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what is reimer tiemann reaction and kolbe reaction

Here’s a clear, exam‑oriented explanation of Reimer–Tiemann reaction and Kolbe reaction (Kolbe–Schmitt reaction) in simple terms.

Reimer–Tiemann Reaction

Definition (Class 11/12 level)
Reimer–Tiemann reaction is the formylation of phenol, where phenol reacts with chloroform (CHCl₃) and aqueous NaOH to give salicylaldehyde (o‑hydroxybenzaldehyde). General reaction

  • Reactants:
    Phenol + CHCl₃ + NaOH (aq), heat

  • Main product:
    o‑Hydroxybenzaldehyde (salicylaldehyde) after acidification.

Key idea (what actually happens)

  1. In strong base, chloroform forms a dichlorocarbene (:CCl₂) , which is an electrophile.
  2. Phenol in NaOH becomes phenoxide ion , which is more reactive.
  3. The dichlorocarbene attacks the ortho position of the aromatic ring.
  4. After hydrolysis and work‑up, you get salicylaldehyde (–CHO group at ortho).

So, in simple words:

Reimer–Tiemann converts phenol → o‑hydroxybenzaldehyde by using **CHCl₃

  • NaOH** and involves a carbene intermediate.

Important points to remember

  • Reagent set: CHCl₃ / NaOH (followed by acidification).
  • Substrate: Mostly phenol and substituted phenols.
  • New group introduced: –CHO (formyl group) at ortho (and small amount para).
  • Used for: Preparation of salicylaldehyde and related aromatic aldehydes.

Kolbe Reaction (Kolbe–Schmitt Reaction)

In Class 11/12 organic chemistry, when teachers say “Kolbe reaction” in the chapter Alcohols, Phenols, and Ethers , they usually mean Kolbe–Schmitt reaction with phenol , not Kolbe electrolysis. Definition
Kolbe (Kolbe–Schmitt) reaction is a carboxylation of sodium phenoxide with CO₂ under pressure , followed by acidification, to form salicylic acid (o‑hydroxybenzoic acid). General reaction

  1. Phenol + NaOH → Sodium phenoxide
  2. Sodium phenoxide + CO₂ (high pressure, 373–393 K) → Sodium salicylate
  3. Acidification (HCl or H₂SO₄) → Salicylic acid (o‑hydroxybenzoic acid)

Key idea (what actually happens)

  • Phenol is converted to phenoxide ion , which is activated for electrophilic substitution.
  • CO₂ acts as a weak electrophile and attacks the ortho position of the ring.
  • After protonation, you get o‑hydroxybenzoic acid (salicylic acid).

So, in simple words:

Kolbe reaction converts phenol → o‑hydroxybenzoic acid (salicylic acid) using NaOH + CO₂ + heat/pressure.

Important points to remember

  • Reagent set: NaOH, CO₂ (under pressure), heat, then acid.
  • Substrate: Phenol (through sodium phenoxide).
  • New group introduced: –COOH (carboxyl group) mainly at ortho.
  • Used for: Industrial preparation of salicylic acid , which is used to make aspirin.

Reimer–Tiemann vs Kolbe (quick exam table)

html

<table>
  <tr>
    <th>Feature</th>
    <th>Reimer–Tiemann Reaction</th>
    <th>Kolbe (Kolbe–Schmitt) Reaction</th>
  </tr>
  <tr>
    <td>Main substrate</td>
    <td>Phenol (phenoxide in base)</td>
    <td>Phenol (sodium phenoxide)</td>
  </tr>
  <tr>
    <td>Key reagents</td>
    <td>CHCl₃ + NaOH (aq), heat; then acidification</td>
    <td>CO₂ (high pressure) + NaOH; then acidification</td>
  </tr>
  <tr>
    <td>Type of group introduced</td>
    <td>–CHO (formyl group → aldehyde)</td>
    <td>–COOH (carboxyl group → acid)</td>
  </tr>
  <tr>
    <td>Main product from phenol</td>
    <td>o‑Hydroxybenzaldehyde (salicylaldehyde)</td>
    <td>o‑Hydroxybenzoic acid (salicylic acid)</td>
  </tr>
  <tr>
    <td>Mechanistic key species</td>
    <td>Dichlorocarbene (:CCl₂)</td>
    <td>CO₂ as electrophile; carboxylation</td>
  </tr>
  <tr>
    <td>Position of substitution</td>
    <td>Mainly ortho (some para)</td>
    <td>Mainly ortho (some para under modified conditions)</td>
  </tr>
  <tr>
    <td>Common exam chapter</td>
    <td>Alcohols, Phenols and Ethers</td>
    <td>Alcohols, Phenols and Ethers</td>
  </tr>
</table>

One‑line memory tricks

  • Reimer–Tiemann :
    “Rei mer–Tiemann gives an al-dehy-deman ” → phenol to aldehyde (–CHO).

  • Kolbe (Kolbe–Schmitt) :
    “Kolbe cooks CO₂ with phenol to make acid ” → phenol to salicylic acid (–COOH).

Bottom note:
Information gathered from public forums or data available on the internet and portrayed here.