Lesson Plan: Alcohols, Phenols & Ethers

Organic Chemistry (Class 12)

Alcohols, Phenols & Ethers

This chapter deals with the -OH group. Understanding Hydrogen Bonding explains their physical properties. In reactions, focus on the Acidity of Phenols (why it's higher than alcohols) and the specific cleavage rules for Ethers.

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⚠️ Prerequisites

GOC: Resonance stabilization of Phenoxide ion vs Alkoxide ion.
Hydrocarbons: Markovnikov addition to alkenes.
Haloalkanes: Nucleophilic substitution ($S_N1/S_N2$) for ether synthesis.

🧠 Study Approach

Structure Determines Property: The -OH group creates Hydrogen Bonds (high Boiling Point, soluble in water). The Phenyl ring in Phenol allows Resonance (High Acidity, Electrophilic Substitution).

Study Sequence

1. Preparation of Alcohols

From Alkenes: Acid catalyzed hydration (Markovnikov) vs Hydroboration-Oxidation (Anti-Markovnikov addition of $H_2O$).
From Carbonyls: Reduction of Aldehydes/Ketones ($NaBH_4, LiAlH_4$). Grignard reagent addition ($HCHO \to 1^\circ$, $RCHO \to 2^\circ$, $RCOR \to 3^\circ$).

2. Preparation of Phenols

From Haloarenes (Dow's Process). From Benzene Sulphonic Acid. From Diazonium Salts.
From Cumene (Isopropylbenzene): The most important commercial method (produces Acetone as byproduct).

3. Physical Properties

Boiling Point: Alcohol > Ether > Hydrocarbon (due to Intermolecular H-bonding).
Solubility: Lower alcohols are soluble in water (H-bonding). Solubility decreases as alkyl chain length increases.

4. Chemical Reactions: Acidity (CRITICAL)

Alcohol vs Phenol: Phenol is more acidic because Phenoxide ion is resonance stabilized. Alcohol's Alkoxide ion is destabilized by +I effect of alkyl group.
Effect of Substituents ($NO_2, CH_3$) on acidity of phenol.

5. Important Mechanisms & Tests

Dehydration: Mechanism to form Ethene ($H^+$ attack $\to$ Carbocation $\to$ Elimination). $3^\circ > 2^\circ > 1^\circ$.
Lucas Test: Distinguishes $1^\circ, 2^\circ, 3^\circ$ alcohols using $HCl/ZnCl_2$. (Turbidity timing).
Oxidation: $PCC$ (Aldehyde) vs $KMnO_4$ (Carboxylic Acid).

6. Reactions of Phenols

Electrophilic Substitution: Nitration (dilute vs conc $HNO_3$), Bromination ($Br_2/CS_2$ vs $Br_2/H_2O$).
Named Reactions: Kolbe's Reaction (Salicylic Acid), Reimer-Tiemann (Salicylaldehyde). Reaction with Zinc dust (Benzene).

7. Ethers: Prep & Properties

Williamson Synthesis: $R-X + R'ONa$. Constraint: $R-X$ must be $1^\circ$ (else Elimination occurs).
Cleavage with HI: If alkyl group is $3^\circ$ $\to$ $I^-$ goes to $3^\circ$ (S_N1). If $1^\circ/2^\circ$ $\to$ $I^-$ goes to smaller group (S_N2).

🎯 How to Practice

Mechanism Drawing: Draw the 3-step mechanism of Acid Catalyzed Hydration of Ethene and Dehydration of Ethanol. These are frequent board exam questions.

Acidity Comparison: Arrange these in increasing acidity: Ethanol, Phenol, p-Nitrophenol, p-Cresol. (Hint: EWG increases acidity, EDG decreases it).

Ether Cleavage Logic: Predict products of Anisole + HI and t-Butyl methyl ether + HI. Justify why the Iodine goes where it goes ($S_N1$ vs $S_N2$).

📝 Quick Revision Notes

Lucas Test (conc. HCl + ZnCl2)

$3^\circ$: Immediate Turbidity

$2^\circ$: Turbidity in 5 mins

$1^\circ$: No Turbidity at room temp

Reimer-Tiemann

Phenol + $CHCl_3$ + $NaOH$ $\to$ Salicylaldehyde (o-hydroxybenzaldehyde)

Kolbe's Reaction

Phenol + $NaOH$ + $CO_2$ $\to$ Salicylic Acid (o-hydroxybenzoic acid)

Ether + HI Cleavage

If $3^\circ$ alkyl group $\to$ $I^-$ attacks $3^\circ$.
Otherwise $\to$ $I^-$ attacks smaller alkyl group.

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