Phenols — acidity (resonance stabilization), Kolbe-Schmidt, Reimer-Tiemann, Fries rearrangement

4.3.5 · Chemistry › Halides and Oxygenated Derivatives

1. Acidity of Phenols

Phenol alcohols se zyada acidic kyun hai? Ek acid strong hota hai jab uski conjugate base stable ho (use H⁺ wapas nahi chahiye hota).

Ethoxide ( $CH_{3} CH_{2} O^{-}$ ) mein, negative charge ek hi oxygen pe atak jaata hai. Koi cheez use spread nahi karti.
Phenoxide ( $C_{6} H_{5} O^{-}$ ) mein, charge resonance ke through ring mein delocalize ho jaata hai. Spread-out charge = lower energy = zyada stable = phenol H⁺ zyada aasaani se chhodata hai.

Substituent effects (exam ke liye 80/20 content yahi hai)

Phenol	$p K_{a}$	Kyun
Phenol	10.0	baseline
p-cresol ( $p$ -CH₃)	10.3	CH₃ donate karta hai → kam acidic
p-nitrophenol	7.1	$-$ NO₂ withdraw karta hai (resonance + inductive) → zyada acidic
2,4-dinitrophenol	4.0	do EWG
2,4,6-trinitrophenol (picric acid)	0.4	teen EWG → mineral acid jaisa strong!

Figure — Phenols — acidity (resonance stabilization), Kolbe-Schmidt, Reimer-Tiemann, Fries rearrangement

2. Kolbe–Schmidt Reaction (→ salicylic acid → aspirin)

KAISE (mechanism, scratch se):

Nucleophile banao: Phenol + NaOH → sodium phenoxide. Phenoxide ring bahut electron-rich hoti hai (extra negative charge).
CO₂ electrophile hai: $O=C=O$ ka carbon electrophilic hota hai.
Electrophilic substitution ortho pe: ortho carbon, CO₂ carbon pe attack karta hai (yeh ek EAS-jaisa / nucleophilic-aromatic-on-electrophile step hai). Ortho prefer hota hai kyunki phenoxide O, incoming carboxylate ke saath Na⁺ ko chelate kar sakta hai, use paas rok ke rakhta hai.
Rearomatise + tautomerise → sodium salicylate. Acidify karo → salicylic acid.

3. Reimer–Tiemann Reaction (→ salicylaldehyde)

KAISE (mechanism — key hai dichlorocarbene):

NaOH, CHCl₃ ko deprotonate karta hai: $CHCl_{3} + OH^{-} \to :CCl_{3}^{-} + H_{2} O$ .
$:CCl_{3}^{-}$ , $Cl^{-}$ chhodata hai → dichlorocarbene $:CCl_{2}$ — ek neutral, electron-deficient electrophile.
Phenoxide ka electron-rich ortho carbon, $:CCl_{2}$ pe attack karta hai → ortho position pe ek Ar–CHCl₂ (benzal chloride) intermediate deta hai.
NaOH se –CHCl₂ ki hydrolysis → –CHO. Acidify karo → salicylaldehyde.

4. Fries Rearrangement (→ acyl phenols)

KAISE (mechanism):

$AlCl_{3}$ , ester carbonyl O ke saath coordinate karta hai, O–acyl bond ko kamzor karta hai.
Ek acylium ion $R-C \equiv O^{+}$ (ya AlCl₃–acyl complex) toot jaata hai — yahi electrophile hai.
Ab free phenol ring ki Friedel–Crafts acylation hoti hai (electron-rich, –OH ek o/p director hai) ortho ya para pe.
Work-up → hydroxy aryl ketone.

5. Quick comparison table

Reaction	Reagent / electrophile	Product group introduced	Position
Kolbe–Schmidt	CO₂ + pressure (Na phenoxide pe)	–COOH	ortho (Na)
Reimer–Tiemann	CHCl₃/NaOH (:CCl₂)	–CHO	ortho (mainly)
Fries	aryl ester + AlCl₃	–COR (ketone)	o (hot) / p (cold)

Recall Feynman: ek 12-saal ke bacche ko explain karo

Socho phenol ek ball (H⁺) hai jo ek glove (oxygen) se halke se chipki hai, jo ek badi trampoline (ring) se judi hai. Jab ball uchhal jaati hai, jo "stickiness" (negative charge) bacha rehta hai woh ek jagah nahi tikta — woh poori trampoline pe uchhalda rehta hai. Kyunki stickiness spread ho gayi, ball aasaani se nikal jaati hai — isliye phenol thoda khatta (acidic) hota hai. Wahi electron-rich trampoline phenol ko guzarti hui molecules (CO₂, ek carbene, ek acyl piece) ko pakadne aur unhe apne edges (ortho/para) pe chipkaane deti hai — yahi Kolbe, Reimer–Tiemann, aur Fries hai.

Active-Recall Flashcards

#flashcards/chemistry

Phenol, ethanol se zyada acidic kyun hai?

Iski conjugate base (phenoxide) resonance ke through negative charge ko ring mein delocalize karti hai; ethoxide nahi kar sakta, isliye uska charge O pe hi lokalized rehta hai.

Phenol, ethanol, aur acetic acid ka approximate pKa kya hai?

Phenol ~10, ethanol ~16, acetic acid ~4.76.

Acetic acid, phenol se zyada acidic kyun hai?

Carboxylate charge ko do electronegative oxygens pe spread karta hai; phenoxide use carbon atoms pe spread karta hai (negative charge rakhne mein bura).

Phenol acidity pe p-NO₂ group ka kya effect hai aur kyun?

Acidity badhta hai (pKa ~7.1); o/p pe EWG, phenoxide ke negative charge ko resonance se stabilize karta hai.

Strong acidification ke liye EWG ortho/para pe kyun hona chahiye (meta pe nahi)?

Resonance delocalization sirf o/p carbons tak pahunchti hai; meta sirf weak inductive effect se kaam kar sakta hai.

Picric acid kya hai aur iska pKa kya hai?

2,4,6-trinitrophenol, pKa ~0.4 — teen EWGs ise mineral acid jaisa strong bana dete hain.

Kolbe–Schmidt: reagents, conditions, product?

Sodium phenoxide + CO₂, ~125 °C aur pressure, phir H⁺ → salicylic acid (2-hydroxybenzoic acid).

Kolbe–Schmidt sodium phenoxide ke saath ortho product kyun deta hai?

Na⁺, phenoxide O aur incoming carboxylate ko chelate karta hai, substitution ko ortho pe direct karta hai.

Reimer–Tiemann: reagents aur actual electrophile?

Phenol + CHCl₃ + NaOH; electrophile dichlorocarbene :CCl₂ hai jo in situ generate hota hai.

Reimer–Tiemann ka product aur kahan?

Salicylaldehyde (2-hydroxybenzaldehyde), –OH ke ortho pe –CHO introduce hota hai.

Fries rearrangement: starting material, catalyst, product?

Aryl ester (ArO–COR) + anhydrous AlCl₃, heat → ortho/para hydroxy aryl ketone.

Fries mein temperature regiochemistry kaise control karta hai?

High temperature → ortho (intramolecular H-bonding); low temperature → para.

Fries mein mechanistic electrophile kya hai?

Acylium ion R–C≡O⁺ (AlCl₃–acyl complex) jo free phenol pe Friedel–Crafts acylation karta hai.

Kaun sa reaction –COOH deta hai, kaun sa –CHO, kaun sa –COR?

Kolbe–Schmidt → –COOH; Reimer–Tiemann → –CHO; Fries → –COR (ketone).

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