CHEMCA
EXAM MASTER REVIEW SHEET
Organic Chemistry: Phenols
1. Structure & Acidic Nature
Phenols ($Ar-OH$) are much more acidic than alcohols due to the resonance stabilization of the phenoxide ion.
The phenoxide ion ($C_6H_5O^-$) is stabilized by resonance, spreading the negative charge over the benzene ring.
Acidity: Phenol > $H_2O$ > Alcohol
- • EWG ($-NO_2, -CN$): Increase acidity (especially at O/P).
- • ERG ($-CH_3, -OCH_3$): Decrease acidity.
Picric acid (2,4,6-trinitrophenol) is more acidic than most carboxylic acids!
2. Methods of Preparation
From Cumene (Industrial)
1. Oxidation of Cumene (Isopropylbenzene) to Cumene Hydroperoxide.
2. Acid hydrolysis yields Phenol + Acetone (useful byproduct).
Other Methods
• From Benzene Sulphonic Acid: Fusion with $NaOH$ at 573K.
• From Diazonium Salts: Warming BDC with water.
• Dow's Process: From Chlorobenzene (requires high $T$ & $P$).
3. Electrophilic Aromatic Substitution
The $-OH$ group is highly activating and Ortho-Para directing.
| Reaction | Reagents | Major Product |
|---|---|---|
| Nitration (dilute) | dil. $HNO_3$, 298K | o-Nitrophenol (Intra H-bond) & p-Nitrophenol |
| Nitration (conc) | conc. $HNO_3 + H_2SO_4$ | Picric Acid (2,4,6-trinitrophenol) |
| Bromination ($H_2O$) | $Br_2 / H_2O$ (Polar) | 2,4,6-Tribromophenol (White Ppt) |
| Bromination ($CS_2$) | $Br_2 / CS_2$, 273K (Non-polar) | p-Bromophenol |
4. Essential Named Reactions
Sodium Phenoxide + $CO_2$ (4-7 atm, 400K) $\xrightarrow{H^+}$ Salicylic Acid.
Used in Aspirin synthesis.
Phenol + $CHCl_3 + aq. NaOH \xrightarrow{H^+}$ Salicylaldehyde.
Intermediate: Dichlorocarbene ($:CCl_2$).
Reaction with Zinc Dust
$C_6H_5OH + Zn \xrightarrow{\Delta} C_6H_6 + ZnO$
Used to convert Phenol back to Benzene.
5. Identification Tests for Phenols
Ferric Chloride Test
Phenol + Neutral $FeCl_3 \rightarrow$ Violet/Purple Coloration.
*Alcohols do not give this test.*
Liebermann's Nitroso Test
Phenol + $NaNO_2 + H_2SO_4 \rightarrow$ Red color $\xrightarrow{NaOH}$ Deep Blue color.
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