Reactions & Acidity of Alcohols & Phenols
Decode the dual reactivity of the Hydroxyl group. Master the Acidity order, the Lucas Test kinetics, PCC vs $KMnO_4$ oxidations, and the iconic Reimer-Tiemann mechanism.
Module Focus: Bond Cleavage Duality
Alcohols can act as both nucleophiles and electrophiles depending on the bond broken. When acting as a nucleophile, the O-H bond breaks (e.g., displaying acidic nature or forming esters). When acting as an electrophile (after protonation), the C-O bond breaks (e.g., reacting with HX in the Lucas test). Understanding which bond breaks explains the complete reversal of reactivity orders between primary and tertiary alcohols.
1. Acidity of Alcohols and Phenols
The acidic character relies on the stability of the conjugate base formed after releasing $H^+$.
Alkyl groups exert a strong $+I$ (electron-donating) effect. They pump electron density onto the alkoxide ion ($R-O^-$), heavily destabilizing it.
Acidity Order: $1^\circ > 2^\circ > 3^\circ$
Trap: Water ($H_2O$) is actually MORE ACIDIC than most alcohols (except methanol)!
Phenols are a million times more acidic than alcohols. The Phenoxide ion ($C_6H_5O^-$) is massively stabilized by the delocalization of the negative charge across the aromatic ring via resonance.
Acidity: Phenols $\gg$ Water $>$ Alcohols
- EWG ($-NO_2, -CN, -X$): Electron-withdrawing groups stabilize the phenoxide ion by dispersing the negative charge. They INCREASE acidity. The effect is maximum at ortho and para positions.
- EDG ($-CH_3, -OCH_3$): Electron-donating groups destabilize the phenoxide ion by intensifying the negative charge. They DECREASE acidity.
2. Reactions of Alcohols (C-O Bond Cleavage)
Reactions involving the cleavage of the C-O bond proceed via a carbocation intermediate. Therefore, the reactivity order is exactly reversed: $3^\circ > 2^\circ > 1^\circ$.
Reacting alcohols with Lucas Reagent (Conc. $HCl$ + Anhydrous $ZnCl_2$) forms insoluble alkyl chlorides, which appear as a white turbidity (cloudiness).
Forms highly stable $3^\circ$ carbocation.
Turbidity appears IMMEDIATELY.
Forms moderately stable $2^\circ$ carbocation.
Turbidity appears after ~5 minutes.
Highly unstable $1^\circ$ carbocation.
NO turbidity at room temp. Requires heating.
Alcohols react with $PCl_5$, $PCl_3$, and $SOCl_2$ (Thionyl chloride) to form alkyl chlorides. As seen in Haloalkanes, reacting with $SOCl_2$ is the best method because the by-products ($SO_2$ and $HCl$) are escapable gases.
3. Oxidation & Dehydrogenation of Alcohols
Oxidation essentially involves the removal of hydrogen from the carbon bearing the $-OH$ group (the $\alpha$-carbon) and the oxygen itself, forming a $C=O$ double bond.
| Alcohol Type | Mild Oxidation (PCC or $CrO_3$) | Strong Oxidation ($KMnO_4$, $K_2Cr_2O_7$) | Hot Copper Tube ($Cu/573\text{K}$) |
|---|---|---|---|
| Primary ($1^\circ$) | Aldehyde | Carboxylic Acid | Aldehyde (Dehydrogenation) |
| Secondary ($2^\circ$) | Ketone | Ketone | Ketone (Dehydrogenation) |
| Tertiary ($3^\circ$) | No Reaction | No Reaction (Under normal conditions) | ALKENE (Dehydration) |
4. Specific Reactions of Phenols
The $-OH$ group in phenol is highly activating and ortho/para directing. Electrophilic Aromatic Substitution (EAS) occurs much faster than in benzene.
A. The Solvent Effect in Bromination
In a highly polar solvent like water, phenol ionizes significantly to form the phenoxide ion, which activates the ring immensely.
(White Precipitate)
In a non-polar solvent at low temperatures, ionization is suppressed. The ring is less activated.
(p-Bromophenol is major)
B. Name Reactions (Extremely High Yield)
Treating phenol with Chloroform ($CHCl_3$) and aqueous NaOH, followed by acidification.
(o-Hydroxybenzaldehyde)
Treating Sodium phenoxide with Carbon Dioxide ($CO_2$) under high pressure and temperature, followed by acidification.
(2-Hydroxybenzoic acid)
C. Oxidation with Chromic Acid ($Na_2Cr_2O_7 / H_2SO_4$)
Phenol undergoes vigorous oxidation with strong oxidizing agents to form a conjugated diketone.
NEET Grand Test: Alcohol & Phenol Rxns
15 High-Yield Questions testing acidity orders, Reimer-Tiemann intermediates, and oxidation anomalies.
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