Q.Write the equations involved in the following reactions:

(i) Reimer-Tiemann reaction — This is an electrophilic aromatic substitution where phenol is treated with chloroform in the presence of a strong base (NaOH) to introduce an aldehyde group at the ortho position. The key intermediate is a dichlorocarbene (:CCl₂), which attacks the electron-rich aromatic ring.

The overall equation is:

$\ce{C6H5OH + CHCl3 + 3NaOH ->[70^\circ C] C6H4(OH)CHO + 3NaCl + 2H2O}$ …

The Core Idea: Why Phenol is Special

Phenol ($C_6H_5OH$) is unusually reactive toward electrophilic substitution because the –OH group donates electrons into the ring by resonance. But in both reactions below, we first treat phenol with a strong base (NaOH or KOH). This deprotonates the –OH to give the phenoxide ion ($C_6H_5O^-$), which is an even stronger activating group. The negative charge on oxygen pushes electrons into the ring so powerfully that the ortho and para positions become highly nucleophilic — ready to attack even weak or unusual electrophiles.

(i) Reimer–Tiemann Reaction

What happens: Phenol is treated with chloroform (CHCl₃) in the presence of aqueous NaOH (or KOH) at about 60–70 °C. After work‑up with acid, the product is salicylaldehyde (2‑hydroxybenzaldehyde).

Step‑by‑step reasoning

1. Generation of the electrophile The base (NaOH) deprotonates chloroform to form the trichloromethyl carbanion ($:CCl_3^-$), which then loses a chloride ion to give dichlorocarbene ($:CCl_2$). This is a highly reactive, electron‑deficient species — a powerful electrophile.

2. Attack on the phenoxide ion The phenoxide ion (formed from phenol + NaOH) attacks the electrophilic carbene at the ortho position (the para position is sterically hindered by the –OH group in the transition state). This gives an intermediate dichloromethyl‑substituted phenol.

3. Hydrolysis to aldehyde The dichloromethyl group ($–CHCl_2$) is hydrolysed by the aqueous base. Two successive nucleophilic substitutions (by OH⁻) replace both chlorines with –OH groups, which then tautomerise to give the aldehyde.

4. Acidification Finally, dilute acid is added to protonate the phenoxide back to phenol, yielding salicylaldehyde.

Overall equation:

(ii) Kolbe’s Reaction (Kolbe–Schmitt Reaction)

What happens: Sodium phenoxide is heated with carbon dioxide under pressure (about 125 °C, 4–7 atm), then acidified to give salicylic acid (2‑hydroxybenzoic acid).

Step‑by‑step reasoning

1. Activation of the ring

   Phenol is first converted to sodium phenoxide ($C_6H_5ONa$) using NaOH. The phenoxide ion is a much stronger nucleophile than phenol itself.

2. Attack by CO₂

   Carbon dioxide is a weak electrophile (the carbon is partially positive due to the two electronegative oxygens). The ortho carbon of the phenoxide ion attacks the carbon of CO₂, forming a carboxylate group at the ortho position. …

Here is the clear solution method for the given reactions, following a concept-first approach.

Method: Mechanism-Based Analysis of Named Reactions

This method focuses on understanding the key reactive intermediate (the electrophile) and the substrate (the aromatic ring) to write the correct chemical equation.

Steps:

1. Identify the Substrate: Determine the starting aromatic compound (usually phenol, $C_6H_5OH$).
2. Identify the Reagents & Conditions: Note the specific chemicals and environment (e.g., $CHCl_3$/NaOH, $CO_2$/NaOH).
3. Identify the Electrophile: Determine the highly reactive species generated in situ (e.g., dichlorocarbene $:CCl_2$, carbon dioxide $CO_2$).
4. Determine the Position of Attack: Recall the directing effect of the substituent already on the ring (e.g., -OH is ortho/para directing).
5. Write the Final Product: Combine the aromatic ring with the new functional group at the correct position, accounting for any subsequent hydrolysis or acidification steps.

(i) Reimer-Tiemann Reaction

Concept: This is an electrophilic aromatic substitution where the electrophile is dichlorocarbene ($:CCl_2$) , a very reactive species. The substrate is phenol.

Step-by-Step Equation:

1. Generation of the Electrophile: Chloroform ($CHCl_3$) reacts with a strong base (NaOH) to form dichlorocarbene.

2. Attack on the Phenoxide Ion: Phenol first reacts with NaOH to form the more reactive phenoxide ion. The dichlorocarbene attacks the ortho position of the phenoxide ion.

3. Hydrolysis to Final Product: The intermediate is hydrolyzed during the work-up (acidification) to yield the final product.

Final Balanced Equation:

• Key Result: The product is Salicylaldehyde (2-hydroxybenzaldehyde). A small amount of the para isomer is also formed.

(ii) Kolbe's Reaction …

Here are the common mistakes students make with the Reimer-Tiemann and Kolbe’s reactions, along with precise corrections.

1. Confusing the Reagent (Reimer-Tiemann)

The Mistake: Students often write the wrong halogenated reagent or forget the base. They might write:

"Phenol + $\ce{CHCl3}$ → Salicylaldehyde"

Why it’s wrong: The reaction requires a strong base ($\ce{NaOH}$ or $\ce{KOH}$) to generate the reactive intermediate (:$\ce{CCl2}$, dichlorocarbene). Without the base, no reaction occurs.

How to Avoid: Memorize the reagent pair: $\ce{CHCl3}$ + $\ce{NaOH}$ (aq). Think of it as a carbene generation step first.

Correct Equation:

2. Forgetting the Ortho-Selectivity (Both Reactions)

The Mistake: Students write the product as para-substituted or as a mixture without specifying the major product.

Why it’s wrong: Both reactions are ortho-selective due to the intramolecular nature of the mechanism (the attacking group is held near the ortho position by the phenoxide ion).

How to Avoid: Always draw the –OH group and place the new group ($\ce{-CHO}$ or $\ce{-COOH}$) ortho to it. The para product is a minor byproduct (often ignored in board exams unless specified).

Key Point: The major product is Salicylaldehyde (ortho-hydroxybenzaldehyde) and Salicylic acid (ortho-hydroxybenzoic acid).

3. Writing the Wrong Product for Kolbe’s Reaction

The Mistake: Students write the product as para-hydroxybenzoic acid or forget that $\ce{CO2}$ adds directly to the ring.

Why it’s wrong: Kolbe’s reaction is an electrophilic substitution where $\ce{CO2}$ (after activation by base) acts as the electrophile. The product is salicylic acid, not benzoic acid.

How to Avoid: Remember the order of addition: Phenoxide ion → attack by $\ce{CO2}$ → ortho-substituted intermediate → acidification gives salicylic acid.

Correct Equation:

4. Mixing Up the Conditions (Temperature/Pressure)

The Mistake: Students write the same conditions for both reactions (e.g., "reflux" for both).

Why it’s wrong: Reimer-Tiemann requires aqueous base and heat (60–80°C). Kolbe’s requires high pressure (4–7 atm) and temperature (125°C) to force $\ce{CO2}$ into the reaction.

How to Avoid: Make a quick table:

| Reaction | Reagent | Conditions |

| :--- | :--- | :--- | …