Intuition The big pictureA carboxylic acid is R - C O O H R\text{-}COOH R - C O O H C = O C=O C = O O - H O\text{-}H O - H everything :
WHY acidic? The O - H O\text{-}H O - H spread over two oxygens (resonance) — a "shared burden" anion is a stable anion.WHY reactive derivatives? The carbonyl carbon is electron-poor (δ + \delta+ δ + -OH \text{-OH} -OH -Cl , -OOCR , -OR , -NR 2 \text{-Cl}, \text{-OOCR}, \text{-OR}, \text{-NR}_2 -Cl , -OOCR , -OR , -NR 2 acid derivatives .
All the chemistry below is just two ideas: stabilise the anion and attack the carbonyl, then kick out a leaving group .
Definition Carboxylic acidAn organic compound containing the carboxyl group − C O O H -COOH − C O O H R − C ( = O ) − O − H R-C(=O)-O-H R − C ( = O ) − O − H
R - C O O H R\text{-}COOH R - C O O H p K a ≈ 4 pK_a \approx 4 p K a ≈ 4 5 5 5 p K a ≈ 16 pK_a \approx 16 p K a ≈ 16
Alkoxide R O − RO^- R O − stuck on one oxygen.Carboxylate R C O O − RCOO^- R C O O − delocalised over two equivalent oxygens by resonance.
A delocalised charge is a low-energy charge. Lower-energy product ⟹ reaction "wants" to happen ⟹ stronger acid.
R − C ⟨ = O O − ⟷ R − C ⟨ O − = O R-C{\Large\langle}^{\;O^-}_{\;=O} \quad\longleftrightarrow\quad R-C{\Large\langle}^{\;=O}_{\;O^-} R − C ⟨ = O O − ⟷ R − C ⟨ O − = O
Intuition Electron-WITHDRAWING groups (EWG) → MORE acidic. Why?An EWG (like C l Cl C l N O 2 NO_2 N O 2 F F F away from the carboxylate, spreading the negative charge even further ⟹ even more stable anion ⟹ stronger acid. This is the inductive effect , and it weakens with distance.
Acid
p K a pK_a p K a Why
C H 3 C O O H CH_3COOH C H 3 C O O H 4.76
baseline
C l C H 2 C O O H ClCH_2COOH C l C H 2 C O O H 2.86
C l Cl C l
C l 2 C H C O O H Cl_2CHCOOH C l 2 C H C O O H 1.29
two C l Cl C l
C l 3 C C O O H Cl_3CCOOH C l 3 C C O O H 0.65
three C l Cl C l
C H 3 C H 2 C O O H CH_3CH_2COOH C H 3 C H 2 C O O H 4.87
alkyl donates → less acidic
Definition Acid derivativeA compound where the − O H -OH − O H R C O O H RCOOH R C O O H L L L R − C ( = O ) − L R-C(=O)-L R − C ( = O ) − L
Derivative
L L L Structure
Reactivity
Acid chloride
− C l -Cl − C l R C O C l RCOCl R C O C l highest
Anhydride
− O O C R -OOCR − O O C R ( R C O ) 2 O (RCO)_2O ( R C O ) 2 O high
Ester
− O R ′ -OR' − O R ′ R C O O R ′ RCOOR' R C O O R ′ low
Amide
− N R 2 -NR_2 − N R 2 R C O N R 2 RCONR_2 R C O N R 2 lowest
Intuition WHY this reactivity order?Two ideas, same direction:
Leaving-group ability : C l − Cl^- C l − − N R 2 \,^-NR_2 − N R 2 Lone-pair donation into C = O C=O C = O : N N N strongly into the carbonyl (good orbital overlap, low electronegativity), making amides very stable/unreactive. C l Cl C l poorly (poor overlap with C). So amide carbonyl is "satisfied," acid chloride carbonyl is "hungry."
Rule: you can always go DOWN the reactivity ladder (chloride → ester → amide), never easily back up. Synthesis flows downhill.
R C O O H + S O C l 2 → R C O C l + S O 2 + H C l RCOOH + SOCl_2 \rightarrow RCOCl + SO_2 + HCl R C O O H + S O C l 2 → R C O C l + S O 2 + H C l R C O C l + R ′ O H → R C O O R ′ + H C l RCOCl + R'OH \rightarrow RCOOR' + HCl R C O C l + R ′ O H → R C O O R ′ + H C l R C O C l + N H 3 → R C O N H 2 + H C l RCOCl + NH_3 \rightarrow RCONH_2 + HCl R C O C l + N H 3 → R C O N H 2 + H C l R C O C l + R C O O − → ( R C O ) 2 O RCOCl + RCOO^- \rightarrow (RCO)_2O R C O C l + R C O O − → ( R C O ) 2 O
Intuition The universal mechanism:addition–elimination
Unlike aldehydes (where the nucleophile just adds ), the acyl carbon already has a leaving group. So:
Nucleophile adds to C = O C=O C = O s p 3 sp^3 s p 3
The C = O C=O C = O re-forms , pushing out the leaving group L − L^- L − L L L N u Nu N u
Carboxylic acids with at least one α-hydrogen react with X 2 X_2 X 2 red P (or P X 3 PX_3 P X 3 α-halo acid .
