Phenols — Revision Notes

⚡ 30-Second Revision

Definition: — $-\text{OH}$ group directly attached to an aromatic ring.
Acidity Order: — Carboxylic Acids > Phenols > Water > Alcohols.
Reason for Acidity: — Resonance stabilization of phenoxide ion.
$ ext{FeCl}_3$ Test: — Phenols give characteristic color (violet/blue/green).
EAS: — $-\text{OH}$ is strong activating, ortho/para director.

- Bromination: Phenol + $ext{Br}_2(\text{aq})$ $ightarrow$ 2,4,6-Tribromophenol. - Nitration: Phenol + dil. $ext{HNO}_3$ $ightarrow$ o/p-nitrophenol; conc. $ext{HNO}_3$ $ightarrow$ Picric acid.

Kolbe's Reaction: — Phenol $xrightarrow{\text{1. NaOH, 2. CO}_2, \text{heat/pressure, 3. H}^+}$ Salicylic acid.
Reimer-Tiemann Reaction: — Phenol $xrightarrow{\text{1. CHCl}_3, \text{NaOH, 340 K, 2. H}^+}$ Salicylaldehyde.
Reduction: — Phenol + $ext{Zn}$ dust $xrightarrow{\text{heat}}$ Benzene.
Oxidation: — Phenol $xrightarrow{\text{mild oxidation}}$ Benzoquinone.

2-Minute Revision

Phenols are aromatic compounds with a hydroxyl group directly bonded to the benzene ring. Their defining characteristic is their enhanced acidity compared to alcohols, attributed to the resonance stabilization of the phenoxide ion.

This means they react with strong bases like $ext{NaOH}$ but not with weaker bases like $ext{NaHCO}_3$. A crucial distinguishing test is the neutral ferric chloride test, where phenols produce a characteristic color.

The $-\text{OH}$ group is a potent activator and an ortho/para director for electrophilic aromatic substitution (EAS), leading to facile reactions like nitration and bromination, often resulting in polysubstitution (e.

g., 2,4,6-tribromophenol with bromine water). Key named reactions include Kolbe's reaction, which yields salicylic acid from sodium phenoxide and carbon dioxide, and Reimer-Tiemann reaction, which forms salicylaldehyde from phenol, chloroform, and $ext{NaOH}$.

Phenols can be prepared industrially from cumene or from benzene sulfonic acid, and reduced to benzene using zinc dust. Remember the acidity order: Carboxylic Acids > Phenols > Water > Alcohols.

5-Minute Revision

Phenols are organic compounds featuring a hydroxyl group ($-\text{OH}$) directly attached to an aromatic ring. This structural feature is key to their unique properties. Their most significant property is their acidity, which is greater than that of alcohols and water but less than carboxylic acids.

This enhanced acidity arises from the resonance stabilization of the phenoxide ion, where the negative charge on the oxygen is delocalized into the aromatic ring. Electron-withdrawing groups on the ring increase acidity, while electron-donating groups decrease it.

For example, p-nitrophenol is more acidic than phenol, while p-cresol is less acidic.

Preparation methods include the industrial cumene process (phenol and acetone from cumene hydroperoxide), Dow's process (from chlorobenzene), from benzene sulfonic acid, and hydrolysis of diazonium salts.

Chemical reactions can be categorized:

1
Reactions of the $- ext{OH}$ group: — Acidic nature (reacts with $ext{NaOH}$), esterification (with acid chlorides/anhydrides), ether formation (Williamson synthesis with phenoxide and primary alkyl halide), and reduction to benzene with zinc dust.
2
Electrophilic Aromatic Substitution (EAS): — The $-\text{OH}$ group is a strong activating and ortho/para directing group. This leads to:

* Nitration: Dilute $ext{HNO}_3$ gives o/p-nitrophenol; concentrated $ext{HNO}_3$ gives 2,4,6-trinitrophenol (picric acid). * Halogenation: Bromine water gives 2,4,6-tribromophenol. Controlled monobromination requires non-polar solvents. * Sulfonation: Forms o- or p-phenolsulfonic acid depending on temperature.

1
Named Reactions:

* Kolbe's Reaction: Sodium phenoxide + $ext{CO}_2$ (heat, pressure) $ightarrow$ Salicylic acid (o-hydroxybenzoic acid). * Reimer-Tiemann Reaction: Phenol + $ext{CHCl}_3$ + $ext{NaOH}$ (340 K) $ightarrow$ Salicylaldehyde (o-hydroxybenzaldehyde). The electrophile is dichlorocarbene ($ext{CCl}_2$). * Coupling Reaction: Phenol + Arenediazonium salt $ightarrow$ Azo dyes.

