Aldehydes, Ketones and Carboxylic Acids

The carbonyl group consists of a carbon atom doubly bonded to an oxygen atom (>C=O). Both carbon and oxygen are $sp^2$ hybridized:

• The carbonyl carbon forms three coplanar $\sigma$-bonds with $120^\circ$ bond angles (trigonal planar geometry). The unhybridized $2p$ orbital of carbon overlaps sideways with a $2p$ orbital of oxygen to form a $\pi$-bond.
• Because oxygen (Pauling electronegativity $3.5$) is far more electronegative than carbon ($2.5$), the $\pi$-electron cloud is strongly shifted toward oxygen: $$\gt\text{C}=\text{O} \longleftrightarrow \gt\text{C}^+-\text{O}^-$$ This permanent polarization imparts a high dipole moment ($\mu \approx 2.3 - 2.8\text{ D}$) and renders the carbonyl carbon strongly electrophilic (susceptible to nucleophilic attack) and the carbonyl oxygen nucleophilic (readily protonated by acids).

1. Controlled Oxidation of Alcohols:
   • Primary alcohols oxidise to aldehydes using mild reagents like Pyridinium Chlorochromate (PCC, Corey's reagent) in $\text{CH}_2\text{Cl}_2$ without over-oxidation to carboxylic acids: $$R\text{CH}_2\text{OH} \xrightarrow{\text{PCC / }\text{CH}_2\text{Cl}_2} R\text{CHO}$$ • Secondary alcohols oxidise cleanly to ketones using acidified $\text{K}_2\text{Cr}_2\text{O}_7$ or chromic anhydride in aqueous acetone (Jones reagent): $$R\text{-CH(OH)-}R' \xrightarrow{\text{CrO}_3 / \text{H}_2\text{SO}_4} R\text{-CO-}R'$$
2. Catalytic Dehydrogenation of Alcohols: Passing alcohol vapours over heated copper gauze at $573\text{ K}$ ($300^\circ\text{C}$): $$R\text{CH}_2\text{OH} \xrightarrow{\text{Cu / } 573\text{ K}} R\text{CHO} + \text{H}_2\uparrow \qquad R\text{-CH(OH)-}R' \xrightarrow{\text{Cu / } 573\text{ K}} R\text{-CO-}R' + \text{H}_2\uparrow$$ Note: Tertiary alcohols undergo dehydration to form alkenes rather than dehydrogenation.
3. Ozonolysis of Alkenes: Cleavage of alkenes with $\text{O}_3$ followed by reductive workup with $\text{Zn}/\text{H}_2\text{O}$: $$R\text{-CH=CH-}R' \xrightarrow{\text{(i) }\text{O}_3, \text{ (ii) }\text{Zn/H}_2\text{O}} R\text{CHO} + R'\text{CHO}$$ $$R_2\text{C=CH}_2 \xrightarrow{\text{(i) }\text{O}_3, \text{ (ii) }\text{Zn/H}_2\text{O}} R_2\text{C=O} + \text{HCHO}$$
4. Hydration of Alkynes (Kucherov Reaction): Addition of water in presence of $20\%\,\text{H}_2\text{SO}_4$ and $1\%\,\text{HgSO}_4$ at $333\text{ K}$: $$\text{HC}\equiv\text{CH} + \text{H}_2\text{O} \xrightarrow{\text{Hg}^{2+}/\text{H}^+} [\text{CH}_2=\text{CH-OH}] \xrightarrow{\text{tautomerisation}} \mathbf{\text{CH}_3\text{CHO}} \text{ (Acetaldehyde)}$$ $$\text{CH}_3\text{-C}\equiv\text{CH} + \text{H}_2\text{O} \xrightarrow{\text{Hg}^{2+}/\text{H}^+} [\text{CH}_3\text{-C(OH)=CH}_2] \xrightarrow{\text{tautomerisation}} \mathbf{\text{CH}_3\text{COCH}_3} \text{ (Acetone)}$$ Markovnikov addition yields ketones from all terminal alkynes except acetylene itself.

