Write the reaction involved in the following: (3 × 1)
(a) Reimer-Tiemann reaction
(b) Kolbe's reaction
(c) Friedel-Crafts acylation of anisole
Write the reaction involved in the following: (3 × 1)
(a) Reimer-Tiemann reaction
(b) Kolbe's reaction
(c) Friedel-Crafts acylation of anisole
(a) Reimer–Tiemann reaction:
Phenol is treated with CHCl₃ and aqueous NaOH at ~340 K; the intermediate substituted benzal chloride is hydrolysed to give mainly salicylaldehyde (2-hydroxybenzaldehyde).
- C₆H₅OH + CHCl₃ + 3NaOH →(340 K)→ o-C₆H₄(OH)(CHO) + 3NaCl + 2H₂O
- (An electrophilic dichlorocarbene, :CCl₂, is the attacking species.)
(b) Kolbe's (Kolbe–Schmitt) reaction:
Sodium phenoxide is heated with CO₂ under pressure (~400 K, 4–7 atm) and the product acidified to give mainly salicylic acid (2-hydroxybenzoic acid).
- C₆H₅O⁻Na⁺ + CO₂ →(400 K, pressure)→ sodium salicylate →(H⁺)→ o-C₆H₄(OH)(COOH)
(c) Friedel–Crafts acylation of anisole:
Anisole (methoxybenzene) reacts with acetyl chloride (or acetic anhydride) in the presence of anhydrous AlCl₃. The –OCH₃ group is o/p-directing, so the acetyl group enters mainly at the para position giving p-methoxyacetophenone (with minor ortho product).
- C₆H₅OCH₃ + CH₃COCl →(anhyd. AlCl₃)→ p-CH₃O–C₆H₄–COCH₃ + HCl
Marking Scheme
- 11 mark (a): phenol + CHCl₃ + NaOH (≈340 K) → salicylaldehyde (o-hydroxybenzaldehyde); credit for showing dichlorocarbene.
- 21 mark (b): sodium phenoxide + CO₂ (400 K, pressure) then H⁺ → salicylic acid (2-hydroxybenzoic acid).
- 31 mark (c): anisole + CH₃COCl / anhyd. AlCl₃ → p-methoxyacetophenone (para major, ortho minor). Deduct if wrong position or reagent.
Hint
Both Reimer–Tiemann (–CHO) and Kolbe (–COOH) put the new group ORTHO to –OH on phenol (giving salicylaldehyde and salicylic acid); anisole's –OCH₃ is o/p-directing, so acylation gives mainly the para ketone.
Quick Oral Answer
Reimer–Tiemann uses CHCl₃ with aqueous NaOH on phenol to introduce a –CHO ortho to –OH, giving salicylaldehyde via dichlorocarbene; Kolbe's reaction carboxylates sodium phenoxide with CO₂ under pressure to give salicylic acid; and Friedel–Crafts acylation of anisole with acetyl chloride/AlCl₃ gives mainly p-methoxyacetophenone because –OCH₃ is ortho/para directing.
Analysis & Explanation
This is a straight 'write the reaction' question, but each part rewards showing reagents, conditions, and the major product with correct regiochemistry.
(a) Reimer–Tiemann — making an ortho aldehyde on phenol:
- Key species: dichlorocarbene :CCl₂, generated from CHCl₃ + NaOH. It is electrophilic and attacks the highly activated (phenoxide) ring ortho to –OH.
- Product after hydrolysis: salicylaldehyde. Marks are often lost for forgetting the –CHO ends up ortho.
(b) Kolbe (Kolbe–Schmitt) — carboxylating phenol:
- Phenol is first converted to the more nucleophilic phenoxide; CO₂ is the weak electrophile, so pressure/heat are needed.
- Product after acidification: salicylic acid — industrially the precursor to aspirin.
(c) Friedel–Crafts acylation of anisole — directing effect:
- The lone pair on the –OCH₃ oxygen makes the ring electron-rich and o/p-directing and activating; the bulky acetyl group prefers the less-hindered para site → p-methoxyacetophenone.
- Note: unlike phenol/aniline, anisole (an ether) undergoes Friedel–Crafts cleanly because oxygen is not strongly deactivated by AlCl₃ the way an –NH₂ would be.
Real-world link: Kolbe's reaction is the classical industrial route to salicylic acid, the raw material for aspirin (acetylsalicylic acid) — tying back to Q6 of this very paper on aspirin.
Common Mistakes
- 1Placing the new –CHO/–COOH at the para position in Reimer–Tiemann/Kolbe — the major product is the ortho isomer (salicylaldehyde / salicylic acid).
- 2Forgetting the conditions: Reimer–Tiemann needs CHCl₃ + aq. NaOH (~340 K); Kolbe needs CO₂ under pressure on sodium phenoxide followed by acidification.
- 3Giving the meta product for Friedel–Crafts acylation of anisole — the –OCH₃ group is o/p-directing, so the major product is para (p-methoxyacetophenone).
Interesting Facts
The Reimer–Tiemann reaction was reported by Karl Reimer and Ferdinand Tiemann in 1876 and remains a standard textbook route to salicylaldehyde.
The Kolbe–Schmitt reaction, developed by Hermann Kolbe and refined by Rudolf Schmitt in the 1880s, is still the industrial method for making salicylic acid, the precursor of aspirin.
Because anisole is even more reactive than benzene towards electrophiles, its Friedel–Crafts acylation proceeds readily and selectively at the para position, a nice demonstration of the activating power of an –OCH₃ group.
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Frequently Asked Questions
Why do both the Reimer–Tiemann and Kolbe reactions give ortho products on phenol?
In both reactions the reactive species attacks the phenoxide ring, which is strongly activated at the ortho and para positions. Intramolecular hydrogen bonding/chelation with the neighbouring –OH group stabilises attack at the ortho position, so the major products are the ortho isomers: salicylaldehyde (Reimer–Tiemann) and salicylic acid (Kolbe).
Why does anisole undergo Friedel–Crafts acylation easily while phenol is troublesome?
In anisole the oxygen lone pairs strongly activate the ring and it is o/p-directing, so acylation gives mainly p-methoxyacetophenone. Phenol's –OH can bond to the Lewis acid AlCl₃, and free phenols tend to give side reactions, so anisole (the methyl ether) is the cleaner substrate for Friedel–Crafts acylation.