From the ethanol in beverages to the phenol in antiseptics — the chemistry of oxygen-containing organic compounds. With interactive acidity (pKa) comparison, distinguishing tests, reaction mechanisms, and 20+ real-life examples.
Before diving in, here's the entire chapter on one page. How preparation connects to physical properties (H-bonding), which connects to reactivity (O-H acidity, C-O cleavage), and to real-world applications.
Alcohols (R-OH) and phenols (Ar-OH) contain the -OH group, but phenol's acidity is much higher (pKa ~10 vs ~16) because phenoxide is resonance-stabilized. Alcohols react via O-H bond cleavage (with Na, acids) and C-O bond cleavage (with HX, PCl₅). Phenols undergo electrophilic aromatic substitution (ring activation). Ethers are relatively inert but cleave with HI. Distinguishing tests (Lucas, Victor Meyer, iodoform) identify specific compounds.
Alcohols: Compounds with -OH group attached to sp³ hybridized carbon (aliphatic). General formula: R-OH.
Phenols: Compounds with -OH group attached to sp² hybridized carbon of an aromatic ring (aryl). General formula: Ar-OH.
Ethers: Compounds with oxygen bonded to two carbon groups. General formula: R-O-R' or R-O-Ar or Ar-O-Ar.
By number of -OH groups:
By carbon attached to -OH (degree):
By carbon skeleton: Allylic (CH₂=CH-CH₂-OH), benzylic (Ph-CH₂-OH), vinylic (CH₂=CH-OH, unstable → tautomerizes to acetaldehyde).
Alcohols = -OH on sp³ C. Phenols = -OH on sp² C (aromatic). Ethers = R-O-R'. Alcohols classified by degree (1°/2°/3°), number of OH groups (mono/di/tri/poly), and carbon skeleton (allylic, benzylic). Phenols classified by number of -OH. Ethers classified by symmetry (simple/mixed).
IUPAC: alkane → alkanol (drop -e, add -ol)
OH gets priority over halogens, alkyl, double bonds (in some cases).
Examples:
IUPAC: phen- + ol, with numbering starting at the -OH carbon
Examples:
IUPAC: alkoxy + alkane (larger alkyl as parent)
Examples:
| Common Name | IUPAC Name | Formula |
|---|---|---|
| Methyl alcohol | Methanol | CH₃OH |
| Ethyl alcohol | Ethanol | C₂H₅OH |
| Isopropyl alcohol | Propan-2-ol | (CH₃)₂CHOH |
| tert-Butyl alcohol | 2-Methylpropan-2-ol | (CH₃)₃COH |
| Ethylene glycol | Ethane-1,2-diol | HOCH₂CH₂OH |
| Glycerol | Propane-1,2,3-triol | HOCH₂CHOHCH₂OH |
| Phenol (carbolic acid) | Phenol | C₆H₅OH |
| Anisole | Methoxybenzene | C₆H₅OCH₃ |
| Diethyl ether | Ethoxyethane | C₂H₅OC₂H₅ |
Alcohols: alkanol (propan-2-ol). Phenols: phen-ol with -OH as C1. Ethers: alkoxy + parent alkane (methoxyethane). OH takes priority over halogens and alkyl groups for numbering.
1. From haloalkanes (hydrolysis):
R-X + KOH (aq) → R-OH + KX
Primary halides → SN2. Tertiary → SN1.