R − C H 2 − C O O H → X 2 , cat. red P R − C H X − C O O H R-CH_2-COOH \xrightarrow{\;X_2,\;\text{cat. red P}\;} R-CHX-COOH R − C H 2 − C O O H X 2 , cat. red P R − C H X − C O O H
Intuition WHY do we need red P? (this is the whole point)Carboxylic acids barely form enols (the O–H proton dominates), so direct halogenation is sluggish. P P P R C H 2 C O B r RCH_2COBr R C H 2 C O B r easily . The enol's nucleophilic α-carbon attacks B r 2 Br_2 B r 2 B r Br B r
In one line: the acid won't enolise, but its acyl halide will — so we make a tiny bit of acyl halide and ride it.
Mechanism skeleton:
R C H 2 C O O H + P B r 3 → R C H 2 C O B r RCH_2COOH + PBr_3 \rightarrow RCH_2COBr R C H 2 C O O H + P B r 3 → R C H 2 C O B r R C H 2 C O B r ⇌ R C H = C ( O H ) B r RCH_2COBr \rightleftharpoons RCH=C(OH)Br R C H 2 C O B r ⇌ R C H = C ( O H ) B r Enol + B r 2 → R C H B r - C O B r Br_2 \rightarrow RCHBr\text{-}COBr B r 2 → R C H B r - C O B r
R C H B r - C O B r + R C H 2 C O O H → R C H B r - C O O H + R C H 2 C O B r RCHBr\text{-}COBr + RCH_2COOH \rightarrow RCHBr\text{-}COOH + RCH_2COBr R C H B r - C O B r + R C H 2 C O O H → R C H B r - C O O H + R C H 2 C O B r
Worked example Predict the productC H 3 C H 2 C O O H CH_3CH_2COOH C H 3 C H 2 C O O H C H 2 CH_2 C H 2 Product: C H 3 C H B r - C O O H CH_3CHBr\text{-}COOH C H 3 C H B r - C O O H (2-bromopropanoic acid).
Why this step? Only the carbon adjacent to − C O O H -COOH − C O O H
Definition Fischer esterificationAcid + alcohol, acid-catalysed (conc. H 2 S O 4 H_2SO_4 H 2 S O 4 R C O O H + R ′ O H ⇌ H + R C O O R ′ + H 2 O RCOOH + R'OH \underset{H^+}{\rightleftharpoons} RCOOR' + H_2O R C O O H + R ′ O H H + ⇌ R C O O R ′ + H 2 O
Intuition WHY catalyst? WHY reversible?The carboxyl carbon is only weakly electrophilic and the alcohol O O O Protonating the carbonyl makes the carbon much more δ + \delta+ δ + To drive forward: use excess alcohol or remove water (Le Chatelier).
C H 3 C O O H + C H 3 C H 2 O H → H + CH_3COOH + CH_3CH_2OH \xrightarrow{H^+} C H 3 C O O H + C H 3 C H 2 O H H + Product: C H 3 C O O C H 2 C H 3 CH_3COOCH_2CH_3 C H 3 C O O C H 2 C H 3 (ethyl ethanoate) + H 2 O H_2O H 2 O Why this step? The − O H -OH − O H acid leaves as water (isotopic labelling proves it: the ester oxygen comes from the alcohol , not the acid). This is the classic ¹⁸O experiment.
Common mistake Steel-man: "Water comes from the alcohol's OH."Why it feels right: alcohols famously lose − O H -OH − O H The fix: ¹⁸O-labelled alcohol ends up in the ester , not in the water. So the alcohol contributes its O–R intact; the acid loses its − O H -OH − O H was the acid.
Common mistake "Acid chlorides and Fischer esterification work the same way."Why it feels right: both make esters. Fix: Fischer is reversible & needs acid catalyst (slow); R C O C l + R ′ O H RCOCl + R'OH R C O C l + R ′ O H fast, irreversible, no catalyst because C l − Cl^- C l −
Common mistake "More resonance structures = more acidic, always."Why it feels right: resonance stabilises anions. Fix: what matters is stabilisation of the conjugate base relative to the acid . Phenol has resonance too but is far weaker (p K a ≈ 10 pK_a\approx10 p K a ≈ 10 where the charge goes, not just how many structures.
Recall Quick self-test (cover the answers!)
Why is acetic acid stronger than ethanol? → carboxylate resonance over two O; alkoxide can't delocalise.
