Chemistry: Class 12 — Chapter 3: Aldehydes, Ketones and Carboxylic Acids

The carbonyl carbon atom is sp2-hybridised and forms three sigma ($\\sigma$) bonds. The fourth valence electron of carbon remains in its p-orbital and forms a $\\pi$-bond with oxygen by overlap with a p-orbital of oxygen. The oxygen atom also has two non-bonding electron pairs. The bond angles are approximately $120^\\circ$, forming a trigonal coplanar structure.

The carbon-oxygen double bond is highly polarised due to the higher electronegativity of oxygen relative to carbon. Hence, the carbonyl carbon is an electrophilic (Lewis acid) centre, and the carbonyl oxygen is a nucleophilic (Lewis base) centre.

• Aldehydes: In the IUPAC system, open-chain aliphatic aldehydes are named by replacing the ending -e of the corresponding alkane with -al. The longest carbon chain is numbered starting from the carbonyl carbon. When the aldehyde group is attached to a ring, the suffix carbaldehyde is added after the full name of the cycloalkane.
• Ketones: IUPAC names of ketones are derived by replacing the ending -e of the corresponding alkane with -one. Numbering begins from the end nearer to the carbonyl group.

1. By oxidation of alcohols: Primary alcohols oxidise to aldehydes, while secondary alcohols oxidise to ketones.
2. By dehydrogenation of alcohols: Volatile alcohol vapours are passed over heavy metal catalysts like $Ag$ or $Cu$ at high temperatures.
3. From hydrocarbons:
   • Ozonolysis of alkenes followed by reaction with zinc dust and water.
   • Hydration of alkynes: Addition of water to ethyne in the presence of $H_2SO_4$ and $HgSO_4$ yields acetaldehyde; other alkynes yield ketones.

1. From acyl chloride (Rosenmund Reduction): Acyl chloride is hydrogenated over a catalyst of palladium on barium sulphate ($Pd/BaSO_4$).$$RCOCl + H_2 \\xrightarrow{Pd/BaSO_4} RCHO + HCl$$
2. From nitriles and esters (Stephen Reaction): Nitriles are reduced to corresponding imines with stannous chloride ($SnCl_2$) in the presence of hydrochloric acid ($HCl$), which on hydrolysis give aldehydes.$$RCN + SnCl_2 + HCl \\rightarrow RCH=NH \\xrightarrow{H_3O^+} RCHO$$Alternatively, nitriles and esters are selectively reduced by diisobutylaluminium hydride (DIBAL-H) to imines followed by hydrolysis to aldehydes.
3. From hydrocarbons (Aromatic Aldehydes):
   • Etard Reaction: Chromyl chloride ($CrO_2Cl_2$) oxidises the methyl group of toluene to a chromium complex, which on hydrolysis gives benzaldehyde.
   • Use of chromic oxide ($CrO_3$): Toluene or substituted toluene is converted to benzylidene diacetate using chromic oxide in acetic anhydride, followed by acid hydrolysis.
   • Side-chain chlorination followed by hydrolysis: Industrial method involving chlorination of toluene to benzal chloride, followed by hydrolysis at $373\\text{ K}$.
   • Gatterman-Koch Reaction: Benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride ($AlCl_3$) or cuprous chloride ($CuCl$).$$C_6H_6 + CO + HCl \\xrightarrow{\\text{Anhyd. } AlCl_3/CuCl} C_6H_5CHO$$

1. From acyl chlorides: Treatment of acyl chlorides with dialkylcadmium (prepared from Grignard reagent and cadmium chloride) yields ketones.$$2R'COCl + R_2Cd \\rightarrow 2R'COR + CdCl_2$$
2. From nitriles: Treating a nitrile with a Grignard reagent followed by hydrolysis yields a ketone.
3. Friedel-Crafts Acylation: Treatment of benzene or substituted benzene with an acid chloride in the presence of anhydrous aluminium chloride ($AlCl_3$) yields an acylarene (ketone).

The polar carbonyl group undergoes nucleophilic addition. A nucleophile attacks the electrophilic carbonyl carbon from a direction perpendicular to the plane of $sp2$ hybridised orbitals, changing the hybridisation of carbon to $sp3$ and forming a tetrahedral alkoxide intermediate, which then captures a proton.