• Rosenmund Reduction: Catalytic hydrogenation of acyl chlorides using palladium deposited on barium sulphate ($\text{Pd-BaSO}_4$), partially poisoned with sulfur or quinoline to prevent further reduction of the aldehyde to alcohol: $$R\text{COCl} + \text{H}_2 \xrightarrow{\text{Pd-BaSO}_4, \text{ quinoline / boiling xylene}} \mathbf{R\text{CHO}} + \text{HCl}$$ Limitation: Formaldehyde ($\text{HCHO}$) cannot be prepared by this method because formyl chloride ($\text{HCOCl}$) is unstable at room temperature.
• Stephen Reaction: Reduction of alkyl or aryl nitriles with stannous chloride and gaseous $\text{HCl}$, followed by steam hydrolysis of the intermediate aldimine hydrochloride: $$R\text{C}\equiv\text{N} + \text{SnCl}_2 + 2\text{HCl} \longrightarrow R\text{CH=NH}\cdot\text{HCl} \xrightarrow{\text{H}_3\text{O}^+} \mathbf{R\text{CHO}} + \text{NH}_4\text{Cl}$$
• Reduction using DIBAL-H: Diisobutylaluminium hydride, $[(i\text{-Bu})_2\text{AlH}]$, selectively reduces nitriles and esters to aldehydes at low temperatures ($-78^\circ\text{C}$): $$R\text{COOR}' \xrightarrow{\text{(i) DIBAL-H / } -78^\circ\text{C}, \text{ (ii) }\text{H}_2\text{O}} R\text{CHO} + R'\text{OH}$$
• Etard Reaction: Oxidation of toluene with chromyl chloride ($\text{CrO}_2\text{Cl}_2$) in carbon disulphide ($\text{CS}_2$) or $\text{CCl}_4$, forming a brown chromium complex, which on aqueous hydrolysis gives benzaldehyde: $$\text{C}_6\text{H}_5\text{CH}_3 + 2\text{CrO}_2\text{Cl}_2 \xrightarrow{\text{CS}_2} \text{C}_6\text{H}_5\text{CH(OCrOHCl}_2)_2 \xrightarrow{\text{H}_3\text{O}^+} \mathbf{\text{C}_6\text{H}_5\text{CHO}} \text{ (Benzaldehyde)}$$
• Gattermann-Koch Reaction: Direct formylation of benzene with carbon monoxide and hydrogen chloride gas in the presence of anhydrous aluminium chloride and cuprous chloride: $$\text{C}_6\text{H}_6 + \text{CO} + \text{HCl} \xrightarrow{\text{anhyd. AlCl}_3 / \text{CuCl}} \mathbf{\text{C}_6\text{H}_5\text{CHO}} + \text{HCl}$$

• From Acyl Chlorides & Dialkylcadmium: Reaction of Grignard reagent with $\text{CdCl}_2$ yields dialkylcadmium, which is less reactive than Grignard reagents and selectively attacks acyl chlorides without reacting further with the resulting ketone: $$2R\text{'Mg}X + \text{CdCl}_2 \longrightarrow R'_2\text{Cd} + 2\text{Mg}X\text{Cl}$$ $$2R\text{COCl} + R'_2\text{Cd} \longrightarrow 2\mathbf{R\text{CO}R'} + \text{CdCl}_2$$
• From Nitriles using Grignard Reagents: Nucleophilic addition of Grignard reagent to nitriles followed by acid hydrolysis: $$\text{CH}_3\text{C}\equiv\text{N} + \text{C}_6\text{H}_5\text{MgBr} \xrightarrow{\text{ether}} \text{CH}_3\text{-C(C}_6\text{H}_5)\text{=NMgBr} \xrightarrow{\text{H}_3\text{O}^+} \mathbf{\text{C}_6\text{H}_5\text{COCH}_3} \text{ (Acetophenone)}$$
• Friedel-Crafts Acylation: Treatment of benzene or substituted arenes with an acyl chloride or acid anhydride in the presence of anhydrous $\text{AlCl}_3$: $$\text{C}_6\text{H}_6 + \text{CH}_3\text{COCl} \xrightarrow{\text{anhyd. AlCl}_3} \mathbf{\text{C}_6\text{H}_5\text{COCH}_3} + \text{HCl}$$

• Boiling Points: Due to dipole-dipole attractions between polar carbonyl molecules, aldehydes and ketones possess significantly higher boiling points than non-polar alkanes and weakly polar ethers of comparable molecular mass. However, because they lack an acidic hydrogen bonded directly to oxygen, they cannot form intermolecular hydrogen bonds among themselves; hence, their boiling points are lower than those of isomeric alcohols and carboxylic acids: $$\text{Butane (b.p. 272.5 K)} < \text{Methoxyethane (281 K)} < \mathbf{\text{Propanal (322 K)}} < \mathbf{\text{Propanone (329 K)}} < \text{Propan-1-ol (370 K)} < \text{Ethanoic acid (391 K)}$$
• Solubility: Lower aldehydes and ketones ($\text{HCHO}, \text{CH}_3\text{CHO}, \text{CH}_3\text{COCH}_3$) are completely miscible with water because their carbonyl oxygen forms strong intermolecular hydrogen bonds with water molecules ($>\text{C}=\text{O}\cdots\text{H-O-H}$). As the hydrophobic hydrocarbon chain lengthens ($>C_5$), water solubility drops precipitously.