Distinguishing Test: Phenols give a characteristic color (violet, blue, green) with neutral ferric chloride ($ext{FeCl}_3$) solution, while alcohols do not. Remember this key test for NEET. Be wary of common misconceptions, especially regarding acidity comparisons and the conditions for mono- vs. polysubstitution in EAS.

Prelims Revision Notes

Phenols are compounds with a hydroxyl ($-\text{OH}$) group directly attached to an aromatic ring. This direct attachment is critical. The carbon atom bearing the $-\text{OH}$ group is $sp^2$ hybridized.

Phenols are more acidic than alcohols and water but less acidic than carboxylic acids. This enhanced acidity is due to the resonance stabilization of the phenoxide ion, where the negative charge on the oxygen is delocalized into the aromatic ring, particularly at the ortho and para positions.

Electron-withdrawing groups (e.g., $-\text{NO}_2$) at ortho/para positions increase acidity, while electron-donating groups (e.g., $-\text{CH}_3$) decrease it. Phenols react with $ext{NaOH}$ but not $ext{NaHCO}_3$.

Preparation Methods:

1
From Haloarenes (Dow's Process): — Chlorobenzene $xrightarrow{\text{NaOH, 623 K, 300 atm}}$ Sodium phenoxide $xrightarrow{\text{H}^+}$ Phenol.
2
From Benzene Sulphonic Acid: — Benzene $xrightarrow{\text{oleum}}$ Benzene sulfonic acid $xrightarrow{\text{NaOH, heat}}$ Sodium phenoxide $xrightarrow{\text{H}^+}$ Phenol.
3
From Diazonium Salts: — Arenediazonium salt $xrightarrow{\text{H}_2\text{O, heat}}$ Phenol.
4
From Cumene (Industrial): — Cumene $xrightarrow{\text{O}_2}$ Cumene hydroperoxide $xrightarrow{\text{H}^+/\text{H}_2\text{O}}$ Phenol + Acetone.

Reactions:

Acidity: — Reacts with active metals (Na) to liberate $ext{H}_2$. Reacts with $ext{NaOH}$.
Esterification: — Phenol + Acid chloride/anhydride $xrightarrow{\text{Pyridine}}$ Ester.
Ether Formation (Williamson): — Phenoxide + Primary alkyl halide $ightarrow$ Ether.
Reduction: — Phenol + $ext{Zn}$ dust $xrightarrow{\text{heat}}$ Benzene.
Oxidation: — Phenol $xrightarrow{\text{Na}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4}$ Benzoquinone.
Electrophilic Aromatic Substitution (EAS): — $-\text{OH}$ is strong activating, ortho/para directing.

* Nitration: Dilute $ext{HNO}_3$ $ightarrow$ o- and p-nitrophenol. Concentrated $ext{HNO}_3/\text{H}_2\text{SO}_4$ $ightarrow$ 2,4,6-Trinitrophenol (Picric acid). * Bromination: $ext{Br}_2(\text{aq})$ $ightarrow$ 2,4,6-Tribromophenol (white ppt). $ext{Br}_2/\text{CS}_2$ (low temp) $ightarrow$ o- and p-bromophenol. * Sulfonation: Conc. $ext{H}_2\text{SO}_4$ (298 K) $ightarrow$ o-phenolsulfonic acid. Conc. $ext{H}_2\text{SO}_4$ (373 K) $ightarrow$ p-phenolsulfonic acid.

Named Reactions:

* Kolbe's Reaction: Sodium phenoxide + $ext{CO}_2$ (398 K, 4-7 atm) $xrightarrow{\text{H}^+}$ Salicylic acid. * Reimer-Tiemann Reaction: Phenol + $ext{CHCl}_3$ + $ext{NaOH}$ (340 K) $xrightarrow{\text{H}^+}$ Salicylaldehyde. Electrophile: Dichlorocarbene ($ext{CCl}_2$). * Coupling Reaction: Phenol + Arenediazonium salt $xrightarrow{\text{alkaline}}$ Azo dye.

Distinguishing Test: Neutral $ext{FeCl}_3$ test: Phenols give characteristic color (violet/blue/green). Alcohols do not.

Vyyuha Quick Recall

Phenols Are Really Acidic, Ortho-Para Directing, Kolbe's Reaction, Reimer-Tiemann, Ferric Chloride Test. (PARA-OPD KRT FC)