Reimer–Tiemann & Kolbe's Reactions of Phenol

Why phenoxide is the reactive species

Both reactions begin by treating phenol with aqueous sodium (or potassium) hydroxide. Because phenol is weakly acidic, this generates the phenoxide ion:

$\ce{C6H5OH + NaOH -> C6H5O^{-}Na^{+} + H2O}$

The phenoxide ion is not merely a deprotonated phenol — it is a far more reactive ring. According to the NIOS account, the negative charge on oxygen is delocalised over the benzene ring, and the resonance structures place that charge specifically on the ortho and para carbons. This is exactly the delocalisation that makes phenol more acidic than alcohols, but it has a second consequence that matters here: those ortho and para carbons become electron-rich and therefore strongly nucleophilic.

In other words, the phenoxide ring is "armed" to attack an electrophile. A neutral phenol can do electrophilic substitution too, but the full negative charge of the phenoxide ion makes the ring reactive enough to engage even weak electrophiles such as carbon dioxide and dichlorocarbene. Keeping this single fact in mind — NaOH first, to make the activated phenoxide — explains the conditions of both named reactions below.

Reimer–Tiemann reaction: overview

In the Reimer–Tiemann reaction, phenol is heated with chloroform (CHCl₃) in the presence of NaOH (or KOH), and the mixture is then acidified. The product is a hydroxy aldehyde — specifically salicylaldehyde (2-hydroxybenzaldehyde, the ortho isomer) as the major product, with a smaller amount of 4-hydroxybenzaldehyde (the para isomer).

$\ce{C6H5OH + CHCl3 + 3NaOH ->[\Delta][\text{then } H^+]} \underset{\text{salicylaldehyde (major)}}{\ce{o-HO-C6H4-CHO}} + 3NaCl + 2H2O}$

The net transformation is the introduction of a –CHO group onto the ring carbon ortho to the hydroxyl. This is formally an electrophilic substitution, but the electrophile is unusual: it is the dichlorocarbene, $\ce{:CCl2}$, a neutral but electron-deficient species generated in situ from chloroform and base.

The dichlorocarbene mechanism

The mechanism explains every condition of the reaction. It proceeds in three conceptual stages: carbene generation, attack on the ring, and hydrolysis of the resulting intermediate.

Step 1 — Generation of dichlorocarbene. Chloroform has a relatively acidic C–H bond (the three electron-withdrawing chlorines stabilise the conjugate base). Hydroxide abstracts this proton to give the trichloromethyl carbanion, which then ejects a chloride ion to leave the neutral, electron-deficient dichlorocarbene:

$\ce{CHCl3 + OH^- -> {:}CCl3^- + H2O}$

$\ce{{:}CCl3^- -> {:}CCl2 + Cl^-}$

Step 2 — Attack on the phenoxide ring. The electron-rich ortho carbon of phenoxide attacks the carbene's empty orbital, forming a new C–C bond. After re-aromatisation, the ring carries a –CHCl₂ (benzal chloride) group at the ortho position.

Step 3 — Hydrolysis. The geminal dichloride is hydrolysed by the alkaline medium to a –CHO group; acidification at the end liberates the phenolic –OH, giving salicylaldehyde.

$\ce{o\text{-}HO\text{-}C6H4\text{-}CHCl2 ->[OH^-][H2O] o\text{-}HO\text{-}C6H4\text{-}CHO}$

CHCl₃ vs CCl₄: aldehyde or acid

A single change of reagent flips the product. The NIOS text states it plainly: using carbon tetrachloride in place of chloroform gives salicylic acid instead of salicylaldehyde. The logic is the count of chlorines that end up on the new carbon. Chloroform delivers a carbon bearing two chlorines (after one C–C bond forms), which hydrolyses to an aldehyde. Carbon tetrachloride, lacking a C–H to remove, instead delivers a carbon that ends up with three chlorines on the ring; that –CCl₃ hydrolyses all the way to a carboxyl group.

$\ce{C6H5OH + CCl4 ->[NaOH][\text{then } H^+] \underset{\text{salicylic acid}}{o\text{-}HO\text{-}C6H4\text{-}COOH}}$

So the same named reaction, run with a more chlorinated reagent, becomes a route to the carboxylic acid. This gives NEET a clean point of contrast that is easy to test: CHCl₃ → –CHO (salicylaldehyde); CCl₄ → –COOH (salicylic acid).

Kolbe (Kolbe–Schmitt) reaction

The Kolbe reaction, also called the Kolbe–Schmitt reaction, carboxylates phenol directly. As the NIOS module describes it, sodium phenoxide is made to absorb carbon dioxide and then heated under a pressure of CO₂ to 398 K. The product is sodium salicylate, which on acidification gives salicylic acid.

$\ce{C6H5O^{-}Na^{+} + CO2 ->[398\,K][\text{pressure}] \underset{\text{sodium salicylate}}{o\text{-}HO\text{-}C6H4\text{-}COONa}}$

$\ce{o\text{-}HO\text{-}C6H4\text{-}COONa ->[H^+] \underset{\text{salicylic acid}}{o\text{-}HO\text{-}C6H4\text{-}COOH}}$

Here CO₂ is the electrophile and the ortho carbon of the phenoxide ring is the nucleophile. Carbon dioxide on its own is a sluggish electrophile, which is precisely why the reaction needs the heavily activated phenoxide ion and applied pressure of CO₂ to drive the carboxylation forward.

Why ortho carboxylation dominates

Although the phenoxide ring is activated at both ortho and para positions, the Kolbe reaction delivers the ortho product (salicylate) as the principal outcome. The accepted explanation is that the sodium ion and the incoming CO₂ are held close together near the phenoxide oxygen, so carboxylation is directed to the adjacent ortho carbon; the proximity of the phenolate oxygen and the developing carboxylate also allows a stabilising interaction in the product. The result, salicylic acid, has its –OH and –COOH on neighbouring carbons — which is exactly what later allows intramolecular reactions in its derivatives.

For NEET it is enough to remember the destination: both the Kolbe reaction and the CCl₄-Reimer–Tiemann route arrive at the same compound, 2-hydroxybenzoic acid (salicylic acid). They differ only in the carbon source — CO₂ in Kolbe, CCl₄ in the Reimer–Tiemann variant.

From salicylic acid to aspirin

The industrial importance of these reactions lies downstream. The NIOS module notes that salicylic acid, by reaction with acetic anhydride, yields aspirin, the common pain reliever. Aspirin is acetylsalicylic acid: the phenolic –OH of salicylic acid is acetylated while the –COOH is retained.

$\ce{o\text{-}HO\text{-}C6H4\text{-}COOH + (CH3CO)2O -> \underset{\text{aspirin}}{o\text{-}CH3COO\text{-}C6H4\text{-}COOH} + CH3COOH}$

This is why phenol is listed among the feedstocks for aspirin manufacture: phenol → (Kolbe) → salicylic acid → (acetic anhydride) → aspirin. The same salicylic acid is also a precursor to dyes and other pharmaceuticals, giving these named reactions a clear real-world anchor that examiners like to reference.

Worked product prediction

Side-by-side comparison

The two reactions share a mechanism family — an activated phenoxide ring attacking a weak electrophile at the ortho position — but differ in reagent, electrophile and product. The table below isolates the points NEET tests most often.