The rate-determining step in carbonyl reactions is the attack of a nucleophile (:$\text{Nu}^-$) perpendicular to the trigonal planar $sp^2$ carbonyl carbon, transforming it into a tetrahedral alkoxide intermediate ($sp^3$), which subsequently captures a proton ($H^+$) to yield the addition product:

Relative Reactivity Order: $$\mathbf{\text{HCHO} > \text{CH}_3\text{CHO} > \text{CH}_3\text{CH}_2\text{CHO} > \text{CH}_3\text{COCH}_3} > \text{CH}_3\text{COC}_2\text{H}_5 > \text{C}_6\text{H}_5\text{CHO} > \text{C}_6\text{H}_5\text{COCH}_3 > \text{C}_6\text{H}_5\text{COC}_6\text{H}_5$$ This order is governed by two complementary factors:

1. Steric Factors: Formaldehyde has only two tiny hydrogen atoms, allowing unhindered nucleophilic attack. In ketones, two bulky alkyl groups severely crowd the planar carbonyl carbon and destabilize the crowded tetrahedral transition state ($sp^2 \to sp^3$).
2. Electronic Factors: Alkyl groups exert an electron-releasing inductive effect ($+I$). In ketones, two alkyl groups disperse and reduce the partial positive charge ($\delta+$) on the carbonyl carbon to a much greater extent than in aldehydes (one alkyl group), diminishing its electrophilic reactivity.
3. Aromatic Deactivation: In benzaldehyde and acetophenone, conjugation of the carbonyl $\pi$-bond with the aromatic benzene ring ($+R$ resonance) donates electron density into the carbonyl group, neutralizing the electrophilic character of the carbonyl carbon and making aromatic aldehydes less reactive than aliphatic aldehydes.

Carbonyl compounds react with nucleophilic amine derivatives ($ ext{H}_2 ext{N}-Z$) under controlled weak acid catalysis ($ ext{pH} pprox 4.5$) via nucleophilic addition followed by rapid elimination of water: $$\gt ext{C}= ext{O} + ext{H}_2 ext{N}-Z \xrightleftharpoons{ ext{H}^+} [\gt ext{C(OH)-NH}-Z] \xrightarrow{- ext{H}_2 ext{O}} \mathbf{\gt ext{C}= ext{N}-Z}$$

• $Z = - ext{OH}$ (Hydroxylamine, $ ext{NH}_2 ext{OH}$) $\longrightarrow$ Oxime ($\gt ext{C=N-OH}$)
• $Z = - ext{NH}_2$ (Hydrazine, $ ext{NH}_2 ext{NH}_2$) $\longrightarrow$ Hydrazone ($\gt ext{C=N-NH}_2$)
• $Z = - ext{NHC}_6 ext{H}_5$ (Phenylhydrazine) $\longrightarrow$ Phenylhydrazone ($\gt ext{C=N-NHC}_6 ext{H}_5$)
• $Z = - ext{NH-C}_6 ext{H}_3( ext{NO}_2)_2$ (2,4-Dinitrophenylhydrazine / Brady's Reagent) $\longrightarrow$ 2,4-DNP derivative: Bright yellow, orange, or red crystalline solids with sharp melting points, used as the definitive analytical test for detecting the carbonyl group.
• $Z = - ext{NHCONH}_2$ (Semicarbazide, $ ext{H}_2 ext{N-NH-CO-NH}_2$) $\longrightarrow$ Semicarbazone ($\gt ext{C=N-NH-CO-NH}_2$).
  Crucial Examiner Trap: Semicarbazide contains two $- ext{NH}_2$ groups, but only the hydrazine $- ext{NH}_2$ acts as a nucleophile. The amide $- ext{NH}_2$ group is involved in resonance with the adjacent carbonyl group ($- ext{C}(= ext{O})- ext{NH}_2 \leftrightarrow - ext{C}( ext{O}^-)= ext{NH}_2^+$), which delocalizes its lone pair and eliminates its nucleophilicity.