2. From alkenes (hydration):
CH₂=CH₂ + H₂O (H⁺) → CH₃CH₂OH (Markovnikov)
CH₂=CH₂ + B₂H₆ then H₂O₂/OH⁻ → CH₃CH₂OH (anti-Markovnikov)
3. From carbonyl compounds (reduction):
Aldehyde + H₂/Ni (or NaBH₄ or LiAlH₄) → primary alcohol
Ketone + H₂/Ni → secondary alcohol
Carboxylic acid + LiAlH₄ → primary alcohol
4. From Grignard reagent + carbonyl:
HCHO + R-MgX → primary alcohol (R-CH₂OH)
RCHO + R'-MgX → secondary alcohol
R-CO-R' + R''-MgX → tertiary alcohol
5. Industrial (from CO + H₂): Methanol from syngas.
1. From chlorobenzene (Dow process):
Ph-Cl + NaOH (300°C, 300 atm) → Ph-ONa → Ph-OH (after acidification)
2. From benzene sulfonic acid:
Ph-SO₃H + NaOH (fuse) → Ph-ONa → Ph-OH
3. Cumene process (industrial):
Benzene + propene (H₃PO₄) → cumene → cumene hydroperoxide → phenol + acetone
This is the major industrial route (gives both phenol and acetone).
4. From diazonium salt:
Ph-N₂⁺Cl⁻ + H₂O (warm) → Ph-OH + N₂ + HCl
1. Williamson synthesis (most important):
R-ONa + R'-X → R-O-R' + NaX
Best for primary alkyl halides (SN2). For tertiary, elimination competes (E2 gives alkene).
Example: CH₃CH₂-ONa + CH₃-I → CH₃CH₂-O-CH₃ + NaI
2. From alcohols (dehydration, for symmetrical ethers):
2R-OH (H₂SO₄, 140°C) → R-O-R + H₂O
At 170°C, alkenes form (elimination).
Alcohols: from haloalkanes (hydrolysis), alkenes (hydration), carbonyls (reduction), Grignard reagents (with formaldehyde/aldehyde/ketone → 1°/2°/3° alcohol). Phenols: Dow process, benzene sulfonic acid, cumene process (main industrial). Ethers: Williamson synthesis (R-ONa + R'-X, best for primary halides).
Alcohols and phenols have higher boiling points than expected for their molecular mass because of intermolecular hydrogen bonding.
Order: alcohols > ethers > alkanes (for similar molecular mass)
Examples:
Ethanol (MW 46) has higher BP than diethyl ether (MW 74) because ethanol H-bonds but ether doesn't (no -OH).
Order among alcohols (same carbon number): 1° > 2° > 3° (more branched = less surface area = lower BP)
Lower alcohols are freely soluble in water because they can H-bond with water.
Solubility decreases with increasing carbon chain (hydrophobic part grows):
Phenols are moderately soluble (catechol, resorcinol are quite soluble; phenol itself ~8% at room temp).
Ethers are slightly soluble in water (can accept H-bonds from water via oxygen, but can't donate).
| Compound | H-bond Donor? | H-bond Acceptor? | Water Solubility |
|---|---|---|---|
| Alcohol (R-OH) | Yes | Yes | High (small R) |
| Phenol (Ar-OH) | Yes | Yes | Moderate |
| Ether (R-O-R') | No | Yes | Slightly soluble |
| Alkane (R-H) | No | No | Insoluble |
Alcohols and phenols have higher BP than ethers and alkanes because they H-bond (both donate and accept). Ethers can only accept H-bonds (from water). Solubility decreases with increasing carbon chain. Branching decreases BP (less surface area).
1. Reaction with Na:
2R-OH + 2Na → 2R-ONa + H₂
Confirms presence of -OH (alcohols, phenols). Not ethers.
2. Reaction with carboxylic acids (esterification):
R-OH + R'-COOH (H₂SO₄ catalyst) → R'-COO-R + H₂O
Ester — pleasant smell (flavor, fragrance).
3. Reaction with acid chlorides:
R-OH + R'-COCl (pyridine) → R'-COO-R + HCl
4. Reaction with Grignard reagent:
R-OH + R'-MgX → R-H + R'-OMgX
Alcohols destroy Grignard reagents (acid-base reaction).
1. Reaction with HX:
R-OH + HX → R-X + H₂O
Reactivity: HI > HBr > HCl > HF. Order of alcohol reactivity: 3° > 2° > 1° (via carbocation for 3°, SN2 for 1°).