• Reactivity: Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to:
  1. Steric reasons: The presence of two relatively large substituents in ketones hinders the approach of the nucleophile.
  2. Electronic reasons: Two electron-donating alkyl groups in ketones reduce the electrophilicity of the carbonyl carbon more effectively than in aldehydes.
• Important Examples:
  • Addition of $HCN$: Yields cyanohydrins. The reaction is catalysed by a base to generate the stronger nucleophile $CN^-$.
  • Addition of sodium hydrogensulphite ($NaHSO_3$): Yields bisulphite addition compounds which are water-soluble and can be converted back to the original carbonyl compound, useful for purification.
  • Addition of Grignard reagents: Yields alcohols.
  • Addition of alcohols: Aldehydes react with one equivalent of monohydric alcohol in the presence of dry $HCl$ to yield hemiacetals, and with a second equivalent to yield acetals. Ketones react with ethylene glycol to form cyclic ethylene glycol ketals.
  • Addition of ammonia and its derivatives ($H_2N-Z$): Reactions are reversible and acid-catalysed, forming derivatives containing $>C=N-Z$ bonds (e.g., imines, oximes, hydrazones, semicarbazones).

• Reduction to alcohols: Reduced to primary (from aldehydes) or secondary (from ketones) alcohols using sodium borohydride ($NaBH_4$) or lithium aluminium hydride ($LiAlH_4$), or by catalytic hydrogenation.
• Reduction to hydrocarbons:
  • Clemmensen Reduction: Carbonyl group is reduced to a $-CH_2-$ group using zinc amalgam and concentrated hydrochloric acid ($Zn-Hg/HCl$).$$>C=O \\xrightarrow{Zn-Hg/HCl} >CH_2 + H_2O$$
  • Wolff-Kishner Reduction: Carbonyl group is reduced to a $-CH_2-$ group by heating with hydrazine ($NH_2NH_2$) followed by potassium hydroxide ($KOH$) in a high-boiling solvent like ethylene glycol.$$>C=O \\xrightarrow{NH_2NH_2} >C=N-NH_2 \\xrightarrow{KOH/\\text{glycol}, \\Delta} >CH_2 + N_2$$

• Aldehydes are easily oxidised to carboxylic acids even by mild oxidising agents like Tollens' reagent and Fehling's reagent. Ketones require vigorous conditions and strong oxidising agents, undergoing C-C bond cleavage.
• Tollens' Test: Warming an aldehyde with freshly prepared ammoniacal silver nitrate solution produces a bright silver mirror due to the reduction of $Ag^+$ to metallic silver.
• Fehling's Test: Heating an aldehyde with Fehling's reagent (mixture of Fehling A: aqueous $CuSO_4$ and Fehling B: alkaline sodium potassium tartarate) produces a red-brown precipitate of cuprous oxide ($Cu_2O$). Aromatic aldehydes do not respond to this test.
• Haloform Reaction: Methyl ketones are oxidised by sodium hypohalite ($NaOX$) to sodium salts of corresponding carboxylic acids (having one carbon atom less) and haloform ($CHX_3$).

• Acidity of $\\alpha$-hydrogens: The $\\alpha$-hydrogen atoms of carbonyl compounds are acidic due to the strong electron-withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.
• Aldol Condensation: Aldehydes and ketones having at least one $\\alpha$-hydrogen react in the presence of dilute alkali to form $\\beta$-hydroxy aldehydes (aldols) or $\\beta$-hydroxy ketones (ketols), which readily lose water to yield $\\alpha,\\beta$-unsaturated carbonyl compounds.
• Cross Aldol Condensation: When aldol condensation is carried out between two different aldehydes and/or ketones, it yields a mixture of four products if both reactants contain $\\alpha$-hydrogens.

• Cannizzaro Reaction: Aldehydes which do not have an $\\alpha$-hydrogen atom (e.g., formaldehyde, benzaldehyde) undergo self-oxidation and reduction (disproportionation) on heating with concentrated alkali, yielding an alcohol and a salt of a carboxylic acid.
• Electrophilic Substitution Reaction: Aromatic aldehydes and ketones undergo electrophilic substitution at the meta-position, as the carbonyl group acts as a deactivating and meta-directing group.