• Clemmensen Reduction: Carbonyl compounds are refluxed with zinc amalgam ($\text{Zn-Hg}$) and concentrated $\text{HCl}$ to deoxygenate the carbonyl group into a methylene group ($>\text{CH}_2$): $$R\text{-CO-}R' + 4[\text{H}] \xrightarrow{\text{Zn-Hg / conc. HCl}} R\text{-CH}_2\text{-}R' + \text{H}_2\text{O}$$ Scope & Limitation: Excellent for acid-stable compounds. Substrates containing acid-sensitive groups (such as $-\text{OH}$, $-\text{OR}$, or $\text{C=C}$) cannot be reduced by Clemmensen reduction because $\text{HCl}$ will protonate or add to them.
• Wolff-Kishner Reduction: Carbonyl compounds are heated with hydrazine ($\text{NH}_2\text{NH}_2$) and potassium hydroxide ($\text{KOH}$) in a high-boiling solvent like ethylene glycol ($453-473\text{ K}$): $$R\text{-CO-}R' + \text{NH}_2\text{NH}_2 \xrightarrow{-\text{H}_2\text{O}} R\text{-C(=N-NH}_2\text{)-}R' \xrightarrow{\text{KOH / ethylene glycol, } \Delta} R\text{-CH}_2\text{-}R' + \mathbf{\text{N}_2\uparrow}$$ Scope & Limitation: Ideal for acid-sensitive substrates. Base-sensitive functional groups (e.g., esters, lactones, or halides undergoing elimination) cannot be used.

1. Tollen's Test (Silver Mirror Test): Ammoniacal silver nitrate solution, $[\text{Ag(NH}_3)_2]^+\text{OH}^-$, is a mild oxidizing agent that readily oxidises both aliphatic and aromatic aldehydes to carboxylate anions while reducing silver(I) to metallic silver, forming a mirror-like coating on clean test-tube walls: $$R\text{CHO} + 2[\text{Ag(NH}_3)_2]^+ + 3\text{OH}^- \xrightarrow{\Delta} R\text{COO}^- + \mathbf{2\text{Ag}\downarrow \text{ (Silver Mirror)}} + 4\text{NH}_3 + 2\text{H}_2\text{O}$$ Ketones do NOT reduce Tollen's reagent.
2. Fehling's Test: Equal volumes of Fehling A (aqueous $\text{CuSO}_4$) and Fehling B (alkaline sodium potassium tartrate / Rochelle salt) are mixed. Aliphatic aldehydes reduce the blue bistartratocuprate(II) complex to an insoluble red precipitate of cuprous oxide ($\text{Cu}_2\text{O}$): $$R\text{CHO} + 2\text{Cu}^{2+} + 5\text{OH}^- \xrightarrow{\Delta} R\text{COO}^- + \mathbf{\text{Cu}_2\text{O}\downarrow \text{ (Red Precipitate)}} + 3\text{H}_2\text{O}$$ Critical Distinction: Aromatic aldehydes (e.g., Benzaldehyde, $\text{C}_6\text{H}_5\text{CHO}$) and ketones do NOT reduce Fehling's solution. This provides a definitive test to distinguish aliphatic aldehydes from benzaldehyde.
3. Iodoform (Haloform) Test: Carbonyl compounds containing a methyl ketone group ($\text{CH}_3\text{C=O}$) or secondary alcohols containing the $\text{CH}_3\text{CH(OH)-}$ unit react with iodine and aqueous sodium hydroxide to yield a bright yellow crystalline precipitate of iodoform ($\text{CHI}_3$, m.p. $119^\circ\text{C}$) with a characteristic medicinal/antiseptic odor: $$\text{CH}_3\text{COR} + 3\text{I}_2 + 4\text{NaOH} \longrightarrow \mathbf{\text{CHI}_3\downarrow \text{ (Yellow)}} + R\text{COONa} + 3\text{NaI} + 3\text{H}_2\text{O}$$ Examples giving positive iodoform test: Acetaldehyde ($\text{CH}_3\text{CHO}$, only aldehyde to give test), Acetone ($\text{CH}_3\text{COCH}_3$), Acetophenone ($\text{C}_6\text{H}_5\text{COCH}_3$), Ethanol, Propan-2-ol, Pentan-2-one.
   Negative: Benzaldehyde, Pentan-3-one, Methanol, Propan-1-ol.

