📍 Chapter Overview
Phenols — Complete Mind Map
Topics Covered: Preparation (Dow's Process, Benzene sulfonic acid, Diazonium, Cumene) · Physical Properties · Acidity (Comparison with Alcohols) · Electrophilic Substitution (o/p directivity) · Named Reactions: Kolbe’s, Reimer-Tiemann · Reaction with Zn dust and oxidation.
🤖 AI Prompt — Chapter Mind Map:
Ruby-red themed chemistry mind map on dark background. Central node: Phenol molecule (Benzene-OH) labeled "PHENOLS". Five branches: (1) "Preparation" — from Cumene (with O2/H+), from Chlorobenzene (NaOH/623K), from Diazonium; (2) "Acidity" — Resonance stabilization of Phenoxide ion shown; (3) "Named Reactions" — Kolbe's (Salicylic acid) and Reimer-Tiemann (Salicylaldehyde); (4) "Electrophilic Substitution" — Nitration (o- and p- isomers), Halogenation (Br2/CS2 vs Br2 water); (5) "Tests" — Neutral FeCl3 (Violet color). High resolution educational infographic.
1. Preparation of Phenols
1.1 From Haloarenes (Dow’s Process)
Reaction: Chlorobenzene + NaOH $\xrightarrow{623K, 300 atm}$ Sodium Phenoxide $\xrightarrow{H^+}$ Phenol. — *NCERT Section 7.4.2 (1)*
This requires drastic conditions due to the unreactivity of aryl halides. (Refer to Module 10 for SNAr reasons).
1.2 From Benzene Sulphonic Acid
1.3 From Benzene Diazonium Salts
Benzene Diazonium Chloride $\xrightarrow{H_2O, \text{ warm}}$ Phenol + $N_2$ + $HCl$. — *NCERT Section 7.4.2 (3)*
1.4 From Cumene (Isopropylbenzene) — Industrial Method
Reaction Name: Preparation from Cumene (NCERT Page 202, Section 7.4.2)
Phenol is produced industrially from Cumene. Cumene $\xrightarrow{O_2 \text{ (air)}}$ Cumene Hydroperoxide $\xrightarrow{H^+}$ Phenol + **Acetone**.
Acetone is an important byproduct obtained in large quantities.
Industrial Preparation of Phenol from Cumene (NCERT Page 202)
Draw a 2-step chemical transformation on white background. Step 1: Cumene (benzene ring with an isopropyl group $-CH(CH_3)_2$) reacting with $O_2$. Arrow pointing to "Cumene hydroperoxide" (benzene ring with $-C(CH_3)_2-O-O-H$). Step 2: H+/H2O treatment. Show the molecule splitting into TWO products: Phenol (Benzene-OH) and Acetone ($CH_3COCH_3$). Highlight that Acetone is an important byproduct. Clear skeletal structures, bold labels, textbook quality.
2. Physical Properties and Acidity
Boiling Points: Alcohols/Phenols have higher BP than ethers/hydrocarbons due to intermolecular H-bonding. — *NCERT Section 7.4.3*
2.1 Why are Phenols more acidic than Alcohols? (NCERT Page 210)
- **$sp^2$ Hybridization:** Carbon attached to -OH in phenol is $sp^2$ (more electronegative), whereas in alcohols it is $sp^3$. Electronegative carbon withdraws electrons, weakening the O-H bond.
- **Resonance Stabilization of Phenoxide Ion:** In phenol, the phenoxide ion ($C_6H_5O^-$) is stabilized by resonance over the benzene ring. In alcohols, the alkoxide ion ($R-O^-$) is not stabilized by resonance.
- **Charge Delocalization:** In phenoxide, the negative charge is delocalized over 3 carbons (ortho/para).
Resonance Structures of Phenoxide Ion (NCERT Page 210)
Draw a resonance diagram for the Phenoxide Ion ($C_6H_5O^-$) on white background. Show 5 canonical structures with double-headed arrows. Structure 1: Hexagon with O- (one negative charge at top). Show curved arrow from O- lone pair to C-O bond and ring pi bond to ortho position. Structures 2-4: Show the negative charge at ortho, para, and the other ortho positions respectively. Structure 5: Resonance hybrid with dashed circle inside hexagon. Label: "Stabilization of Phenoxide ion by resonance — reason for high acidity". White background, bold structures, chemistry textbook quality.
