Phenols & Ethers — Preparation, Properties & Reactions

• Colourless crystalline solid, B.P. = 182°C; melts at 41°C.
• Slightly soluble in water (6 g per 100 mL at 25°C) — hydrogen bonding with water. Miscible above 66°C.
• Has a distinctive medicinal/carbolic smell; corrosive to skin.
• B.P. higher than toluene (same mol. mass) due to H-bonding, but lower than benzyl alcohol (same H-bonding but more flexible chain).

Phenol is a much stronger acid than alcohols (${\text{pK}}_a\approx 10$), but weaker than carboxylic acids (${\text{pK}}_a\approx 5$).

$$\underset{{\text{pK}}_a\approx 5}{\underbrace{\text{RCOOH}}}>\underset{{\text{pK}}_a\approx 10}{\underbrace{\text{ArOH}}}>\underset{{\text{pK}}_a=15.7}{\underbrace{{\text{H}}_2\text{O}}}>\underset{{\text{pK}}_a\approx 16\text{–}18}{\underbrace{\text{ROH}}}$$

Why is phenol more acidic than alcohols? The phenoxide ion (${\text{C}}_6{\text{H}}_5{\text{O}}^{-}$) is stabilised by resonance — the negative charge is delocalised into the aromatic ring (ortho and para positions). No such stabilisation exists for alkoxide ions.

Effect of substituents on phenol acidity

• Electron-withdrawing groups (NO₂, CN, Cl at ortho/para) — stabilise phenoxide → increase acidity
• Electron-donating groups (CH₃, OCH₃ at ortho/para) — destabilise phenoxide → decrease acidity
• p-nitrophenol > m-nitrophenol > phenol > p-methylphenol (cresol)

A. Reaction with NaOH (confirms acidity)

$${\text{C}}_6{\text{H}}_5\text{OH}+\text{NaOH}\to {\text{C}}_6{\text{H}}_5\text{ONa}+{\text{H}}_2\text{O}$$

Unlike alcohols, phenol reacts with cold dilute NaOH (not just Na metal). It does NOT react with ${\text{Na}}_2{\text{CO}}_3$ (too weak a base) — this distinguishes phenol from carboxylic acids (which DO react with ${\text{Na}}_2{\text{CO}}_3$).

B. Kolbe's Reaction (carboxylation)

$${\text{C}}_6{\text{H}}_5\text{ONa}+{\text{CO}}_2\stackrel{125°\text{C},4\text{–}7\text{atm}}{\to }\text{sodium salicylate}\stackrel{{\text{H}}^{+}}{\to }\text{salicylic acid (2-hydroxybenzoic acid)}$$

Electrophilic ${\text{CO}}_2$ attacks the electron-rich ortho/para positions of phenoxide. Product: salicylic acid (aspirin precursor).

C. Reimer-Tiemann Reaction (formylation)

$${\text{C}}_6{\text{H}}_5\text{OH}+{\text{CHCl}}_3\stackrel{\text{NaOH},\mathrm{\Delta }}{\to }\text{salicylaldehyde (2-hydroxybenzaldehyde)}+\text{NaCl}$$

Electrophile = dichlorocarbene ($:{\text{CCl}}_2$) generated from ${\text{CHCl}}_3/\text{NaOH}$. Attacks ortho position. Product is an aldehyde (hence "formylation").

D. Electrophilic Aromatic Substitution (EAS)

The $-\text{OH}$ group is a powerful ortho/para director and ring activator (lone pair donation into ring). Phenol reacts under much milder conditions than benzene:

• Bromination: ${\text{C}}_6{\text{H}}_5\text{OH}+3{\text{Br}}_2(\text{aq})\to \text{2,4,6-tribromophenol (white ppt)}+3\text{HBr}$ — no catalyst needed, aqueous bromine water, all three ortho/para positions react.
• Nitration: ${\text{C}}_6{\text{H}}_5\text{OH}+{\text{dil. HNO}}_3\to \text{o- and p-nitrophenol mixture}$ — mild conditions (no need for mixed acid).

E. Fries Rearrangement

$$\text{Phenyl ester}\stackrel{{\text{AlCl}}_3,\mathrm{\Delta }}{\to }\text{o- and p-hydroxyaryl ketone (phenolic ketone)}$$

e.g., Phenyl acetate $\stackrel{{\text{AlCl}}_3}{\to }$ o-hydroxyacetophenone + p-hydroxyacetophenone. Higher temp favours ortho product; lower temp favours para product.

F. Azo Coupling (with diazonium salt)

$${\text{ArN}}_2^{+}+{\text{C}}_6{\text{H}}_5\text{OH}\stackrel{\text{alkaline}}{\to }\text{Ar}-\text{N=N}-{\text{C}}_6{\text{H}}_4\text{OH (azo dye, orange-yellow)}$$

Electrophilic aromatic substitution at the para position of phenol (activated ring). Azo coupling requires an electron-rich ring (phenol, aniline).

G. FeCl₃ Test for Phenol

Phenol gives a violet/purple colour with neutral ${\text{FeCl}}_3$ solution (forms ferric phenoxide complex). This is a specific test to distinguish phenol from aliphatic alcohols.

Preparation — Williamson Synthesis (most important)

$$\text{R-ONa}+\text{R'-X}\to \text{R-O-R'}+\text{NaX}$$

${\text{S}}_{\text{N}}2$ mechanism — must use primary alkyl halide $\text{R'-X}$ (secondary/tertiary halides give elimination instead). The alkoxide is the nucleophile.

To make unsymmetrical ethers: choose the larger group as $\text{R}$ (from alkoxide) and the smaller primary group as $\text{R'}$ (from halide). Example: to make methylphenyl ether (anisole): ${\text{C}}_6{\text{H}}_5\text{ONa}+{\text{CH}}_3\text{I}\to {\text{C}}_6{\text{H}}_5{\text{OCH}}_3$.

Physical Properties of Ethers

• Much lower boiling points than isomeric alcohols — no $\text{O}-\text{H}$ bond, no H-bonding between ether molecules.
• Slightly soluble in water — oxygen can accept H-bonds from water (H-bond acceptor but not donor).
• Good solvents for organic reactions (especially diethyl ether) — dissolve many organic compounds, unreactive.
• Highly flammable (low boiling point, high vapour pressure).

Chemical Properties of Ethers

Ethers are generally unreactive — the C–O–C linkage is stable. Key reactions:

• Cleavage by HI or HBr (strong acid): $\text{R-O-R'}+\text{HI}\to \text{R-OH}+\text{R'I}$ (or $2\text{R-I}$ with excess HI). HI > HBr > HCl in reactivity for ether cleavage. The less sterically hindered carbon is attacked by ${\text{X}}^{-}$.
• Formation of peroxides (danger!): Diethyl ether forms explosive hydroperoxides on prolonged exposure to air and light. Always check before distilling old ether.
• Anisole (methoxybenzene): The $-{\text{OCH}}_3$ group directs EAS to ortho and para positions (ring activation). Anisole undergoes Friedel-Crafts, nitration, halogenation under milder conditions than benzene.

Comparison: Phenol vs Ether (for anisole)