Lucas test: 3° alcohol + HCl (Lucas reagent) → immediate turbidity. 2° alcohol → turbidity in 5 min. 1° alcohol → no reaction at room temp.
2. Reaction with PCl₃, PCl₅, SOCl₂:
3R-OH + PCl₃ → 3R-Cl + H₃PO₃
R-OH + PCl₅ → R-Cl + POCl₃ + HCl
R-OH + SOCl₂ → R-Cl + SO₂ + HCl (SOCl₂ = thionyl chloride, best — no side products)
3. Dehydration (H₂SO₄, heat):
R-CH₂-CH₂-OH → R-CH=CH₂ + H₂O
3° alcohol → easiest (H₂SO₄, 20°C). 2° needs ~100°C. 1° needs ~170°C.
Regiochemistry: Zaitsev's rule → more substituted alkene (major).
4. Oxidation:
1° alcohol → aldehyde (PCC, Pyridinium chlorochromate — mild) → carboxylic acid (KMnO₄, K₂Cr₂O₇/H⁺ — strong)
2° alcohol → ketone (KMnO₄, K₂Cr₂O₇/H⁺)
3° alcohol → no oxidation (no H on C-OH)
Choose an alcohol → see its Lucas test and iodoform test result.
Alcohols react via O-H cleavage (with Na, acids — esterification) or C-O cleavage (with HX → R-X, PCl₅ → R-Cl, SOCl₂, dehydration → alkene). Oxidation: 1° → aldehyde → acid; 2° → ketone; 3° → no oxidation. Lucas test distinguishes 1°/2°/3°. Iodoform test for CH₃-CH(OH)-R.
Phenols are more acidic than alcohols (pKa phenol ~10, alcohol ~16) because:
Reactions showing acidity:
Substituent effects on phenol acidity:
Choose a compound to see its pKa value and acidity ranking.
1. Electrophilic aromatic substitution (ring activated):
-OH is strongly activating and ortho/para directing.
2. Reaction with Zn (distillation):
Phenol + Zn → benzene + ZnO
3. Kolbe's reaction:
Phenol + NaOH + CO₂ (then acidify) → salicylic acid
4. Reimer-Tiemann reaction:
Phenol + CHCl₃ + NaOH → salicylaldehyde (ortho-hydroxybenzaldehyde)
5. Esterification:
Phenol + acetic anhydride (pyridine) → phenyl acetate
6. Oxidation:
Phenol + strong oxidizer → p-benzoquinone
7. Hydrogenation:
Phenol + H₂/Ni → cyclohexanol
Phenols are more acidic than alcohols (resonance stabilization of phenoxide). Phenols react with NaOH (alcohols don't). Ring is strongly activated — undergoes electrophilic substitution easily (bromination without catalyst). Key reactions: Kolbe (salicylic acid), Reimer-Tiemann (salicylaldehyde). Substituent effects: electron-withdrawing groups increase acidity, electron-donating decrease.
Ethers are relatively unreactive (inert):
This makes ethers excellent solvents for organic reactions.
1. Cleavage by HX (C-O bond breaks):
R-O-R + HX → R-X + R-OH (one equivalent)
R-O-R + 2HX → 2R-X + H₂O (excess)
Reactivity: HI > HBr > HCl (HCl rarely works).
Mechanism: Protonation of ether O → nucleophilic attack by X⁻ → SN1 or SN2 depending on R group.
2. Reaction with PCl₅:
R-O-R + PCl₅ → 2R-Cl + POCl₃
3. Formation of peroxide (auto-oxidation):
R-O-R + O₂ (in light) → hydroperoxide (R-O-O-R or R-O-O-H)
Peroxides are explosive! Old ether samples can be dangerous.
Storage: in dark bottles, with antioxidants (e.g., BHT), over KOH.
4. Electrophilic substitution of anisole:
Anisole (Ph-OCH₃) undergoes electrophilic aromatic substitution — -OCH₃ is ortho/para directing.
