Complete Guide to Phenol: Synthesis, Properties & Reactions
Phenol ($\text{C}_6\text{H}_5\text{OH}$), commonly known as Carbolic Acid, is a cornerstone of aromatic chemistry. Its unique acidity and highly activating hydroxyl group make it a favorite topic for JEE Main, Advanced, NEET, and Board exams.
1. Preparation of Phenol
Phenol is synthesized through several industrial and laboratory methods, with the Cumene process being the most significant industrial route.
Key Synthesis Highlights
- Cumene Process: Aerial oxidation of Cumene followed by acid hydrolysis (yields Acetone as a valuable by-product).
- Dow's Process: Chlorobenzene reacts with $\text{NaOH}$ at high temperature and pressure ($623\text{ K}$, $300\text{ atm}$).
- From Benzene Sulphonic Acid: Fusing Sodium Benzene Sulphonate with molten $\text{NaOH}$.
- From Diazonium Salts: Warming aqueous solution of Benzene Diazonium Chloride.
2. Physical Properties of Phenol
The physical behavior of phenol is dominated by the presence of the $-\text{OH}$ group, which allows for extensive intermolecular hydrogen bonding.
High-Yield Trends
- Boiling Point: Significantly higher than corresponding hydrocarbons/haloarenes due to intermolecular H-bonding.
- Solubility: Moderately soluble in water due to H-bonding, but solubility increases at higher temperatures.
- Acidity: Phenol is more acidic than alcohols but less acidic than carboxylic acids. (Phenoxide ion is resonance stabilized).
3. Chemical Reactions of Phenol
Phenol undergoes Electrophilic Aromatic Substitution (EAS) at exceptionally high rates due to the strong $+M$ effect of the hydroxyl group.
Must-Know Named Reactions
- Reimer-Tiemann Reaction: Phenol + $\text{CHCl}_3$ + aq. $\text{NaOH} \rightarrow$ Salicylaldehyde.
- Kolbe’s Reaction: Phenol + $\text{NaOH} + \text{CO}_2 \rightarrow$ Salicylic acid.
- Nitration: Dilute $\text{HNO}_3$ gives $o$- and $p$-nitrophenol; Conc. $\text{HNO}_3$ gives Picric Acid ($2,4,6\text{-trinitrophenol}$).
- Bromination: $\text{Br}_2/\text{CS}_2$ (low polarity) gives mono-substituted products; $\text{Br}_2/\text{H}_2\text{O}$ gives white ppt of $2,4,6\text{-tribromophenol}$.
- Reaction with Zn dust: Reduction to Benzene.
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