Why are phenols more acidic than alcohols?

Phenols are significantly more acidic than alcohols primarily due to the resonance stabilization of their conjugate base, the phenoxide ion. When a phenol loses a proton, the resulting negative charge on the oxygen atom can be delocalized into the aromatic ring through resonance. This delocalization spreads out the charge, making the phenoxide ion more stable than the alkoxide ion formed from an alcohol. Alkoxide ions lack this resonance stabilization, as the negative charge remains localized on the oxygen, making them strong bases and alcohols very weak acids. The greater stability of the phenoxide ion drives the equilibrium towards proton dissociation, hence increasing acidity.

How can you distinguish between a phenol and an alcohol in the lab?

The most common and reliable laboratory test to distinguish between a phenol and an alcohol is the Ferric Chloride ($ ext{FeCl}_3$) test. Phenols react with neutral ferric chloride solution to produce a characteristic violet, blue, or green coloration due to the formation of a colored complex. Alcohols, on the other hand, do not give this test and typically show no color change. Another method is the reaction with $ ext{NaOH}$: phenols react with strong bases like $ ext{NaOH}$ to form sodium phenoxide, while alcohols generally do not. However, phenols do not react with $ ext{NaHCO}_3$, which distinguishes them from carboxylic acids.

What is the role of the $- ext{OH}$ group in electrophilic aromatic substitution reactions of phenols?

The $- ext{OH}$ group in phenols is a powerful activating group and an ortho/para director for electrophilic aromatic substitution (EAS). This is because the lone pair of electrons on the oxygen atom can be donated into the aromatic ring via resonance. This electron donation increases the electron density of the ring, particularly at the ortho and para positions, making these positions highly susceptible to attack by electrophiles. The increased electron density also stabilizes the intermediate carbocation formed during EAS, lowering the activation energy for the reaction. This strong activation means phenols react very readily with electrophiles, often leading to polysubstitution if not carefully controlled.

Explain the Kolbe's reaction and its significance.

Kolbe's reaction, also known as Kolbe-Schmidt reaction, is a crucial industrial method for synthesizing salicylic acid. In this reaction, sodium phenoxide is heated with carbon dioxide under pressure (typically 4-7 atm) at about 398 K. The carbon dioxide acts as a weak electrophile, attacking the ortho position of the highly activated phenoxide ion. Subsequent acidification of the intermediate product yields salicylic acid (o-hydroxybenzoic acid). The significance of Kolbe's reaction lies in its role as a key step in the synthesis of aspirin (acetylsalicylic acid), a widely used analgesic and anti-inflammatory drug, and other pharmaceutical compounds.

Why does phenol react with bromine water to give 2,4,6-tribromophenol, while benzene requires a Lewis acid catalyst for bromination?

Phenol reacts with bromine water to give 2,4,6-tribromophenol without a Lewis acid catalyst because the hydroxyl ($- ext{OH}$) group strongly activates the benzene ring towards electrophilic substitution. The lone pair on the oxygen atom donates electrons into the ring via resonance, significantly increasing the electron density at the ortho and para positions. This makes the ring highly nucleophilic and reactive enough to attack the relatively weak electrophile ($ ext{Br}^+$ or $ ext{Br}_2$) present in bromine water. In contrast, benzene is a less reactive aromatic system and requires a strong electrophile, which is generated by the interaction of bromine with a Lewis acid catalyst like $ ext{FeBr}_3$, to undergo bromination.

Phenols

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Phenols are a class of organic compounds characterized by a hydroxyl group ( $-\text{OH}$ ) directly attached to an aromatic hydrocarbon ring. This direct attachment to the benzene ring significantly alters the chemical and physical properties of the hydroxyl group compared to its presence in aliphatic alcohols. The aromatic ring influences the acidity of the phenolic hydroxyl group, making phenols g…

Quick Summary

Phenols are organic compounds where a hydroxyl ( $-\text{OH}$ ) group is directly attached to an aromatic ring. This direct attachment imparts unique properties, most notably enhanced acidity compared to alcohols, due to the resonance stabilization of the phenoxide ion.

They are typically colorless solids or liquids with characteristic odors, sparingly soluble in water but forming hydrogen bonds. Key preparation methods include Dow's process from haloarenes, fusion of benzene sulfonic acid with $ext{NaOH}$ , hydrolysis of diazonium salts, and the industrial cumene process.

Chemically, phenols exhibit reactions of both the $-\text{OH}$ group (e.g., esterification, ether formation, reduction to benzene with zinc dust) and the aromatic ring. The $-\text{OH}$ group is a strong activating and ortho/para directing group for electrophilic aromatic substitution, leading to facile nitration, halogenation, and sulfonation.

Important named reactions include Kolbe's reaction (forming salicylic acid) and Reimer-Tiemann reaction (forming salicylaldehyde). Phenols are widely used as antiseptics, in polymer synthesis (Bakelite), and in pharmaceuticals.

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Key Concepts

Resonance Stabilization of Phenoxide Ion

When phenol loses its proton, it forms a phenoxide ion ( $ext{C}_6\text{H}_5\text{O}^-$ ). The negative charge…

Activating and Directing Nature of

-\text{OH}

Group

The hydroxyl group is a powerful electron-donating group by resonance. The lone pair on the oxygen atom can…

Distinguishing Phenols from Alcohols: Ferric Chloride Test

The Ferric Chloride test is a qualitative test used to detect the presence of phenolic hydroxyl groups. When…

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.