Ethers are relatively inert (don't react with most reagents) and can dissolve a wide range of organic compounds (both polar and non-polar to some extent). Diethyl ether is a classic solvent for Grignard reactions because it:
Ethers are relatively inert (no -OH, no carbonyl). Cleave with HI (RI + ROH), PCl₅. Auto-oxidize to peroxides (dangerous). Anisole (Ph-OCH₃) undergoes electrophilic substitution (ring activated). Excellent solvents for Grignard reactions (inert to the reagent).
Alcohols and phenols + Na → H₂ gas evolved (bubbles, often ignited with pop sound).
Ethers don't react (no -OH).
So this distinguishes alcohols/phenols from ethers.
Reagent: Lucas reagent = conc. HCl + anhydrous ZnCl₂
R-OH + HCl/ZnCl₂ → R-Cl (turbidity — alkyl chloride is insoluble)
Results:
Carbocation stability → SN1 rate → 3° > 2° > 1° → Lucas test results.
Reagent: HI/P, then AgNO₂, then HNO₂ (multiple steps)
Results:
Based on the different nitroalkanes formed from each class.
Reagent: I₂ + NaOH (or KI + NaOCl)
Compound with CH₃-CH(OH)- or CH₃-CO- group → yellow precipitate of CHI₃ (iodoform).
Positive for:
Negative for:
Phenol + Br₂ water → 2,4,6-tribromophenol (white precipitate).
Alcohols don't react with Br₂ water (no ring to activate).
So this distinguishes phenol from alcohol.
Phenol + FeCl₃ (neutral) → violet/blue-green color
This is a characteristic test for phenols (and enols). Due to formation of a colored iron-phenoxide complex.
Alcohols don't give this test.
| Test | Purpose | Result/Colour |
|---|---|---|
| Sodium metal | -OH group | H₂ bubbles (alcohols/phenols) |
| Lucas test | 1°/2°/3° alcohol | Immediate (3°), 5 min (2°), no reaction (1°) |
| Victor Meyer | 1°/2°/3° alcohol | Red (1°), blue (2°), colorless (3°) |
| Iodoform | CH₃-CH(OH)-R | Yellow CHI₃ precipitate |
| Br₂ water | Phenol | White ppt (2,4,6-tribromophenol) |
| FeCl₃ | Phenol | Violet/blue-green colour |
Mistake: "Lucas test is positive for all
alcohols."
Correction: Lucas test is only
positive for 2° and 3° alcohols at room temperature. 1° alcohols
need heat (SN1 not feasible for primary). This is exactly what
makes Lucas test useful — it distinguishes 1°/2°/3°.
Sodium test: -OH presence (alcohols/phenols). Lucas test: 3° (immediate), 2° (5 min), 1° (no reaction). Victor Meyer: red (1°), blue (2°), colorless (3°). Iodoform test: CH₃-CH(OH)-R or CH₃-CO-R. FeCl₃ test: phenol (violet). Br₂ water: phenol (white ppt).
"1° oxi → aldehyde → acid, 2° oxi → ketone, 3° no oxi"
3° has no H on C-OH → cannot be oxidized.
"Phenol POPULAR — resonance-stabilized, acidic. Alcohol AVERAGE — only pKa 16."
Phenol + NaOH → reacts (soluble phenoxide).
Alcohol + NaOH → no reaction.
"3rd grade gets IMMEDIATE, 2nd grader waits 5 min, 1st grader never passes at room temp"
3° immediate, 2° 5 min, 1° no reaction.
"RBC" — Red (1°), Blue (2°), Colorless (3°)
RBC = Red Blood Cells — think 1° is "primary red".
"CH₃-CH(OH)-R or CH₃-CO-R → yellow iodoform"
Needs CH₃ directly attached to CH(OH) or CO.
"I breaks Ethers, Cl barely touches"
Reactivity: HI > HBr > HCl. HI is strong, good Nu.
