dihydric, trihydric, polyhydric
  • Examples: methanol (mono), ethane-1,2-diol, propane-1,2,3-triol
• By hybridisation of the carbon bearing the $\text{–OH}$
  • $\text{sp}^3$ system (alkyl) → primary/secondary/tertiary; allylic & benzylic are special $\text{sp}^3$ cases.
  • $\text{sp}^2$ system → vinylic alcohols (enols) & phenols.
• Ethers
  • Simple/symmetrical if $R<em>1 = R</em>2$; mixed/unsymmetrical if $R<em>1 \neq R</em>2$.

7.2 Nomenclature Essentials

• Alcohols (IUPAC): replace terminal “e” of parent alkane by “ol”; number chain from end nearest $\text{–OH}$, e.g. $\text{CH}<em>3\text{CH}</em>2\text{OH} \;\rightarrow$ ethanol.
• Polyols: keep “e” then add diol/triol, giving locants – e.g. $\text{HO–CH}<em>2\text{CH}</em>2–\text{OH}$ → ethane-1,2-diol.
• Phenols: simplest = phenol; o-, m-, p- prefixes common; dihydroxy benzenes = benzene-1,2-diol etc.
• Ethers (IUPAC): larger alkyl = parent; smaller written as alkoxy prefix – e.g. $\text{CH}<em>3\text{CH}</em>2\text{OCH}_3$ → 1-methoxypropane. Common: list groups alphabetically + “ether” (ethyl methyl ether).

7.3 Structure & Bond Parameters

• Alcohol C–O formed by $\text{sp}^3<em>C – sp^3</em>O$ $\sigma$ bond; bond angle slightly < $109.5^{\circ}$ (repulsion of two lone pairs).
• Phenol C–O bond length $\approx 136\,\text{pm}$ (< methanol) due to (i) partial $\pi$-character via resonance, (ii) $sp^2$ hybrid carbon.
• Ethers: O is $sp^3$; two lone pairs + two $\sigma$ bonds → tetrahedral; $R–O–R$ angle slightly > $109.5^{\circ}$ (bulk of R groups); $\text{C–O}$ ~ $141\,\text{pm}$.

7.4 Preparation of Alcohols

• From Alkenes
  • Acid-catalysed hydration (Markovnikov) via protonation, carbocation, nucleophilic attack.
  • Hydroboration-oxidation (BH$<em>3$/H$</em>2$O$_2$, $\text{OH}^-$) gives anti-Markovnikov alcohols in excellent yields; Nobel work by H. C. Brown.
• From Carbonyls
  • Catalytic hydrogenation (Pt/Pd/Ni) of aldehydes (→ 1° alcohols) / ketones (→ 2°).
  • NaBH$<em>4$ or LiAlH$</em>4$ achieve same at RT.
• From Carboxylic Acids & Esters
  • Strong reduction with LiAlH$_4$ → 1° alcohols.
  • Industrial: convert acid → ester → hydrogenate (Cu-Cr catalyst).
• From Grignard Reagents ($RMgX$)
  • $+$ methanal → 1°; $+$ other aldehyde → 2°; $+$ ketone → 3° after acidic work-up.

7.5 Preparation of Phenols

• Haloarenes: $\text{C}<em>6\text{H}</em>5\text{Cl} +$ $\text{NaOH, 623\,K, 320 atm} \rightarrow$ $\text{C}<em>6\text{H}</em>5\text{ONa}$ → H$^+$ → phenol.
• Benzenesulphonic acid route: sulphonation $\rightarrow$ Na salt $\rightarrow$ $\text{NaOH (fusion)}$ $\rightarrow$ acidification.
• Diazonium salts: $\text{Ar–N}<em>2^+\text{Cl}^- + \text{H}</em>2\text{O} \rightarrow$ phenol + $\text{N}_2 + \text{HCl}$.
• Cumene process (industrial): cumene $\xrightarrow[\text{air}]{\text{cat.}}$ cumene hydroperoxide $\xrightarrow[]{\text{H}^+}$ phenol $+$ acetone.

