10.3 Acidity of Water, Phenol, and Ethanol

This section explores the acidity of phenols compared to water and ethanol, and details their characteristic reactions including those of related compounds like naphthols.

Acidity refers to the ability of a compound to donate a proton ( $H^{+}$ ), which is directly related to the stability of the resulting conjugate base.

Water ( $H_{2} O$ ): Water is a very weak acid. Its autoionization constant ( $K_{w}$ ) is $1 \times 1 0^{- 14}$ , and its acid dissociation constant ( $K_{a}$ ) is approximately $1.8 \times 1 0^{- 16}$ .

Phenol ( $C_{6} H_{5} O H$ ): Phenol is a stronger acid than water, with a $K_{a}$ of $1.3 \times 1 0^{- 10}$ . This increased acidity is due to the stability of its conjugate base, the phenoxide ion ( $C_{6} H_{5} O^{-}$ ).

Resonance Stabilization of the Phenoxide Ion

When phenol loses a proton, the resulting negative charge on the oxygen atom is not localized. Instead, it is delocalized (spread out) into the pi-system of the benzene ring through resonance. This distribution of charge stabilizes the phenoxide ion, making the formation of the ion (and thus, the donation of the proton) more favorable compared to water or ethanol.

Dissociation of Phenol: $C_{6} H_{5} O H ⇌ C_{6} H_{5} O^{-} + H^{+}$

Resonance Structures of Phenoxide Ion:

Figure 10.3.1 — Resonance structures showing delocalization of negative charge in the phenoxide ion.

Compound	Formula	$K_{a}$ Value	Relative Acidity	Reason for Stability of Conjugate Base
Phenol	$C_{6} H_{5} O H$	$1.3 \times 1 0^{- 10}$	Weakly Acidic	Resonance stabilization of phenoxide ion
Water	$H_{2} O$	$1.8 \times 1 0^{- 16}$	Very Weak Acid	No resonance stabilization for $O H^{-}$
Ethanol	$C_{2} H_{5} O H$	$1.3 \times 1 0^{- 16}$	Least Acidic	Electron donating ethyl group destabilizes ethoxide

Reactions of Phenol

Phenol undergoes several characteristic reactions due to the activating effect of the hydroxyl group.

Reaction with Sodium Hydroxide

Phenol reacts with sodium hydroxide to form sodium phenoxide (sodium phenolate), a salt.

$C_{6} H_{5} OH + NaOH \to C_{6} H_{5} ONa + H_{2} O$

Reaction with Diazonium Salt (Azo Coupling)

This reaction forms azo-dyes, which are colored synthetic compounds containing an azo group ( $- N = N -$ ) that links two aromatic rings.

Procedure:

Phenol is dissolved in a sodium hydroxide solution, forming the highly reactive sodium phenoxide ion.
The solution is cooled in ice to about 0-5 °C.
A cold solution of benzene diazonium chloride is added slowly.

Observation: A yellow-orange solution or precipitate is formed.

Product: The product is an azo compound, such as p-hydroxyazobenzene.

Overall Reaction:

$C_{6} H_{5} OH + C_{6} H_{5} N_{2} Cl 0 - 5°C NaOH p - HO C_{6} H_{4} N_{2} C_{6} H_{5} + HCl$

Mechanism:

Figure 10.3.2 — Mechanism of azo coupling reaction between phenol and benzene diazonium chloride.

10.5 Reactions of Naphthols

Naphthols are derivatives of naphthalene containing a hydroxyl group. They behave chemically like simple phenols.

1. Reaction with Sodium Hydroxide

Like phenols, naphthols are acidic and react with strong bases to form sodium naphthoxide salts.

1-Naphthol (α-naphthol):

Figure 10.5.1 — Reaction of 1-naphthol with sodium hydroxide forming sodium 1-naphthoxide.

2-Naphthol (β-naphthol):

Figure 10.5.2 — Reaction of 2-naphthol with sodium hydroxide forming sodium 2-naphthoxide.

2. Reaction with Bromine Water (Bromination)

Naphthols undergo electrophilic substitution and decolorize bromine water. The position of substitution depends on the isomer.

1-Naphthol: Substitution occurs at the activated 2- and 4-positions.

Figure 10.5.3 — Bromination of 1-naphthol at positions 2 and 4.

2-Naphthol: Substitution occurs at position 1, which is adjacent to the -OH group. Position 3 is less reactive, and other positions are sterically hindered by the fused ring.

Figure 10.5.4 — Bromination of 2-naphthol at position 1.

3. Reaction with Nitric Acid (Nitration)

Nitration occurs similarly to bromination.

1-Naphthol: Reaction with nitric acid yields a mixture of 2-nitro-1-naphthol and 4-nitro-1-naphthol.

Figure 10.5.5 — Nitration of 1-naphthol yielding 2-nitro and 4-nitro products.

4. Reaction with Diazonium Chloride (Azo Coupling)

Naphthols react with diazonium salts to give intensely colored azo compounds. The reaction is performed under the same cold, basic conditions as with phenol.

1-Naphthol: The diazonium salt attacks at position 4.

Figure 10.5.6 — Azo coupling of 1-naphthol at position 4.

2-Naphthol: The diazonium salt attacks at position 1.

Figure 10.5.7 — Azo coupling of 2-naphthol at position 1.

This section explores the acidity of phenols compared to water and ethanol, and details their characteristic reactions including those of related compounds like naphthols.