"R-ONa + R'-X → R-O-R'"
Best for primary R'-X (SN2). For tertiary → elimination dominates.
"Kolbe Carboxylates, Reimer-Tiemann Aldehydes"
Kolbe: phenol + CO₂ → salicylic acid.
Reimer-Tiemann: phenol + CHCl₃ → salicylaldehyde.
"FeCl₃ = Violet, Br₂ = White ppt"
Both tests distinguish phenol from alcohol.
Use "RBC" for Victor Meyer (Red 1°, Blue 2°, Colorless 3°), "I breaks Ethers" for HI cleavage, and "3rd gets immediate" for Lucas test. Phenol: FeCl₃ violet or Br₂ white ppt.
Q1. Why is phenol more acidic than ethanol?
Phenol (pKa ~10) is more acidic than ethanol (pKa ~16) because:
(1) Phenoxide ion (PhO⁻) is resonance-stabilized — negative charge delocalized into ring
(2) Ethoxide ion (C₂H₅O⁻) has no such stabilization — charge localized on O
Phenol + NaOH → sodium phenoxide (soluble) — reaction occurs
Ethanol + NaOH → no reaction (ethanol too weak an acid)
Q2. Give one chemical test to distinguish between 1-propanol and 2-propanol.
Reagent: conc. HCl + anhydrous ZnCl₂
2-propanol (2°) → turbidity in ~5 minutes (secondary → SN1)
1-propanol (1°) → no turbidity at room temp (needs heat)
2-propanol (CH₃-CHOH-CH₃) has CH₃-CH(OH)-R → iodoform test positive (yellow CHI₃)
1-propanol (CH₃CH₂CH₂OH) → no CH₃-CH(OH) → iodoform negative
Q3. How is phenol converted to (a) salicylic acid, (b) salicylaldehyde?
Phenol + NaOH → sodium phenoxide
Sodium phenoxide + CO₂ (CO₂ gas passed, 130°C, 4-7 atm) → sodium salicylate
Sodium salicylate + HCl → salicylic acid (2-hydroxybenzoic acid)
Phenol + CHCl₃ (chloroform) + NaOH → sodium salicylate (formylation)
Acidification gives salicylaldehyde (2-hydroxybenzaldehyde)
Mechanism: CHCl₃ + NaOH → dichlorocarbene (:CCl₂) → attacks phenol → hydrolysis
Salicylic acid → aspirin (acetylsalicylic acid). Salicylaldehyde → flavors, chelating agents.
Q4. Write the reaction of 1-propanol with (a) Na, (b) PCl₅, (c) hot conc. H₂SO₄.
2CH₃CH₂CH₂OH + 2Na → 2CH₃CH₂CH₂ONa + H₂↑
Product: sodium propoxide + hydrogen gas
CH₃CH₂CH₂OH + PCl₅ → CH₃CH₂CH₂Cl + POCl₃ + HCl
Product: 1-chloropropane
CH₃CH₂CH₂OH → CH₃CH=CH₂ + H₂O (dehydration)
Product: propene (only one alkene possible from 1-propanol)
Q5. How is ethanol converted to: (a) ethene, (b) ethanal, (c) ethanoic acid, (d) diethyl ether, (e) ethyl ethanoate?
CH₃CH₂OH --(conc. H₂SO₄, 170°C)--> CH₂=CH₂ + H₂O (dehydration)
CH₃CH₂OH --(PCC, or Cu at 300°C)--> CH₃CHO + H₂ (controlled oxidation)
PCC = pyridinium chlorochromate — mild oxidant, stops at aldehyde
CH₃CH₂OH --(KMnO₄/H⁺, or K₂Cr₂O₇/H⁺, heat)--> CH₃COOH (strong oxidation)
Via aldehyde intermediate → acid
2CH₃CH₂OH --(conc. H₂SO₄, 140°C)--> C₂H₅-O-C₂H₅ + H₂O (dehydration)
Lower T (140°C) favors ether; higher T (170°C) favors alkene
CH₃CH₂OH + CH₃COOH --(H₂SO₄)--> CH₃COOC₂H₅ + H₂O (esterification)
Product: ethyl ethanoate (ethyl acetate) — pleasant smelling ester
Q6. (a) What is Williamson synthesis? (b) Why does it give poor yield with tertiary alkyl halides? (c) Give one important use of ethers.