The hydrogen atoms attached to the carbon adjacent to the carbonyl group ($\alpha$-hydrogens) are weakly acidic ($\text{p}K_a \approx 19-20$) because the resulting conjugate carbanion (enolate ion) is resonance-stabilized by delocalization of the negative charge onto the electronegative carbonyl oxygen: $$\text{-CH}_2\text{-C(=O)-} + :\text{B}^- \xrightleftharpoons{} [\text{-C}^-\text{H-C(=O)-} \longleftrightarrow \text{-CH=C(O}^-\text{)-}] + \text{HB}$$

• Self-Aldol Condensation: Aldehydes and ketones possessing at least one $\alpha$-hydrogen undergo self-addition in the presence of dilute base (e.g., $10\%\,\text{NaOH}, \text{Ba(OH)}_2$) to form $\beta$-hydroxyaldehydes (aldols) or $\beta$-hydroxyketones (ketols). Upon heating, these readily dehydrate via an $E1cB$ elimination mechanism to give conjugated $\alpha,\beta$-unsaturated carbonyl compounds: $$2\,\text{CH}_3\text{CHO} \xrightarrow{\text{dil. NaOH}} \text{CH}_3\text{-CH(OH)-CH}_2\text{CHO} \text{ (3-hydroxybutanal)} \xrightarrow{\Delta, -\text{H}_2\text{O}} \mathbf{\text{CH}_3\text{-CH=CH-CHO}} \text{ (But-2-enal / Crotonaldehyde)}$$
• Cross-Aldol Condensation: When aldol condensation is conducted between two different carbonyl compounds both possessing $\alpha$-hydrogens, a complex mixture of four condensation products is obtained. However, if one carbonyl partner lacks $\alpha$-hydrogens (e.g., benzaldehyde or formaldehyde), it acts exclusively as the electrophilic acceptor, yielding a single predominant cross-product (Claisen-Schmidt Condensation): $$\text{C}_6\text{H}_5\text{CHO} + \text{CH}_3\text{CHO} \xrightarrow{\text{dil. NaOH, } 293\text{ K}} \text{C}_6\text{H}_5\text{-CH(OH)-CH}_2\text{CHO} \xrightarrow{\Delta, -\text{H}_2\text{O}} \mathbf{\text{C}_6\text{H}_5\text{-CH=CH-CHO}} \text{ (Cinnamaldehyde)}$$

Aldehydes that completely lack $lpha$-hydrogens (e.g., $ ext{HCHO}, ext{C}_6 ext{H}_5 ext{CHO}, ( ext{CH}_3)_3 ext{C-CHO}$, furfural) cannot form enolates. When heated with concentrated alkali ($50\%\, ext{NaOH}$ or $ ext{KOH}$), they undergo intermolecular self-oxidation-reduction (disproportionation), in which one molecule is reduced to an alcohol while the second is oxidised to a carboxylic acid salt:

• Self-Cannizzaro: $$2\,\text{HCHO} + \text{conc. NaOH} \xrightarrow{\Delta} \mathbf{\text{CH}_3\text{OH}} \text{ (Methanol)} + \mathbf{\text{HCOONa}} \text{ (Sodium Formate)}$$ $$2\,\text{C}_6\text{H}_5\text{CHO} + \text{conc. KOH} \xrightarrow{\Delta} \mathbf{\text{C}_6\text{H}_5\text{CH}_2\text{OH}} \text{ (Benzyl alcohol)} + \mathbf{\text{C}_6\text{H}_5\text{COOK}} \text{ (Potassium Benzoate)}$$ Mechanism Highlight: Rapid nucleophilic attack of $ ext{OH}^-$ on carbonyl carbon forms a tetrahedral dianion, which transfers a hydride ion ($: ext{H}^-$) directly to the carbonyl carbon of a second aldehyde molecule in the rate-determining step.
• Cross-Cannizzaro Reaction: When an equimolar mixture of benzaldehyde and formaldehyde is treated with concentrated $ ext{NaOH}$, formaldehyde is oxidised exclusively to formic acid while benzaldehyde is reduced to benzyl alcohol: $$\text{C}_6\text{H}_5\text{CHO} + \text{HCHO} + \text{conc. NaOH} \longrightarrow \mathbf{\text{C}_6\text{H}_5\text{CH}_2\text{OH}} + \mathbf{\text{HCOONa}}$$ Rationale: Formaldehyde has no electron-donating aryl ring and is sterically unhindered; hence, nucleophilic attack of $ ext{OH}^-$ occurs far more rapidly on formaldehyde, generating the hydride-donating intermediate.

