Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

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.

2-Minute Revision

Phenols are aromatic compounds with a hydroxyl group directly attached to the benzene ring. Their physical properties include being colorless liquids or low-melting solids with a characteristic odor. They exhibit high boiling points due to strong intermolecular hydrogen bonding, making them less volatile than hydrocarbons of similar molecular mass.

Solubility in water is limited, though they are soluble in organic solvents. The most significant chemical property is their acidity, which is greater than alcohols but less than carboxylic acids, primarily due to the resonance stabilization of the phenoxide ion.

Electron-withdrawing groups enhance this acidity, while electron-donating groups diminish it. Phenols readily undergo electrophilic aromatic substitution (EAS) reactions, as the -OH group is a powerful activating and ortho-para directing group.

Key EAS reactions include nitration and halogenation, where reaction conditions dictate mono- or poly-substitution. Important named reactions are Kolbe's reaction, yielding salicylic acid, and Reimer-Tiemann reaction, producing salicylaldehyde.

Phenols can also be reduced to benzene using zinc dust and oxidize to quinones.

5-Minute Revision

Phenols are compounds featuring a hydroxyl group directly bonded to an aromatic ring. This structural feature dictates their unique physical and chemical characteristics. Physically, simple phenols are often colorless liquids or crystalline solids with a distinctive 'carbolic' odor.

Their boiling points are notably high due to extensive intermolecular hydrogen bonding, requiring more energy to overcome these attractions during phase change. For instance, *o*-nitrophenol, capable of intramolecular hydrogen bonding, is more volatile than *p*-nitrophenol, which forms intermolecular hydrogen bonds.

Phenols are sparingly soluble in water, as the polar -OH group can hydrogen bond with water, but the non-polar benzene ring limits overall miscibility. They are soluble in most organic solvents.

Chemically, phenols are acidic, a property that distinguishes them from neutral alcohols. This acidity (pKa ~10) arises from the resonance stabilization of the phenoxide ion, the conjugate base formed upon proton loss.

The negative charge on the oxygen is delocalized across the benzene ring, enhancing stability. Electron-withdrawing groups (e.g., $-\text{NO}_2$, halogens) increase acidity by further stabilizing the phenoxide ion, especially at ortho and para positions.

Conversely, electron-donating groups (e.g., $-\text{CH}_3$, $-\text{OCH}_3$) decrease acidity. Phenols react with strong bases like $\text{NaOH}$ but generally not with weaker bases like $\text{NaHCO}_3$, unlike carboxylic acids.

The -OH group is a strong activating and ortho-para directing group for electrophilic aromatic substitution (EAS) reactions. For example, nitration with dilute $\text{HNO}_3$ yields *o*- and *p*-nitrophenols, while concentrated $\text{HNO}_3$ leads to 2,4,6-trinitrophenol (picric acid).

Bromination with $\text{Br}_2/\text{CS}_2$ gives mono-bromophenols, but with $\text{Br}_2/\text{H}_2\text{O}$, 2,4,6-tribromophenol is formed due to the highly activated phenoxide ion. Sulfonation also occurs, with temperature influencing ortho/para product distribution.

Importantly, phenols generally do not undergo Friedel-Crafts reactions due to complex formation with the Lewis acid catalyst.

Two crucial named reactions are Kolbe's reaction and Reimer-Tiemann reaction. Kolbe's reaction converts sodium phenoxide to salicylic acid using $\text{CO}_2$ under pressure. Reimer-Tiemann reaction transforms phenol into salicylaldehyde using $\text{CHCl}_3$ and $\text{NaOH}$.

Phenols can also be reduced to benzene by heating with zinc dust and are susceptible to oxidation, forming quinones upon exposure to air or strong oxidizing agents. These properties make phenols versatile intermediates in organic synthesis.

Prelims Revision Notes

Physical Properties

State — Colorless liquids or low-melting crystalline solids. Turn colored on exposure to air/light due to oxidation.
Odor — Characteristic 'carbolic' odor.
Boiling Point — High due to strong intermolecular hydrogen bonding. Higher than hydrocarbons, ethers, haloarenes of similar MW.

* *o*-Nitrophenol (intramolecular H-bond) is more volatile (lower BP) than *p*-nitrophenol (intermolecular H-bond).

Solubility — Sparingly soluble in water (H-bonding with water), soluble in organic solvents. Solubility decreases with increasing size of non-polar part.

Chemical Properties

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Acidity — Phenols are acidic (pKa ~10).

* Reason: Resonance stabilization of phenoxide ion (conjugate base). * Order: Alcohols < Phenols < Carboxylic Acids. * Effect of Substituents: * EWG (e.g., -NO$_2$, -CN, -CHO, -COOH, -X): Increase acidity (stabilize phenoxide ion).

Effect strongest at ortho/para. * EDG (e.g., -CH$_3$, -OCH$_3$, -NH$_2$): Decrease acidity (destabilize phenoxide ion). * Reactions: React with active metals (Na) to liberate H$_2$. React with strong bases (NaOH) to form phenoxides.

Do NOT react with weak bases (NaHCO$_3$).

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Electrophilic Aromatic Substitution (EAS) — OH is a strong activating and ortho-para directing group (+M effect).

* Halogenation (Bromination): * $\text{Br}_2/\text{CS}_2$ (low temp): Mono-substitution (o- & p-bromophenol). * $\text{Br}_2/\text{H}_2\text{O}$: Tri-substitution (2,4,6-tribromophenol, white ppt).

Phenoxide ion is highly activated. * Nitration: * Dilute $\text{HNO}_3$: o- & p-nitrophenol. * Conc. $\text{HNO}_3$/Conc. $\text{H}_2\text{SO}_4$: 2,4,6-trinitrophenol (Picric Acid). * Sulphonation: Conc.

$\text{H}_2\text{SO}_4$. * Low temp: *o*-phenolsulfonic acid. * High temp: *p*-phenolsulfonic acid. * Friedel-Crafts: Generally NOT feasible due to complex formation with Lewis acid catalyst ($\text{AlCl}_3$).

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Kolbe's Reaction — Phenol $\xrightarrow{\text{1. NaOH, CO}_2, \text{Heat}; \text{2. H}^+}$ Salicylic Acid (o-hydroxybenzoic acid).

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Reimer-Tiemann Reaction — Phenol $\xrightarrow{\text{1. CHCl}_3, \text{NaOH}; \text{2. H}^+}$ Salicylaldehyde (o-hydroxybenzaldehyde). Active electrophile: dichlorocarbene (:\text{CCl}_2).

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Reduction — Phenol + Zn dust $\xrightarrow{\text{Heat}}$ Benzene + ZnO.

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Oxidation — Phenol $\xrightarrow{\text{Na}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4}$ p-Benzoquinone. Turns colored on air exposure.

1
Coupling Reaction — Phenol + Arenediazonium salt $\xrightarrow{\text{Weakly alkaline}}$ Azo dye (colored product).

Vyyuha Quick Recall

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).