• Nomenclature: In the IUPAC system, aliphatic carboxylic acids are named by replacing the ending -e of the corresponding alkane with -oic acid.
• Structure of Carboxyl Group: The bonds to the carboxyl carbon lie in one plane separated by about $120^\\circ$. The carboxylic carbon is less electrophilic than the carbonyl carbon because of resonance stabilisation of the carboxyl group.

1. From primary alcohols and aldehydes: Using strong oxidising agents like potassium permanganate ($KMnO_4$) or Jones reagent (chromium trioxide in acidic media).
2. From alkylbenzenes: Vigorous oxidation of alkylbenzenes with acidic or alkaline $KMnO_4$ oxidises the entire side chain to a carboxyl group, regardless of chain length.
3. From nitriles and amides: Nitriles are hydrolysed to amides and then to acids in the presence of $H^+$ or $OH^-$ as catalyst.
4. From Grignard reagents: Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids, which on acidification yield carboxylic acids.
5. From acyl halides and anhydrides: Acid chlorides hydrolyse with water to give carboxylic acids. Anhydrides hydrolyse to yield corresponding acids.
6. From esters: Acidic hydrolysis of esters gives carboxylic acids directly; basic hydrolysis gives carboxylates, which yield acids upon acidification.

• Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids with unpleasant odours. Higher acids are wax-like solids.
• They have higher boiling points than aldehydes, ketones, and even alcohols of comparable molecular masses due to extensive association through intermolecular hydrogen bonding, forming stable cyclic dimers in the vapour phase and in aprotic solvents.
• Lower aliphatic carboxylic acids (up to four carbon atoms) are miscible in water. Solubility decreases with increasing carbon chain length. Benzoic acid is nearly insoluble in cold water.

• Acidity: Carboxylic acids are stronger acids than alcohols and phenols. The conjugate base (carboxylate ion) is stabilised by two equivalent resonance structures where the negative charge is delocalised over two highly electronegative oxygen atoms.
  • Effect of Substituents: Electron-withdrawing groups (EWG) increase acidity by stabilising the carboxylate anion. Electron-donating groups (EDG) decrease acidity by destabilising it.
  • Acidity order: $CF_3COOH > CCl_3COOH > CHCl_2COOH > NO_2CH_2COOH > NC-CH_2COOH > FCH_2COOH > ClCH_2COOH > HCOOH$
• Reactions Involving Cleavage of O-H Bond: Carboxylic acids react with metals to evolve hydrogen gas, and with alkalies, carbonates, and hydrogencarbonates to form salts and evolve carbon dioxide.
• Reactions Involving Cleavage of C-OH Bond:
  • Anhydride formation: Heating with mineral acids ($H_2SO_4$) or phosphorus pentoxide ($P_2O_5$) yields anhydrides.
  • Esterification: Reacting with alcohols or phenols in the presence of concentrated $H_2SO_4$ or $HCl$ gas.
  • Reactions with $PCl_5, PCl_3,$ and $SOCl_2$: Hydroxyl group is replaced by chlorine to form acyl chlorides. Thionyl chloride ($SOCl_2$) is preferred because the byproducts ($SO_2$ and $HCl$) are gases.
  • Reaction with ammonia: Forms ammonium salts which on heating lose water to yield amides.
• Reactions Involving -COOH Group:
  • Reduction: Reduced to primary alcohols using lithium aluminium hydride ($LiAlH_4$) or diborane ($B_2H_6$). Sodium borohydride ($NaBH_4$) does not reduce the carboxyl group.
  • Decarboxylation: Sodium salts of carboxylic acids lose carbon dioxide to form hydrocarbons when heated with sodalime ($NaOH$ and $CaO$ in a 3:1 ratio).
• Substitution Reactions in Hydrocarbon Part:
  • Halogenation (Hell-Volhard-Zelinsky Reaction): Carboxylic acids with an $\\alpha$-hydrogen are halogenated at the $\\alpha$-position by treatment with chlorine or bromine in the presence of a small amount of red phosphorus.$$R-CH_2-COOH \\xrightarrow{\\text{(i) } X_2/\\text{Red P, (ii) } H_2O} R-CH(X)-COOH \\quad (X = Cl, Br)$$
  • Ring Substitution: Aromatic carboxylic acids undergo electrophilic substitution at the meta-position. They do not undergo Friedel-Crafts reaction because the carboxyl group is deactivating and the catalyst $AlCl_3$ bonds to the carboxyl group.