7.6 Physical Properties

• Boiling Points
  • Rise with C-number; fall with branching (weaker van der Waals).
  • $\text{–OH}$
    hydrogen-bonding → much higher $b.p.$ than ethers/alkanes: e.g. $\text{C}<em>2\text{H}</em>5\text{OH}$ $b.p.$ $=351\,\text{K}$ vs propane $=231\,\text{K}$.
• Solubility
  • Ability to H-bond with water; decreases with hydrophobic chain length.
  • Lower alcohols miscible; phenol moderately soluble; ethers miscible up to $\approx$ $C_4$.

7.7 Acidity Trends

• Alcohols: very weak acids; order 1^{\circ} > 2^{\circ} > 3^{\circ}; still weaker than water (alkoxides stronger bases than $\text{OH}^-$).
• Phenols
  • $pK<em>a$ phenol $\approx 10$ (10$^6$ times stronger than ethanol $pK</em>a 15.9$).
  • Electron-withdrawing $–NO<em>2$ (o,p) stabilise phenoxide via resonance → stronger acid (e.g. 2,4,6-trinitrophenol $pK</em>a 2.8$).
  • Electron-donating alkyl weaken acidity (cresols > $pK_a\,10.1–10.2$).

7.8 Reactions of Alcohols (by Bond Cleavage)

A. O–H cleavage

• With active metals: $2ROH + 2Na → 2RONa + H_2$.
• Esterification (Fischer): $ROH + R'COOH \xrightarrow[{\Delta}]{{\text{H}<em>2\text{SO}</em>4}} R'COOR + H<em>2O$; with $\text{Ac}</em>2\text{O}$ or $R'COCl/pyridine$ (acetylation).

B. C–O cleavage / electrophilic pathways

• HX (Lucas test): $ROH + HCl/ZnCl_2 → RCl$ (3° > 2° > 1°).
• $PBr<em>3,\;PCl</em>3$: $3ROH + PBr<em>3 → 3RBr + H</em>3PO_3$.
• Dehydration (acid, $\Delta$): $RCH<em>2CH</em>2OH \xrightarrow[]{\text{H}<em>2\text{SO}</em>4,443\,K} RCH=CH<em>2 + H</em>2O$; order of ease 3° > 2° > 1°; mechanism $E1$ via carbocation.
• Oxidation
  • 1° $\xrightarrow[\text{PCC}]{\text{mild}}$ aldehyde $\xrightarrow[\text{KMnO}_4]{\text{strong}}$ acid.
  • 2° → ketone (CrO$<em>3$, PCC, KMnO$</em>4$).
  • 3° resistant; strong oxidants cleave $C–C$.
• Vapour phase Cu (573 K): dehydrogenation – 1° → aldehyde, 2° → ketone, 3° → alkene.

7.9 Reactions of Phenols

• Electrophilic substitution (ortho/para orienting)
  • Nitration: phenol + dil. $HNO<em>3$ (298 K) → p- & o-nitrophenol; conc. $HNO</em>3$ → 2,4,6-trinitrophenol (picric acid).
  • Bromination: in $\text{CCl}<em>4$ low T → o- & p-bromophenol; in $\text{Br}</em>2/\text{H}_2\text{O}$ → 2,4,6-tribromophenol (white ppt.).
• Kolbe (carboxylation): $\text{C}<em>6\text{H}</em>5\text{ONa} + CO_2 (473\,K,6\,atm) → o\text{-}$salicylic acid.
• Reimer–Tiemann: phenol + $CHCl_3/NaOH$ → salicylaldehyde (ortho-formylation via dichlorocarbene).
• Zn-dust: phenol → benzene + ZnO.
• Oxidation: phenol + $H<em>2CrO</em>4$ → p-benzoquinone; slow air oxidation darkens colour.

7.10 Ethers

Preparation

1. Acidic dehydration of 1° alcohols (443 K lower → ether; 443 K higher → alkene): $2RCH<em>2OH \xrightarrow[]{H</em>2SO<em>4,413\,K} RCH</em>2OCH_2R$.
2. Williamson synthesis (SN2): $R'ONa + R–X → R–O–R' + NaX$
   • Best when $R–X$ is 1°; 3° gives elimination.
   • Phenoxide + 1° alkyl halide → aryl alkyl ether.
   • Limitation: cannot use 3° halide with strong base (E2 dominates).

