Why are phenols acidic, unlike alcohols?

Phenols are acidic because the phenoxide ion, formed after losing a proton, is stabilized by resonance. The negative charge on the oxygen atom can be delocalized over the entire benzene ring, spreading out the charge and making the conjugate base more stable. In contrast, for alcohols, the alkoxide ion formed after proton loss has its negative charge localized solely on the oxygen, making it less stable and thus alcohols are much weaker acids, essentially neutral.

How do electron-withdrawing groups affect the acidity of phenols?

Electron-withdrawing groups (EWGs) like $-\text{NO}_2$ or halogens increase the acidity of phenols. They do this by further stabilizing the phenoxide ion. EWGs pull electron density away from the oxygen, either through resonance (like $-\text{NO}_2$ at ortho/para positions) or inductive effect, which helps to delocalize and disperse the negative charge on the phenoxide ion, making it even more stable and thus making the parent phenol a stronger acid.

Why does phenol give 2,4,6-tribromophenol with bromine water but only monobromophenol with $\text{Br}_2/\text{CS}_2$?

The difference lies in the solvent. In aqueous solution, phenol ionizes slightly to form the phenoxide ion ($\text{C}_6\text{H}_5\text{O}^-$). The phenoxide ion is much more highly activated towards electrophilic substitution than neutral phenol due to the stronger electron-donating resonance effect of the negatively charged oxygen. This extreme activation leads to rapid tri-substitution. In contrast, in a non-polar solvent like $\text{CS}_2$, phenol does not ionize, and its activating effect is moderated, leading to mono-substitution.

What are Kolbe's reaction and Reimer-Tiemann reaction, and what are their products?

Kolbe's reaction involves treating sodium phenoxide with carbon dioxide under pressure, followed by acidification, to yield salicylic acid (o-hydroxybenzoic acid). It's a key method for synthesizing aspirin precursors. Reimer-Tiemann reaction involves treating phenol with chloroform in the presence of an aqueous alkali, followed by hydrolysis and acidification, to produce salicylaldehyde (o-hydroxybenzaldehyde). This reaction introduces an aldehyde group at the ortho position.

Can phenols undergo Friedel-Crafts reactions?

Generally, phenols do not undergo Friedel-Crafts alkylation or acylation reactions under typical conditions. The hydroxyl group of phenol coordinates with the Lewis acid catalyst (e.g., $\text{AlCl}_3$), forming a complex. This complex deactivates the benzene ring towards electrophilic attack and can also lead to side reactions or decomposition of the catalyst. Therefore, Friedel-Crafts reactions are not a practical method for synthesizing alkylated or acylated phenols.

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Version 1Updated 22 Mar 2026

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Phenols are organic compounds characterized by a hydroxyl group (-OH) directly attached to an aromatic hydrocarbon group. This direct attachment significantly influences both their physical and chemical properties, distinguishing them markedly from aliphatic alcohols. The delocalization of the lone pair electrons of the oxygen atom into the aromatic ring via resonance imparts unique characteristic…

Quick Summary

Phenols are organic compounds where a hydroxyl group is directly attached to an aromatic ring. Physically, they are typically colorless liquids or low-melting solids with a characteristic odor. Their high boiling points are due to strong intermolecular hydrogen bonding.

They are sparingly soluble in water but soluble in organic solvents. Chemically, their most defining feature is their acidity, which is significantly greater than alcohols but less than carboxylic acids.

This acidity arises from the resonance stabilization of the phenoxide ion. Electron-withdrawing groups increase acidity, while electron-donating groups decrease it. The -OH group is a strong activating and ortho-para directing group for electrophilic aromatic substitution reactions like nitration, halogenation, and sulfonation.

Important named reactions include Kolbe's reaction (forming salicylic acid) and Reimer-Tiemann reaction (forming salicylaldehyde). Phenols can also be reduced to benzene with zinc dust and undergo oxidation to quinones.

They are crucial intermediates in the synthesis of pharmaceuticals, dyes, and polymers.

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Key Concepts

Comparative Acidity of Phenols

Phenols are more acidic than alcohols but less acidic than carboxylic acids. This hierarchy is crucial for…

Effect of Substituents on Phenol Acidity

Substituents on the benzene ring significantly alter phenol's acidity. Electron-withdrawing groups (EWGs)…

Electrophilic Aromatic Substitution (EAS) Directing Effect of -OH

The hydroxyl group (-OH) is a powerful activating group and an ortho-para director in EAS reactions. This is…

Structure — OH directly on benzene ring.

Acidity — Phenol > Alcohol, Phenol < Carboxylic Acid. Stabilized phenoxide ion.

Substituents — EWG (e.g., -NO

_2

) increases acidity; EDG (e.g., -CH

_3

) decreases acidity.

Boiling Point — High due to intermolecular H-bonding.

Solubility — Sparingly soluble in water.

EAS — OH is ortho-para directing & activating.

- Bromination: $\text{Br}_2/\text{CS}_2 \rightarrow$ mono-Br; $\text{Br}_2/\text{H}_2\text{O} \rightarrow$ 2,4,6-tri-Br. - Nitration: Dil. $\text{HNO}_3 \rightarrow$ o/p-nitro; Conc. $\text{HNO}_3 \rightarrow$ 2,4,6-trinitro (Picric Acid).

Kolbe's Reaction — Phenol

\xrightarrow{\text{1. NaOH, CO}_2, \text{Heat}; \text{2. H}^+}

Salicylic Acid.

Reimer-Tiemann Reaction — Phenol

\xrightarrow{\text{1. CHCl}_3, \text{NaOH}; \text{2. H}^+}

Salicylaldehyde.

Reduction — Phenol + Zn dust

\xrightarrow{\text{Heat}}

Benzene.

Oxidation — Phenol

\xrightarrow{\text{Oxidizing agent}}

Quinone.

Phenols Are Cool, Especially Named Hydroxyls:

Phenols Are Cool: Acidity, Colbe's, Reimer-Tiemann.

Especially Named Hydroxyls: EAS (Electrophilic Aromatic Substitution), Nitration, Halogenation.

For Acidity order: Alcohol < Phenol < Carboxylic Acid (APC - A Primary Class). For Bromination: CS $_2$ for Single; Water for Whole (tri-substitution).

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