Order acid chloride/ester/amide/anhydride by reactivity → R C O C l > RCOCl > R C O C l > > > > > > >
Role of red P in HVZ? → forms acid bromide, which enolises so α-C can grab halogen.
Where does the ester O come from in Fischer? → the alcohol .
How to push Fischer equilibrium right? → excess alcohol / remove water.
Recall Feynman: explain to a 12-year-oldImagine the acid is a kid holding a hot potato (a negative charge). An alcohol is just one kid holding it — uncomfortable, won't let go. A carboxylic acid is two friends sharing the potato, so they're happy to drop the hand that holds the hydrogen — that's why it's "acidic," it gives away its H easily.
The "derivatives" are like swapping the kid's backpack: you can trade the − O H -OH − O H
Why is a carboxylic acid more acidic than an alcohol? Its conjugate base (carboxylate) delocalises the negative charge over two equivalent oxygens by resonance; alkoxide cannot.
What does smaller p K a pK_a p K a Stronger acid (larger
K a K_a K a , equilibrium favours dissociation).
Effect of electron-withdrawing groups on acidity? Increase acidity by stabilising the carboxylate anion through the inductive effect (stronger when closer).
Rank acid derivatives by reactivity. Acid chloride > anhydride > ester > amide.
Why are amides the least reactive derivative? Nitrogen donates its lone pair strongly into the carbonyl (good overlap, low electronegativity) and
− N R 2 ^-NR_2 − N R 2 is a poor leaving group.
General mechanism of acid derivative reactions? Nucleophilic acyl substitution = addition (tetrahedral intermediate) then elimination of leaving group.
Reagent to make an acid chloride from an acid? S O C l 2 SOCl_2 S O C l 2 (also
P C l 3 PCl_3 P C l 3 ,
P C l 5 PCl_5 P C l 5 ).
What is the HVZ reaction? α-halogenation of carboxylic acids (with α-H) using
X 2 X_2 X 2 + catalytic red P, giving α-halo acids.
Role of red phosphorus in HVZ? Converts the acid to its acid halide, which enolises readily so the α-carbon can attack
X 2 X_2 X 2 .
Requirement for HVZ to work? The acid must have at least one α-hydrogen.
What conditions does Fischer esterification need? Carboxylic acid + alcohol + acid catalyst (conc.
H 2 S O 4 H_2SO_4 H 2 S O 4 ); reversible.
In Fischer esterification, where does the ester oxygen originate? From the alcohol (proven by ¹⁸O labelling); the acid loses its OH as part of water.
How do you drive Fischer esterification to completion? Use excess alcohol or remove water (Le Chatelier).
Memory aid for Fischer mechanism steps? PADPED — Protonate, Add, Deprotonate, Protonate, Eliminate, Deprotonate.
Why is making an ester from an acid chloride irreversible while Fischer is reversible? C l − Cl^- C l − is an excellent leaving group (no catalyst needed); in Fischer,
− O H -OH − O H /water is a poor LG so all steps are equilibria.
Resonance and delocalisation — basis of carboxylic acid acidityInductive effect — substituent effects on p K a pK_a p K a Nucleophilic acyl substitution — the master mechanism for all derivativesKeto–enol tautomerism — the enol step in HVZLe Chatelier's principle — driving Fischer esterification forwardAldehydes and ketones — contrast: addition vs addition–eliminationSaponification — base hydrolysis of esters (downstream of this note)
nucleophile attacks then kicks out L
Electron-withdrawing groups
Acid chloride, most reactive
Intuition Hinglish mein samjhoDekho, carboxylic acid (R - C O O H R\text{-}COOH R - C O O H H + H^+ H + do oxygen par resonance se baant jaata hai. Charge jitna zyada spread, anion utna zyada stable, aur acid utna zyada strong. Isiliye acetic acid (p K a ≈ 4.76 pK_a\approx4.76 p K a ≈ 4.76 p K a ≈ 16 pK_a\approx16 p K a ≈ 16 C l Cl C l
Derivatives ek hi family hain — bas − O H -OH − O H − C l -Cl − C l − O O C R -OOCR − O O C R − O R -OR − O R − N R 2 -NR_2 − N R 2 chloride > anhydride > ester > amide . Reason — chloride ka C l − Cl^- C l − nucleophilic acyl substitution bolte hain.
HVZ reaction mein hum acid ke alpha-carbon par halogen lagate hain, lekin red phosphorus zaroori hai. Kyun? Kyunki acid khud enol nahi banata, par red P use acid bromide bana deta hai jo aaram se enolise ho jaata hai, aur uska alpha-carbon B r 2 Br_2 B r 2
Fischer esterification mein acid + alcohol, conc. H 2 S O 4 H_2SO_4 H 2 S O 4 alcohol se aata hai , aur acid apna − O H -OH − O H PADPED .