Physical Properties

• Boiling points similar to alkanes, far lower than isomeric alcohols (no intermolecular H-bonding).
• Moderately polar; miscibility with water comparable to alcohol of same $M_r$ due to O···H–O bonding.

Reactions

1. Cleavage with HX (excess, $\Delta$):
   • Dialkyl: $R–O–R + 2HI → 2RI + H_2O$.
   • Aryl-alkyl: $Ar–O–R + HI → ArOH + RI$ (cleavage at alkyl oxygen).
   • Order HI > HBr ≫ HCl.
   • Mechanistic path: protonation → $SN2$ (1°) or $SN1$ (3°) attack by I$^-$.
2. Electrophilic substitution on aromatic ethers (anisole prototype)
   • Halogenation, nitration, Friedel–Crafts alkylation/acylation: $–OCH_3$ activates ring, directing o/p.

7.11 Commercially Important Alcohols

• Methanol (wood spirit)
  • Prepared by catalytic hydrogenation of CO at $350–400\,\text{K}, 50–100\,\text{atm}$, $\text{ZnO–Cr}<em>2\text{O}</em>3$.
  • Colourless, $b.p.$ $=337\,K$; toxic – blindness/death by oxidation to methanal/methanoic acid in body.
  • Solvent, formaldehyde manufacture, fuel.
• Ethanol
  • Fermentation of sugars by invertase/zymase (yeast) up to $\approx 14\%$; hydration of ethene industrially.
  • $b.p.$ $=351\,K$; solvent, beverages, chemical feedstock.
  • Denatured by $CuSO_4$ (colour) + pyridine (odour).

7.12 Named / Signature Reactions

• Lucas Test: $ROH + HCl/ZnCl_2$ – rate of turbidity 3° > 2° > 1°.
• Kolbe’s Electrophilic Carboxylation.
• Reimer–Tiemann Ortho-formylation.
• Williamson Ether Synthesis.
• Fischer Esterification.

7.13 Concept Check & Problem Types (In-Text Examples)

• Predict products for hydration, Grignard addition, dehydration.
• Arrange compounds by boiling point or acidity; justify via H-bonding/resonance.
• Devise syntheses using named reactions (e.g., prepare t-butyl ethyl ether: use $t$-butoxide $+ EtBr$, not $EtONa + tBuBr$).

7.14 Key Numerical / Statistical Data

• $\theta_{\text{tetrahedral}} = 109^{\circ}28'$ (approx $109.5^{\circ}$).
• Fusion of chlorobenzene: $623\,\text{K}$ and $320$ atm.
• Kolbe: $473\,K$, $6$–$7$ atm $CO_2$.
• Cumene oxidation & cleavage moderate $\approx 423\,K$.
• Williamson generally at $\approx 373–393\,K$ in polar aprotic solvents.

7.15 Ethical / Safety Notes

• Methanol poisoning: treat by IV dilute ethanol – competes for alcohol dehydrogenase, allowing renal clearance of methanol.
• Industrial phenol & acetone production significantly tied to petroleum; greener routes being explored.

7.16 Real-World Applications

• Detergents: long-chain $\text{–SO}_3^-$ prepared from alkyl phenols.
• Antiseptics: phenol, picric acid.
• Fragrances & solvents: anisole, diethyl ether (historically as anaesthetic).
• Cumene process links plastics (phenol-formaldehyde resins) and polycarbonates.

7.17 Quick Summary / Mnemonics

• "HIPO" order HX reactivity with ethers: H\textbf{I} > H\textbf{Br} > H\textbf{Cl} (\textbf{O}).
• Alcohol dehydration: low $T$ $\rightarrow$ ether, high $T$ $\rightarrow$ alkene.
• Phenols Love $E$ (electrophiles) ortho/para because of $$\pi$$ donation of lone pair.
• Williamson: get the smaller/less hindered alkyl halide.