Reaction: R-ONa + R'-X → R-O-R' + NaX
Alkoxide (R-ONa) + alkyl halide (R'-X) → ether + sodium halide.
Example: CH₃CH₂-ONa + CH₃-I → CH₃CH₂-O-CH₃ + NaI (methoxyethane)
This is the best method for preparing both symmetrical and unsymmetrical ethers.
With tertiary alkyl halides, elimination (E2) competes with substitution (SN2).
Reason: strong base (alkoxide) + 3° halide (hindered C-Br) → elimination more favorable than SN2. Major product: alkene (from E2), not ether.
So Williamson synthesis is only good for 1° and 2° halides; with 3° halides, use a different method.
Diethyl ether: solvent for Grignard reactions, historical anesthetic.
MTBE: gasoline octane booster.
THF: industrial solvent for polymers, Grignard reactions.
Anisole: perfumes, flavors.
Q7. Describe the reactions of phenol with: (a) Br₂ water, (b) FeCl₃, (c) NaOH, (d) Zn dust, (e) dil. HNO₃.
Phenol + Br₂ (aq) → 2,4,6-tribromophenol (white precipitate)
No catalyst needed (unlike benzene) — ring is strongly activated by -OH group
Phenol + FeCl₃ (neutral) → violet/blue-green color
Formation of iron phenoxide complex — characteristic test for phenol
Phenol + NaOH → sodium phenoxide + H₂O (clear solution)
Demonstrates acidic nature of phenol (alcohols don't react with NaOH)
Phenol + Zn (distillation) → benzene + ZnO
Deoxygenation — removes the -OH group. Zinc dust distillation is a general method for removing phenolic -OH.
Phenol + dil. HNO₃ (room temp) → o-nitrophenol + p-nitrophenol (mixture)
Ring is activated — mild nitration sufficient
Selected exercise questions from NCERT Class 12 Chemistry Chapter 7. Click each to reveal the solution.
(a) Propan-2-ol (isopropanol) — OH on C2 of propane
(b) Butan-1-ol (n-butanol) — OH on C1 of butane
(c) 2-Methylpropan-2-ol (t-butanol) — central C bonded to 3 methyls and OH
(d) Prop-2-en-1-ol (allyl alcohol) — 3-carbon chain with double bond at C2 and OH at C1
(a) 2-Chloropropan-1-ol: Propane-1-ol with Cl on C2
HOCH₂-CH(Cl)-CH₃
(b) 4-Methoxyphenol: Phenol with OCH₃ at C4
p-CH₃O-C₆H₄-OH (para-methoxyphenol)
(c) 3-Bromo-2-methylbut-1-ene-1-ol: The structure would be complex — CnH(2n-2) chain with double bond at C1, OH at C1, Br at C3, methyl at C2. HO-C(CH₃)=C(Br)-CH₃... not stable (enol form). In practice, such compounds tautomerize to the keto form.
(a) Pentan-1-ol from 1-pentene:
CH₃CH₂CH₂CH=CH₂ + B₂H₆ then H₂O₂/OH⁻ → CH₃CH₂CH₂CH₂CH₂OH (anti-Markovnikov)
Or hydration (H⁺/H₂O) gives pentan-2-ol (Markovnikov).
