Phenols — acidity (resonance stabilization), Kolbe-Schmidt, Reimer-Tiemann, Fries rearrangement
1. Acidity of Phenols
WHY is phenol more acidic than alcohols? An acid is strong when its conjugate base is stable (it doesn't want the H⁺ back).
- In ethoxide (), the negative charge is stuck on one oxygen. Nothing spreads it out.
- In phenoxide (), the charge is delocalized into the ring by resonance. Spread-out charge = lower energy = more stable = phenol gives up H⁺ more readily.
Substituent effects (this is the 80/20 exam content)
| Phenol | Why | |
|---|---|---|
| Phenol | 10.0 | baseline |
| p-cresol (-CH₃) | 10.3 | CH₃ donates → less acidic |
| p-nitrophenol | 7.1 | NO₂ withdraws (resonance + inductive) → more acidic |
| 2,4-dinitrophenol | 4.0 | two EWG |
| 2,4,6-trinitrophenol (picric acid) | 0.4 | three EWG → as strong as a mineral acid! |
2. Kolbe–Schmidt Reaction (→ salicylic acid → aspirin)
HOW (mechanism, from scratch):
- Make the nucleophile: Phenol + NaOH → sodium phenoxide. Phenoxide ring is very electron-rich (extra negative charge).
- CO₂ is the electrophile: the carbon of is electrophilic.
- Electrophilic substitution at ortho: the ortho carbon attacks the CO₂ carbon (this is an EAS-like / nucleophilic-aromatic-on-electrophile step). Ortho is favoured because the phenoxide O can chelate the Na⁺ to the incoming carboxylate, holding it nearby.
- Rearomatise + tautomerise → sodium salicylate. Acidify → salicylic acid.
3. Reimer–Tiemann Reaction (→ salicylaldehyde)
HOW (mechanism — the key is dichlorocarbene):
- NaOH deprotonates CHCl₃: .
- loses → dichlorocarbene — a neutral, electron-deficient electrophile.
- Phenoxide's electron-rich ortho carbon attacks → gives an Ar–CHCl₂ (benzal chloride) intermediate at the ortho position.
- Hydrolysis of –CHCl₂ by NaOH → –CHO. Acidify → salicylaldehyde.
4. Fries Rearrangement (→ acyl phenols)
HOW (mechanism):
- coordinates to the ester carbonyl O, weakening the O–acyl bond.
- An acylium ion (or AlCl₃–acyl complex) breaks off — this is the electrophile.
- Friedel–Crafts acylation of the now-free phenol ring (electron-rich, –OH is an o/p director) at ortho or para.
- Work-up → hydroxy aryl ketone.
5. Quick comparison table
| Reaction | Reagent / electrophile | Product group introduced | Position |
|---|---|---|---|
| Kolbe–Schmidt | CO₂ + pressure (on Na phenoxide) | –COOH | ortho (Na) |
| Reimer–Tiemann | CHCl₃/NaOH (:CCl₂) | –CHO | ortho (mainly) |
| Fries | aryl ester + AlCl₃ | –COR (ketone) | o (hot) / p (cold) |
Recall Feynman: explain to a 12-year-old
Imagine phenol is a ball (H⁺) lightly stuck to a glove (oxygen) attached to a big trampoline (the ring). When the ball pops off, the leftover "stickiness" (negative charge) doesn't stay in one spot — it bounces all over the trampoline. Because the stickiness is spread out, the ball comes off easily — that's why phenol is sour-ish (acidic). The same electron-rich trampoline lets phenol grab passing molecules (CO₂, a carbene, an acyl piece) and stick them onto its edges (ortho/para) — that's Kolbe, Reimer–Tiemann, and Fries.
Active-Recall Flashcards
#flashcards/chemistry
Why is phenol more acidic than ethanol?
Approximate pKa of phenol vs ethanol vs acetic acid?
Why is acetic acid more acidic than phenol?
Effect of a p-NO₂ group on phenol acidity and why?
Why must the EWG be ortho/para (not meta) for strong acidification?
What is picric acid and its pKa?
Kolbe–Schmidt: reagents, conditions, product?
Why does Kolbe–Schmidt give ortho with sodium phenoxide?
Reimer–Tiemann: reagents and the actual electrophile?
Reimer–Tiemann product and where?
Fries rearrangement: starting material, catalyst, product?
Fries: how does temperature control regiochemistry?
Mechanistic electrophile in Fries?
Which reaction gives –COOH, which –CHO, which –COR?
Connections
- Resonance and Mesomeric Effect
- Acid Strength and Conjugate Base Stability (pKa)
- Electrophilic Aromatic Substitution
- Friedel-Crafts Acylation
- Carbenes and Dichlorocarbene
- Aspirin and Salicylic Acid (applications)
- Inductive vs Mesomeric Effects of Substituents
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, phenol ka pura khel ek baat pe tika hai: oxygen ka lone pair benzene ring ke andar "ghus" jaata hai (resonance). Jab phenol apna H⁺ chhodta hai, toh jo phenoxide banta hai uska negative charge sirf oxygen pe nahi rehta — wo ring ke ortho aur para carbons pe spread ho jaata hai. Charge jab phaila hua ho, toh anion zyada stable hota hai, aur stable anion matlab acid strong. Isiliye phenol (pKa ~10) alcohol (pKa ~16) se zyada acidic hai. Lekin carboxylic acid (pKa ~5) phenol se bhi strong hai, kyunki uska charge do oxygens pe baithta hai — oxygen carbon se behtar negative charge sambhalta hai.
Substituent effect simple hai: agar ortho/para pe electron-withdrawing group (jaise –NO₂) ho, to wo negative charge ko aur sambhal leta hai → acidity badhti hai (picric acid pKa 0.4, almost mineral acid jaisa!). Electron-donating group (–CH₃, –OCH₃) ulta charge ko dhakka deta hai → acidity kam. Meta position se sirf inductive thoda help karta hai, resonance nahi — isiliye o/p strong matter karte hain.
Ab reactions: phenol ki ring electron-rich hai, toh wo electrophiles ko pakad ke ortho/para pe chipka leti hai. Kolbe–Schmidt: sodium phenoxide + CO₂ (pressure, ~125°C) → salicylic acid (–COOH ortho). Yahi aage aspirin banta hai. Reimer–Tiemann: CHCl₃ + NaOH se banta hai dichlorocarbene (:CCl₂) — yahi asli electrophile hai, CHCl₃ khud kuch nahi karta — aur milta hai salicylaldehyde (–CHO ortho). Fries: phenol ka ester + AlCl₃ heat karo, acyl group O se ring pe shift ho jaata hai → hydroxy aryl ketone (–COR). Yaad rakho: Hot = Ortho, Cold = Para.
Trick yaad rakhne ka: K-CO₂-acid, R-Carbene-aldehyde, F-ester-ketone. Bas itna pakka kar lo, exam me yeh chapter point dilwa dega.