Acidity & Reactions

Phenol (C₆H₅OH) is a benzene ring bearing a hydroxyl group, and it is significantly more acidic than a typical aliphatic alcohol. While ethanol has a pKa of about 16, phenol has a pKa of approximately 10. This dramatic difference arises because the phenoxide anion formed after deprotonation is stabilised by resonance: the negative charge on oxygen is delocalised into the aromatic ring across five positions, spreading electron density and lowering the energy of the anion. Understanding this resonance stabilisation is the key to predicting how substituents on the ring will further modify phenol's acidity. Substituents attached to the benzene ring exert both inductive (through-bond) and mesomeric (through-resonance) effects that either reinforce or weaken phenol's inherent acidity. Electron-withdrawing groups (EWGs) such as –NO₂, –CN, and –CHO stabilise the phenoxide anion even further, making the substituted phenol more acidic (lower pKa). Electron-donating groups (EDGs) such as –CH₃, –OCH₃, and –NH₂ destabilise the phenoxide anion by increasing electron density, making the compound less acidic (higher pKa). The position of the substituent — ortho, meta, or para — matters because mesomeric effects operate only at ortho and para positions while inductive effects act at all positions but diminish with distance. Beyond its acid–base chemistry, phenol undergoes several named reactions of great industrial and academic importance. The Reimer–Tiemann reaction introduces an aldehyde group ortho to the –OH, the Kolbe–Schmitt reaction carboxylates sodium phenoxide to produce salicylic acid (the precursor to aspirin), and zinc dust reduction converts phenol back to benzene by reductive removal of the –OH group. Together, these reactions illustrate how the activated aromatic ring and the –OH group cooperate to enable unique transformations unavailable to simple alcohols.

Key takeaways
  • Why Phenol Is More Acidic Than Alcohols
  • Effect of Electron-Withdrawing Substituents
  • Effect of Electron-Donating Substituents
  • Reimer–Tiemann Reaction
  • Kolbe–Schmitt Reaction
  • Zinc Dust Reduction

Why Phenol Is More Acidic Than Alcohols

When phenol loses a proton, the resulting phenoxide ion (C₆H₅O⁻) is stabilised by resonance. The lone pair on oxygen overlaps with the π system of the ring, delocalising the negative charge onto the ortho and para ring carbons. This delocalisation lowers the potential energy of the anion compared with an isolated alkoxide, shifting the equilibrium toward dissociation. The pKa of phenol ≈ 10 versus ethanol ≈ 16 quantifies this effect. Phenol dissolves in NaOH(aq) to form sodium phenoxide but does NOT dissolve in Na₂CO₃(aq) because carbonic acid (pKa ≈ 6.4) is a stronger acid than phenol — phenol cannot displace carbonate.

Effect of Electron-Withdrawing Substituents

Groups such as –NO₂ withdraw electron density from the ring by both inductive and mesomeric mechanisms. At the para position, –NO₂ extends the resonance system so that the negative charge on the phenoxide oxygen can be further delocalised onto the nitro group's oxygen atoms. This extra stabilisation makes 4-nitrophenol far more acidic (pKa ≈ 7.1) than phenol itself. At the meta position, the mesomeric effect is absent; only the inductive withdrawal operates, giving 3-nitrophenol a pKa ≈ 8.4, less acidic than the para isomer but still more acidic than phenol. Multiple nitro groups compound the effect: 2,4,6-trinitrophenol (picric acid) has pKa ≈ 0.4, approaching mineral acid strength.

Effect of Electron-Donating Substituents

Alkyl groups (–CH₃) and groups with lone pairs (–OCH₃, –NH₂) push electron density into the ring, which destabilises the phenoxide anion by increasing the electron density that must be accommodated on oxygen. 4-methylphenol (cresol) has pKa ≈ 10.2, slightly less acidic than phenol. 4-methoxyphenol has pKa ≈ 10.2 as well, because the –OCH₃ mesomeric donation competes. The practical rule: EWG at ortho/para → more acidic; EDG at ortho/para → less acidic; meta position effects are dominated by induction alone.

Reimer–Tiemann Reaction

When phenol is treated with chloroform (CHCl₃) in alkaline solution (NaOH, 60–70 °C), an aldehyde group is introduced predominantly at the ortho position. The mechanism proceeds via dichlorocarbene (:CCl₂), generated by base-induced α-elimination from CHCl₃. The electrophilic carbene attacks the electron-rich ortho carbon of the phenoxide anion, forming a dichloromethyl intermediate that hydrolyses to give salicylaldehyde (2-hydroxybenzaldehyde) as the major product and 4-hydroxybenzaldehyde as the minor. Reaction: C₆H₅OH + CHCl₃ + 3NaOH → 2-HO-C₆H₄-CHO + 3NaCl + 2H₂O. This is a rare example of electrophilic substitution by a neutral carbene intermediate.