The carboxyl group consists of a carbonyl group attached directly to a hydroxyl group:

• The carboxyl carbon and both oxygens are $sp^2$ hybridized, creating a coplanar array of atoms with bond angles of approximately $120^\circ$.
• The unshared electron pair on the hydroxyl oxygen overlaps with the carbonyl $\pi$-system: $$-C(=O)-OH \longleftrightarrow -C(O^-)=OH^+$$ This resonance interaction decreases the electrophilicity of the carbonyl carbon compared to aldehydes and ketones; consequently, carboxylic acids do not undergo typical nucleophilic addition reactions characteristic of aldehydes and ketones (they do not form bisulfite adducts or oximes).

1. Vigorous Oxidation of Primary Alcohols & Aldehydes: $$R\text{CH}_2\text{OH} \xrightarrow{\text{KMnO}_4 / \text{KOH, } \Delta} R\text{COOK} \xrightarrow{\text{H}_3\text{O}^+} \mathbf{R\text{COOH}}$$ Acidified potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4$) or Jones reagent ($\text{CrO}_3 / \text{H}_2\text{SO}_4$) gives identical conversions.
2. Oxidation of Alkylbenzenes: Vigorous heating of alkylbenzenes with alkaline $\text{KMnO}_4$ oxidises the entire side chain into a carboxyl group, irrespective of the chain length, yielding benzoic acid: $$\text{C}_6\text{H}_5\text{-CH}_3 \xrightarrow{\text{(i) alkaline KMnO}_4, \Delta, \text{ (ii) }\text{H}_3\text{O}^+} \mathbf{\text{C}_6\text{H}_5\text{COOH}} \text{ (Benzoic Acid)}$$ $$\text{C}_6\text{H}_5\text{-CH}_2\text{CH}_2\text{CH}_3 \xrightarrow{\text{(i) alkaline KMnO}_4, \Delta, \text{ (ii) }\text{H}_3\text{O}^+} \mathbf{\text{C}_6\text{H}_5\text{COOH}} + 2\,\text{CO}_2 + 3\,\text{H}_2\text{O}$$ Critical Rule: At least one benzylic hydrogen atom must be present on the $lpha$-carbon. Tert-butylbenzene, $\text{C}_6\text{H}_5\text{-C(CH}_3)_3$, has no benzylic hydrogen and completely resists $\text{KMnO}_4$ oxidation!
3. Hydrolysis of Nitriles & Amides: Nitriles undergo stepwise acid- or base-catalyzed hydrolysis through an amide intermediate to carboxylic acids: $$R\text{-C}\equiv\text{N} \xrightarrow{\text{H}_2\text{O} / \text{H}^+} R\text{CONH}_2 \xrightarrow{\text{H}_2\text{O} / \text{H}^+, \Delta} \mathbf{R\text{COOH}} + \text{NH}_4^+$$
4. Carbonation of Grignard Reagents: Bubbling carbon dioxide through an ethereal solution of a Grignard reagent followed by acid hydrolysis provides a powerful method to ascend the homologous series by one carbon atom: $$R\text{Mg}X + \text{O=C=O} \xrightarrow{\text{dry ether}} R\text{-COO}^-\text{Mg}X^+ \xrightarrow{\text{H}_3\text{O}^+} \mathbf{R\text{COOH}} + \text{Mg(OH)}X$$
5. Hydrolysis of Acid Derivatives: Acyl chlorides, acid anhydrides, and esters hydrolyze readily to carboxylic acids: $$R\text{COCl} + \text{H}_2\text{O} \longrightarrow R\text{COOH} + \text{HCl}$$ $$(R\text{CO})_2\text{O} + \text{H}_2\text{O} \longrightarrow 2\,R\text{COOH}$$ $$R\text{COOR}' + \text{H}_2\text{O} \xrightleftharpoons{\text{H}^+} R\text{COOH} + R'\text{OH} \quad (\text{Fischer hydrolysis})$$ $$\text{Alkaline hydrolysis (Saponification)}: R\text{COOR}' + \text{NaOH} \longrightarrow R\text{COONa} + R'\text{OH} \xrightarrow{\text{H}^+} R\text{COOH}$$

Carboxylic acids ionise in water to establish an equilibrium with hydronium ions and carboxylate anions: $$R\text{COOH} + \text{H}_2\text{O} \xrightleftharpoons{} R\text{COO}^- + \text{H}_3\text{O}^+ \qquad K_a = \frac{[R\text{COO}^-][\text{H}_3\text{O}^+]}{[R\text{COOH}]}, \quad \text{p}K_a = -\log_{10} K_a$$ The smaller the $\text{p}K_a$, the stronger the acid.

