Carboxylic acids — acidity, derivatives (acid chlorides, anhydrides, esters, amides), Hell-Volhard-Zelinsky, esterificat

4.3.8 · Chemistry › Halides and Oxygenated Derivatives

1. Acidity — derived, memorised nahi

$R - C ⟨_{= O}^{O^{-}} ⟷ R - C ⟨_{O^{-}}^{= O}$

Substituent effects (acidity questions ka 80/20)

Acid	$p K_{a}$	Kyun
$C H_{3} C O O H$	4.76	baseline
$C l C H_{2} C O O H$	2.86	$C l$ withdraws (inductive)
$C l_{2} C H C O O H$	1.29	do $C l$
$C l_{3} C C O O H$	0.65	teen $C l$ , bahut acidic
$C H_{3} C H_{2} C O O H$	4.87	alkyl donate karta hai → kam acidic

Figure — Carboxylic acids — acidity, derivatives (acid chlorides, anhydrides, esters, amides), Hell-Volhard-Zelinsky, esterificat

2. Acid derivatives — ek family, ek trick

Derivative	$L$	Structure	Reactivity
Acid chloride	$- C l$	$R C O C l$	sabse zyada
Anhydride	$- O O C R$	$(R C O)_{2} O$	zyada
Ester	$- O R^{'}$	$R C O O R^{'}$	kam
Amide	$- N R_{2}$	$R C O N R_{2}$	sabse kam

Key conversions (sab nucleophilic acyl substitution hain)

$R C O O H + S O C l_{2} \to R C O C l + S O_{2} + H C l$ (chloride banao)
$R C O C l + R^{'} O H \to R C O O R^{'} + H C l$ (fast esterification)
$R C O C l + N H_{3} \to R C O N H_{2} + H C l$ (amide)
$R C O C l + R C O O^{-} \to (R C O)_{2} O$ (anhydride)

3. Hell–Volhard–Zelinsky (HVZ) — α-halogenation

Mechanism skeleton:

$R C H_{2} C O O H + P B r_{3} \to R C H_{2} C O B r$ (acid bromide)
$R C H_{2} C O B r ⇌ R C H = C (O H) B r$ (enol)
Enol + $B r_{2} \to R C H B r - C O B r$ (α-bromo acyl bromide)
$R C H B r - C O B r + R C H_{2} C O O H \to R C H B r - C O O H + R C H_{2} C O B r$ (catalyst regenerate)

4. Fischer esterification — mechanism derive karo

5. Active recall

Recall Quick self-test (answers cover karo!)

Acetic acid ethanol se stronger kyun hai? → carboxylate resonance do O pe; alkoxide delocalise nahi kar sakta.
Acid chloride/ester/amide/anhydride ko reactivity se order karo → $R C O C l >$ anhydride $>$ ester $>$ amide.
HVZ mein red P ka role? → acid bromide banata hai, jo easily enolise hota hai taaki α-C halogen grab kar sake.
Fischer mein ester oxygen kahan se aata hai? → alcohol se.
Fischer equilibrium ko right kaise push karein? → excess alcohol / water remove karo.

Recall Feynman: ek 12-saal ke bacche ko samjhao

Socho acid ek baccha hai jo ek hot potato (negative charge) pakde hua hai. Ek alcohol sirf ek baccha hai jo usse pakde hai — uncomfortable, chodega nahi. Ek carboxylic acid do doston ki sharing hai potato ki, toh woh khush hain us haath ko chhodne mein jo hydrogen pakde hai — isliye yeh "acidic" hai, apna H aasaani se de deta hai. "Derivatives" jaise bacche ka backpack swap karna hai: tum $- O H$ backpack ko chloride, ester, ya amide backpack se trade kar sakte ho. Chloride dhila hai aur aasaani se gir jaata hai (bahut reactive); amide backpack kaske bandha hai (bahut stable). Swap karne ke liye ek aur baccha (nucleophile) aata hai, pakad leta hai, aur purana backpack gir jaata hai — yahi hai nucleophilic acyl substitution.

Carboxylic acid ek alcohol se zyada acidic kyun hoti hai?

Uska conjugate base (carboxylate) negative charge ko resonance ke through do equivalent oxygens pe delocalise karta hai; alkoxide nahi kar sakta.

Chhote

p K_{a}

ka matlab kya hai?

Stronger acid (bada

K_{a}

, equilibrium dissociation favour karta hai).

Electron-withdrawing groups ka acidity pe kya effect hai?

Acidity badhate hain carboxylate anion ko inductive effect ke through stabilise karke (paas hone pe zyada strong).

Acid derivatives ko reactivity se rank karo.

Acid chloride > anhydride > ester > amide.

Amides sabse kam reactive derivative kyun hain?

Nitrogen apna lone pair strongly carbonyl mein donate karta hai (achha overlap, kam electronegativity) aur

^{-} N R_{2}

ek bura leaving group hai.

Acid derivative reactions ka general mechanism kya hai?

Nucleophilic acyl substitution = addition (tetrahedral intermediate) phir leaving group ka elimination.

Acid se acid chloride banane ka reagent?

S O C l_{2}

(aur

P C l_{3}

P C l_{5}

HVZ reaction kya hai?

Carboxylic acids ki α-halogenation (with α-H)

X_{2}

+ catalytic red P se, α-halo acids deta hai.

HVZ mein red phosphorus ka role?

Acid ko uske acid halide mein convert karta hai, jo aasaani se enolise hota hai taaki α-carbon

X_{2}

ko attack kar sake.

HVZ kaam karne ki requirement?

Acid mein kam se kam ek α-hydrogen hona chahiye.

Fischer esterification ko kya conditions chahiye?

Carboxylic acid + alcohol + acid catalyst (conc.

H_{2} S O_{4}

); reversible.

Fischer esterification mein ester oxygen kahan se aata hai?

Alcohol se (¹⁸O labelling se proven); acid apna OH water ke roop mein kho deta hai.

Fischer esterification ko completion tak kaise drive karte ho?

Excess alcohol use karo ya water remove karo (Le Chatelier).

Fischer mechanism steps ka memory aid?

PADPED — Protonate, Add, Deprotonate, Protonate, Eliminate, Deprotonate.

Acid chloride se ester banana irreversible kyun hai jabki Fischer reversible hai?

C l^{-}

ek excellent leaving group hai (koi catalyst nahi chahiye); Fischer mein,

- O H

/water ek poor LG hai isliye sab steps equilibria hain.