3. Chemical Reactions
3.1 Electrophilic Substitution (Halogenation)
**Reaction Name: Halogenation of Phenol (NCERT Page 211, Section 7.5.3)**
- With $Br_2$ in $CS_2$ (low temp): Mixture of o- and p-bromophenols (p-isomer major).
- With **Bromine Water**: Immediate formation of **2,4,6-tribromophenol** (white precipitate). This happens because water makes phenol more ionizable (to phenoxide), making it highly activated.
3.2 Nitration
Phenol + dil. $HNO_3$ $\rightarrow$ o-nitrophenol + p-nitrophenol. (Separated by steam distillation — o-isomer is steam volatile due to **intramolecular** H-bonding; p-isomer is not due to **intermolecular** H-bonding).
3.3 Kolbe’s Reaction
Reaction Name: Kolbe’s Reaction (NCERT Page 211, Section 7.5.3)
Phenol treated with $NaOH$ (forms phenoxide, which is more reactive than phenol) $\xrightarrow{CO_2, \text{then } H^+}$ **Salicylic acid** (2-hydroxybenzoic acid).
3.4 Reimer-Tiemann Reaction
Reaction Name: Reimer-Tiemann Reaction (NCERT Page 212, Section 7.5.3)
Phenol $\xrightarrow{CHCl_3 + aq. NaOH, \text{then } H^+}$ **Salicylaldehyde** (2-hydroxybenzaldehyde).
Reimer-Tiemann Reaction Mechanism (NCERT Page 212)
Draw the sequence for Reimer-Tiemann transformation on white background. Phenol $\xrightarrow{CHCl_3, NaOH}$ show intermediate with $-CHCl_2$ at ortho position. $\xrightarrow{NaOH}$ show hydrolysis of $-CHCl_2$ to aldehyde. $\xrightarrow{H^+}$ Final product Salicylaldehyde (Benzene ring with $-OH$ at C1 and $-CHO$ at C2). Bold skeletal structures, textbook reactive arrows. Label: "Intermediate: Dichlorocarbene active species (though not shown in reaction chain)".
3.5 Other Reactions
- **Reaction with Zinc Dust:** Phenol + Zn $\xrightarrow{\Delta}$ **Benzene** + ZnO. — *NCERT P212*
- **Oxidation:** Phenol $\xrightarrow{Na_2Cr_2O_7 / H_2SO_4}$ **Benzoquinone** (dark colored mixture).
⚡ QUICK REVISION
| Reaction | Reagents | Major Product |
|---|
| Dow’s Process | NaOH, 623K, 300atm | Phenol |
| Cumene Route | O2, then H+/H2O | Phenol + Acetone |
| Kolbe’s | NaOH, CO2 | Salicylic acid |
| Reimer-Tiemann | CHCl3, NaOH | Salicylaldehyde |
| FeCl3 Test | Neutral FeCl3 (aq) | **Violet / Purple coloration** |
E Q1. Arrange the following in decreasing order of acidity: Phenol, o-nitrophenol, p-nitrophenol, o-cresol. *(NCERT Reference)*
M Q2. Why is o-nitrophenol steam volatile but p-nitrophenol is not? Draw structures of H-bonding to explain. *(NCERT P211)*
M Q3. Give the chemical test to distinguish between Phenol and Ethanol.
H Q4. Write the chemical equation for the synthesis of aspirin from phenol. *(Hint: salicylic acid + acetic anhydride)*
- Saying phenol is less acidic than alcohol — WRONG! Phenol ($pKa \approx 10$) is much more acidic than ethanol ($pKa \approx 16$).
- Assuming meta-nitrophenol is more acidic than para. Para-influence involves resonance stabilization, making it more acidic.
- Forgetting that Phenol + Br2/water gives **white precipitate** of 2,4,6-tribromophenol.