• Electron-Withdrawing Groups (EWGs, $-I, -M$): Disperse the negative charge of the carboxylate anion by inductive electron withdrawal, stabilizing the conjugate base and shifting the ionisation equilibrium to the right $\implies$ Strongly Increase Acidity (lower $ ext{p}K_a$). $$\text{EWG Power}: -\text{CF}_3 > -\text{NO}_2 > -\text{CN} > -\text{F} > -\text{Cl} > -\text{Br} > -\text{I} > -\text{Ph}$$ $$\mathbf{\text{CF}_3\text{COOH} > \text{CCl}_3\text{COOH} > \text{CHCl}_2\text{COOH} > \text{CH}_2\text{ClCOOH} > \text{CH}_2\text{BrCOOH} > \text{CH}_2\text{ICOOH} > \text{CH}_3\text{COOH}}$$ $(\text{p}K_a: \text{CF}_3\text{COOH} = 0.23, \text{CCl}_3\text{COOH} = 0.65, \text{CH}_2\text{ClCOOH} = 2.87, \text{CH}_3\text{COOH} = 4.76)$
• Distance Dependence of Inductive Effect: As the distance between the halogen and the carboxyl group increases, the $-I$ stabilizing effect diminishes rapidly: $$\mathbf{\text{CH}_3\text{CH}_2\text{CH(Cl)COOH} > \text{CH}_3\text{CH(Cl)CH}_2\text{COOH} > \text{CH}_2\text{(Cl)CH}_2\text{CH}_2\text{COOH} > \text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}}$$ $(\text{2-chlorobutanoic} > \text{3-chlorobutanoic} > \text{4-chlorobutanoic} > \text{butanoic acid})$
• Electron-Donating Groups (EDGs, $+I$): Alkyl groups push electron density toward the carboxylate group, intensifying the negative charge on oxygen, destabilizing the anion, and decreasing acidity: $$\mathbf{\text{HCOOH} > \text{CH}_3\text{COOH} > \text{CH}_3\text{CH}_2\text{COOH} > (\text{CH}_3)_2\text{CHCOOH}}$$

The Ortho-Effect: Nearly all ortho-substituted benzoic acids are distinctly stronger acids than benzoic acid itself, regardless of whether the ortho-substituent is electron-withdrawing ($- ext{NO}_2, - ext{Cl}$) or electron-donating ($- ext{CH}_3, - ext{OCH}_3$)!

• Acid Anhydride Formation: Heating carboxylic acids with phosphorus pentoxide ($\text{P}_2\text{O}_5$) or conc. $\text{H}_2\text{SO}_4$ causes bimolecular dehydration: $$2\,R\text{COOH} \xrightarrow{\text{P}_2\text{O}_5, \Delta} (R\text{CO})_2\text{O} + \text{H}_2\text{O}$$
• Fischer Esterification: Carboxylic acids react with alcohols in the presence of an inorganic acid catalyst (conc. $\text{H}_2\text{SO}_4$ or dry $\text{HCl}$ gas): $$R\text{COOH} + R'\text{OH} \xrightleftharpoons{\text{conc. H}_2\text{SO}_4} R\text{COOR}' + \text{H}_2\text{O}$$ Mechanism: Nucleophilic acyl substitution. Isotopic tracer experiments with $\text{O}^{18}$-labelled alcohol ($R'\text{O}^{18}\text{H}$) prove that the ester oxygen originates exclusively from the alcohol molecule, while the $\text{-OH}$ is cleaved from the carboxylic acid.
• Acyl Chloride Formation: Reaction with $\text{PCl}_5, \text{PCl}_3$, or thionyl chloride ($\text{SOCl}_2$): $$R\text{COOH} + \text{SOCl}_2 \longrightarrow \mathbf{R\text{COCl}} + \mathbf{\text{SO}_2\uparrow} + \mathbf{\text{HCl}\uparrow}$$ Why $\text{SOCl}_2$ is preferred: Both byproducts ($\text{SO}_2$ and $\text{HCl}$) are volatile gases that escape from the reaction mixture, leaving the acyl chloride in near-quantitative purity without demanding fractional distillation.
• Amide Formation & Phthalimide Synthesis: Reaction with ammonia yields ammonium salts, which on heating eliminate water to form amides: $$R\text{COOH} + \text{NH}_3 \xrightleftharpoons{} R\text{COO}^-\text{NH}_4^+ \xrightarrow{\Delta} \mathbf{R\text{CONH}_2} + \text{H}_2\text{O}$$ With phthalic acid, reaction with ammonia followed by strong heating gives phthalimide: $$\text{C}_6\text{H}_4(\text{COOH})_2 \xrightarrow{+2\text{NH}_3} \text{C}_6\text{H}_4(\text{COONH}_4)_2 \xrightarrow{\Delta, -2\text{H}_2\text{O}} \text{C}_6\text{H}_4(\text{CONH}_2)_2 \xrightarrow{\text{strong }\Delta, -\text{NH}_3} \mathbf{\text{Phthalimide}}$$