(b) 3-Methylbutan-2-ol from 2-methylbut-2-ene:
CH₃-C(CH₃)=CH-CH₃ + H₂O/H⁺ → CH₃-CH(CH₃)-CH(OH)-CH₃ (Markovnikov addition — H to carbon with more H)
(c) Hexan-1-ol from 1-hexene:
CH₃(CH₂)₃CH=CH₂ + B₂H₆ then H₂O₂/OH⁻ → CH₃(CH₂)₄CH₂OH (anti-Markovnikov)
Cumene process:
Step 1 — Cumene formation:
Benzene + propene --(H₃PO₄, 250°C, 30 atm)--> Cumene (isopropylbenzene, Ph-CH(CH₃)₂)
Step 2 — Oxidation:
Cumene + O₂ → Cumene hydroperoxide (Ph-C(CH₃)₂-O-O-H)
Step 3 — Cleavage (H⁺):
Cumene hydroperoxide + H⁺ → Phenol + Acetone
Ph-C(CH₃)₂-O-O-H + H₂O (H⁺) → Ph-OH + (CH₃)₂C=O + H₂O₂ — wait, no
Actually: Ph-C(CH₃)₂-O-O-H --(H⁺, H₂O)--> Ph-OH + CH₃COCH₃ (acetone)
Overall: cumene + O₂ → phenol + acetone
Advantage: Two valuable products — phenol AND acetone. Major industrial route (95% of world phenol).
Answer: ortho-Nitrophenol is steam volatile; para-nitrophenol is not.
Reason: ortho-Nitrophenol forms intramolecular hydrogen bonds (between -OH and -NO₂ groups on adjacent carbons) — this reduces intermolecular H-bonding, making it more volatile. para-Nitrophenol forms intermolecular H-bonds (between -OH of one molecule and -NO₂ of another) — this increases intermolecular forces, making it less volatile.
Result: ortho is steam volatile (distils over), para remains behind. This is how they're separated.
Answer: In ortho-nitrophenol, intramolecular H-bonding between -OH and -NO₂ stabilizes the undissociated form (phenol), making it harder to lose the proton. So ortho-nitrophenol is less acidic (higher pKa = 7.23).
In para-nitrophenol, there is no intramolecular H-bonding possible (too far apart). The -NO₂ stabilizes the phenoxide ion via -M effect (resonance), making it easier to lose the proton. So para-nitrophenol is more acidic (lower pKa = 7.15).
Note: The difference is small (7.23 vs 7.15) but noticeable. Similar trend for ortho vs para-cresol.
Order of acidity: p-nitrophenol > o-nitrophenol > phenol
Reason:
• Phenol (pKa 10.0) is the weakest — phenoxide is stabilized only by resonance into the ring.
• o-Nitrophenol (pKa 7.23) — NO₂ group stabilizes phenoxide via -M effect. But intramolecular H-bonding in the unionized form slightly reduces acidity.
• p-Nitrophenol (pKa 7.15) — NO₂ group stabilizes phenoxide via -M effect with NO intramolecular H-bonding interference → most acidic.
p-Nitrophenol is slightly stronger than o-isomer. Both much stronger than phenol.
(a) Br₂ in CS₂ at low temperature (0°C):
Phenol + Br₂/CS₂ → o-bromophenol + p-bromophenol (monobromination)
Low T + non-polar solvent → only monosubstitution (controlled)
(b) Br₂ water (aq, room temp):
Phenol + 3Br₂ (aq) → 2,4,6-tribromophenol (white ppt) + 3HBr
High Br₂ concentration + polar solvent → trisubstitution (all o/p positions go)
(c) Dil. HNO₃ (room temp):
Phenol + dil. HNO₃ → o-nitrophenol + p-nitrophenol
Mild nitration — ring is activated enough that dilute acid works
(a) Kolbe's reaction: Phenol is treated with CO₂ in the presence of NaOH (sodium phenoxide) at 130°C, 4-7 atm, to give sodium salicylate, which on acidification gives salicylic acid (2-hydroxybenzoic acid).
Ph-OH + NaOH → Ph-ONa
Ph-ONa + CO₂ (130°C, 5 atm) → sodium salicylate
Sodium salicylate + H⁺ → salicylic acid
Salicylic acid is used to make aspirin.