Kolbe–Schmitt Reaction

Dry sodium phenoxide (C₆H₅ONa) reacts with CO₂ under pressure (4–7 atm) at about 125 °C to give sodium salicylate. On acidification, salicylic acid (2-hydroxybenzoic acid) is obtained. The reaction is an electrophilic aromatic substitution in which CO₂ acts as the electrophile, directed to the ortho position by the strongly activating –O⁻ group. The mechanism involves CO₂ coordination to the phenoxide oxygen, followed by a [3,3]-sigmatropic-like rearrangement (Fries-type intramolecular step) delivering the carboxylate to the ortho carbon. Potassium phenoxide under higher temperature (200 °C) gives para-hydroxybenzoic acid instead, illustrating how reaction conditions tune regioselectivity.

Zinc Dust Reduction

When phenol vapour is passed over heated zinc dust (Zn, ~300 °C), the hydroxyl group is reductively removed and benzene is produced: C₆H₅OH + Zn → C₆H₆ + ZnO. Zinc acts as a reducing agent, donating electrons to facilitate C–O bond cleavage. This reaction is historically important as evidence for phenol's structure and as a route to pure benzene from phenolic coal-tar fractions. It also demonstrates that the –OH group is directly attached to the ring (not a side chain), since aliphatic alcohols undergo dehydration rather than this clean reduction under similar conditions.

The Key Equation

\ceC6H5OH+CHCl3+3NaOH−>[(60−70°C)]2−HO−C6H4−CHO+3NaCl+2H2O\ce{C6H5OH + CHCl3 + 3NaOH ->[(60-70°C)] 2-HO-C6H4-CHO + 3NaCl + 2H2O}
  • \ce{C6H5OH} — Phenol (mol): The aromatic substrate; its activated ring and phenoxide form facilitate electrophilic attack.
  • \ce{CHCl3} — Chloroform (mol): Source of dichlorocarbene (:CCl₂) upon treatment with strong base.
  • \ce{:CCl2} — Dichlorocarbene (—): Reactive electrophilic intermediate that attacks the ortho position of the phenoxide ring.
  • \ce{2-HO-C6H4-CHO} — Salicylaldehyde (mol): Major ortho product; industrially used in perfumery and as a pharmaceutical intermediate.

Worked Example

Problem. Arrange the following compounds in decreasing order of acidity (most acidic first) and justify your answer: (A) Phenol, (B) 4-nitrophenol, (C) 4-methylphenol, (D) 2,4-dinitrophenol.

pKa: D⏟4.1<B⏟7.1<A⏟10.0<C⏟10.2\text{pKa: } \underbrace{D}_{4.1} < \underbrace{B}_{7.1} < \underbrace{A}_{10.0} < \underbrace{C}_{10.2}

Answer. Decreasing acidity order: 2,4-Dinitrophenol > 4-Nitrophenol > Phenol > 4-Methylphenol

Figure
Diagram
Resonance Stabilisation of Phenoxide IonNegative charge delocalised over five positionsStructure 1O⁻charge on O⇌Structure 2O⁻ortho C⇌Structure 3O⁻para C⇌Structure 4O⁻ortho CpKa Comparison of Substituted Phenols04812pKaPicricacid0.42,4-Di-NO₂-PhOH4.14-NO₂PhOH7.1PhenolPhOH10.04-CH₃PhOH10.2EthanolEtOH16Lower pKa = stronger acid. EWG at ortho/para lower pKa; EDG raise pKa relative to phenol.
Origins · 1 minHermann Kolbe and how this was discovered
HK
Hermann Kolbe
1818–1884 · German

Kolbe pioneered the synthesis of organic compounds from inorganic starting materials, famously synthesising acetic acid from its elements. His Kolbe–Schmitt reaction (1860), developed with Rudolf Schmitt, converts sodium phenoxide and CO₂ under pressure into sodium salicylate — the industrial precursor to aspirin (acetylsalicylic acid). This reaction remains one of the earliest examples of directed electrophilic carboxylation of an aromatic ring and is still used commercially today. Kolbe was famously dismissive of structural theory and wrote a scathing attack on van't Hoff's proposal of tetrahedral carbon — a theory now considered one of chemistry's greatest breakthroughs. History judged van't Hoff correct, and van't Hoff later became the first Nobel Prize laureate in Chemistry (1901).

  • Phenols: Structure and Properties — Understanding phenol's structure — sp² oxygen conjugated with the π ring — is a prerequisite for explaining its acidity and reactivity.
  • Electrophilic Aromatic Substitution (EAS) — The Reimer–Tiemann and Kolbe reactions are electrophilic aromatic substitutions; knowing the EAS mechanism clarifies how carbene and CO₂ attack the ring.
  • Resonance, Inductive, and Mesomeric Effects — Substituent effects on phenol acidity are explained entirely through these electronic effects acting on the phenoxide anion.
  • Salicylic Acid and Aspirin Synthesis — Kolbe's reaction produces salicylic acid, which is then acetylated to form aspirin — a direct industrial application of phenol chemistry.
  • Acid–Base Equilibria and pKa — Interpreting pKa values and understanding how structural features shift acid–base equilibria is central to comparing phenol acidity with alcohols and other acids.