1. Soda Lime Decarboxylation: Sodium salts of carboxylic acids heated with soda lime (a $3:1$ mixture of $\text{NaOH}$ and $\text{CaO}$) eliminate carbon dioxide to form an alkane with one fewer carbon atom than the parent acid: $$R\text{COONa} + \text{NaOH} \xrightarrow{\text{CaO}, \Delta} \mathbf{R\text{-H}} + \text{Na}_2\text{CO}_3$$ $$\text{CH}_3\text{COONa} + \text{NaOH} \xrightarrow{\text{CaO}, \Delta} \mathbf{\text{CH}_4\uparrow} + \text{Na}_2\text{CO}_3$$ Role of $\text{CaO}$: Keeps $\text{NaOH}$ dry (hygroscopic protection) and raises the fusion temperature, preventing the glass test tube from melting.
2. Kolbe's Electrolysis: Electrolysis of an aqueous solution of sodium or potassium salt of a carboxylic acid yields a symmetrical alkane with an even number of carbon atoms at the anode: $$2\,R\text{COOK} + 2\,\text{H}_2\text{O} \xrightarrow{\text{electrolysis}} \mathbf{R-R} + 2\,\text{CO}_2\uparrow + \text{H}_2\uparrow + 2\,\text{KOH}$$
3. Reduction to Primary Alcohols: Carboxylic acids are reduced cleanly to primary alcohols using lithium aluminium hydride ($\text{LiAlH}_4$) or diborane ($\text{B}_2\text{H}_6$): $$R\text{COOH} \xrightarrow{\text{(i) LiAlH}_4 / \text{ether, (ii) }\text{H}_3\text{O}^+} R\text{CH}_2\text{OH}$$ Crucial Note: $\text{NaBH}_4$ does NOT reduce carboxylic acids or esters. Diborane is often preferred as it does not reduce other functional groups like nitro or ester.

Carboxylic acids possessing $lpha$-hydrogens react smoothly with chlorine or bromine in the presence of a catalytic amount of red phosphorus ($ ext{red P}$), followed by hydrolysis, to yield $lpha$-halocarboxylic acids: $$R\text{-CH}_2\text{-COOH} \xrightarrow{\text{(i) }\text{X}_2 / \text{red P, (ii) }\text{H}_2\text{O}} \mathbf{R\text{-CH(X)-COOH}} \quad (\text{X} = \text{Cl}, \text{Br})$$ $$\text{CH}_3\text{COOH} \xrightarrow{\text{(i) }\text{Cl}_2 / \text{red P, (ii) }\text{H}_2\text{O}} \mathbf{\text{CH}_2\text{ClCOOH}} \xrightarrow{\text{excess }\text{Cl}_2} \text{CHCl}_2\text{COOH} \longrightarrow \text{CCl}_3\text{COOH}$$ Limitations: Formic acid ($\text{HCOOH}$) and benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$) have no $lpha$-hydrogens and do not undergo the HVZ reaction.

The carboxyl group ($- ext{COOH}$) is strongly deactivating and meta-directing due to its $-M$ and $-I$ electron-withdrawing effects:

• Nitration: $ ext{C}_6 ext{H}_5 ext{COOH} + ext{conc. HNO}_3 / ext{conc. H}_2 ext{SO}_4 \xrightarrow{\Delta} \mathbf{m ext{-nitrobenzoic acid}}$
• Bromination: $ ext{C}_6 ext{H}_5 ext{COOH} + ext{Br}_2 / ext{FeBr}_3 \xrightarrow{\Delta} \mathbf{m ext{-bromobenzoic acid}}$
• No Friedel-Crafts Reaction: Benzoic acid does NOT undergo Friedel-Crafts alkylation or acylation because the strongly deactivating $- ext{COOH}$ group complexes with and poisons the Lewis acid catalyst ($ ext{AlCl}_3$).