(b) Reimer-Tiemann reaction: Phenol is treated with CHCl₃ and NaOH to give salicylaldehyde (2-hydroxybenzaldehyde) as the major product.
Phenol + CHCl₃ + 3NaOH → sodium salicylate (formylation via dichlorocarbene intermediate)
Acidification → salicylaldehyde
Dichlorocarbene (:CCl₂) is the reactive intermediate, generated from CHCl₃ + NaOH.
Why ethers are inert:
(1) No -OH group — cannot form H-bonds (no reaction with Na, NaOH)
(2) No carbonyl group — cannot undergo nucleophilic addition (no reaction with HCN, NaHSO₃)
(3) The C-O-C bond is stable to bases, oxidants (except strong), and most reducing agents
(4) The oxygen lone pairs are tied up in C-O bonds — weak basicity
Reactions that DO occur: cleavage by HI, auto-oxidation to peroxides.
Uses:
• Solvents (diethyl ether, THF) — especially for Grignard reactions
• Diethyl ether: historical anesthetic (1846-1950s)
• MTBE: gasoline additive (octane booster)
• Anisole: perfume, flavor
• THF: polymer industry (PVC, PU), Grignard reactions
• 1,4-Dioxane: solvent in cosmetics, inks
(a) 1-Propanol vs 2-propanol:
• Lucas test: 2-propanol turbid in 5 min, 1-propanol no reaction at room temp
• Iodoform test: 2-propanol positive (CH₃-CHOH-CH₃), 1-propanol negative
• Victor Meyer: 1-propanol red, 2-propanol blue
(b) Phenol vs ethanol:
• FeCl₃ test: phenol gives violet color, ethanol no color
• Br₂ water: phenol gives white ppt (2,4,6-tribromophenol), ethanol no reaction
• NaOH: phenol dissolves (forms phenoxide), ethanol no reaction (too weak acid)
• NaHCO₃: phenol no reaction (weaker than H₂CO₃), carboxylic acids react. Neither phenol nor ethanol reacts with NaHCO₃.
(c) Methanol vs ethanol:
• Iodoform test: ethanol positive (CH₃-CH₂OH has CH₃-C-OH), methanol negative (no CH₃-CH-OH)
• Victor Meyer: both 1° → red. Not useful for distinction.
• Boiling point: ethanol 78°C, methanol 65°C (small difference)
• Iodoform test is the best distinguishing test.
| Ethanol | pKa ≈ 16 | Weakest (alkoxide not stabilized) |
| Water | pKa ≈ 15.7 | Reference |
| Phenol | pKa ≈ 10 | Phenoxide resonance-stabilized |
| p-Nitrophenol | pKa ≈ 7.15 | NO₂ stabilizes phenoxide |
| Picric acid | pKa ≈ 0.4 | Very strong (3 NO₂ groups) |
| Carboxylic acid | pKa ≈ 4-5 | Stronger than phenol |
| Lucas reagent | HCl + ZnCl₂ | 3° immediate, 2° 5 min, 1° no rxn |
| Iodoform test | I₂ + NaOH | Positive for CH₃-CH(OH)-R |
| Victor Meyer | HI/P + AgNO₂ + HNO₂ | Red (1°), blue (2°), colorless (3°) |
| FeCl₃ test | Neutral FeCl₃ | Violet color for phenol |
| R-OH | Alcohol (general) |
| Ar-OH | Phenol (aryl alcohol) |
| R-O-R' | Ether |
| Ph | Phenyl group (C₆H₅) |
| pKa | Acid dissociation constant (lower = stronger acid) |
| PCC | Pyridinium chlorochromate (mild oxidant) |
| Lucas reagent | Conc. HCl + anhydrous ZnCl₂ |
| THF | Tetrahydrofuran (cyclic ether) |
| MTBE | Methyl tert-butyl ether |
| BHT | Butylated hydroxytoluene (antioxidant) |
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
All key reactions & facts at one place — perfect for quick revision before exams.