Revision: Carbonyl Compounds and Carboxylic Acids Chemistry HSC Science Class 12 Tamil Nadu Board of Secondary Education

In IUPAC system in aldehyde the suffix ‘e’ of alkane is replaced by ‘al’, e.g.,
CH₃—CH₂—CH=O; Propanal

\[ \underset{\text{2-methylpropanal}}{\mathrm{CH}_3 - \underset{\underset{\displaystyle \mathrm{CH}_3}{|}}{\mathrm{CH}} - \mathrm{CH} = \mathrm{O}} \]

In ketones, the suffix ‘e’ of alkane is replaced by ‘one’.

For example,

\[ \underset{\text{Butan-2-one}}{\mathrm{CH}_3 - \mathrm{CH}_2 - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{CH}_3} \quad \quad \underset{\text{Propanone (Acetone)}}{\mathrm{CH}_3\mathrm{COCH}_3} \]

• Carbon is sp² hybridised (trigonal planar, bond angle ≈ 120°).
• C = O bond consists of one σ bond and one π bond.
• Oxygen is more electronegative, so it pulls electron density towards itself →
  C gets a partial positive charge (δ⁺) and O gets a partial negative charge (δ⁻).
• This makes the carbonyl group polar.
• Hence, the carbon atom becomes electrophilic and is susceptible to nucleophilic attack.

Preparation of aliphatic aldehydes and ketones 

By oxidation of alcohols:

\[\ \begin{array}{r@{\;}c@{\;}l} \mathrm{R} & & \\ & \backslash & \\ & & \mathrm{CH}-\mathrm{OH} \\ & / & \\ \mathrm{R}' & & \end{array} + [\mathrm{O}] \xrightarrow[\text{Or KMnO}_4]{\text{K}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4} \begin{array}{r@{\;}c@{\;}l} \mathrm{R} & & \\ & \backslash & \\ & & \mathrm{C}=\mathrm{O} \\ & / & \\ \mathrm{R}' & & \end{array} + \mathrm{H}_2\mathrm{O}\]

When,

• R' = H then 1° alcohol to aldehyde. 
• R' = alkyl group then 2º alcohol to ketone.

By dehydrogenation of alcohols:

\[ \begin{array}{r@{\;}c@{\;}l} \mathrm{R} & & \\ & \backslash & \\ & & \mathrm{CH}-\mathrm{OH} \\ & / & \\ \mathrm{R}' & & \end{array} \xrightarrow[\text{573 K}]{\text{Cu}} \begin{array}{r@{\;}c@{\;}l} \mathrm{R} & & \\ & \backslash & \\ & & \mathrm{C}=\mathrm{O} \\ & / & \\ \mathrm{R}' & & \end{array} + \mathrm{H}_2 \]

When

• R' = H then 1° alcohol to aldehyde.
• R' = alkyl group the 2° alcohol to ketone.

By acid chloride:

\[ \mathrm{R} - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{Cl} + \mathrm{H}_2 \xrightarrow[\text{Rosenmund Reduction}]{\text{Pd}-\text{BaSO}_4} \underset{\text{Aldehyde}}{\mathrm{R} - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{H}} + \mathrm{HCl} \]

2RMgX + CdCl₂ → R₂Cd + 2MgXCl

\[\ 2\mathrm{R}' - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{Cl} + \mathrm{R}_2\mathrm{Cd} \longrightarrow 2\mathrm{R}' - \underset{\text{Ketone}}{\overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}}} - \mathrm{R} + \mathrm{CdCl}_2 \]

From nitriles and esters:

\[ \mathrm{R} - \mathrm{CN} \xrightarrow[\text{(ii) }\mathrm{H}_2\mathrm{O}]{\text{(i) }\mathrm{AlH}(i\text{Bu})_2} \underset{\text{Aldehyde}}{\mathrm{R} - \mathrm{CHO}} \]

\[ \mathrm{CH}_3(\mathrm{CH}_2)_9 - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{OC}_2\mathrm{H}_5 \xrightarrow[\text{(ii) }\mathrm{H}_2\mathrm{O}]{\text{(i) DIBAL-H}} \mathrm{CH}_3(\mathrm{CH}_2)_9 - \underset{\text{Aldehyde}}{\overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{H}} \]

From hydrocarbons

By ozonolysis:

By hydration:

• Most aldehydes are liquids (except HCHO = gas); ketones of lower order are colourless liquids with a pleasant odour.
• Higher BP than corresponding hydrocarbons but lower than alcohols (no H-bonding between molecules, but dipole-dipole interactions).
• Lower members are soluble in water (H-bonding with water); higher members are insoluble (large alkyl groups).

• Oxidation: Aldehydes → RCOOH (same C) with dil. HNO₃/KMnO₄/K₂Cr₂O₇; Ketones undergo C–C cleavage with CrO₃, giving two carboxylic acids.
• Reduction: Clemmensen (Zn-Hg/conc. HCl) and Wolf-Kishner (NH₂-NH₂ then KOH/HOCH₂CH₂OH, Δ) both convert >C=O to >CH₂.
• Electrophilic Substitution: –CHO is EWG (inductive + resonance); deactivates ring; directs electrophile (NO₂⁺) to meta position.
• Lab Tests: Schiff (pink), Tollens (silver mirror), Fehling (red Cu₂O ppt) — for aldehydes; Sodium nitroprusside (red) — for ketones.
• Nucleophilic Reactions: HCN → cyanohydrin; NaHSO₃ → bisulphite adduct; ROH/dry HCl → hemiacetal/acetal/ketal; NaOI → haloform (CHI₃); dil. NaOH → Aldol condensation; No α-H + NaOH → Cannizzaro reaction.

• Formaldehyde: Used in making Bakelite (phenol-formaldehyde resin), as a preservative (formalin = 40% HCHO).
• Acetaldehyde: Used in the preparation of acetic acid and ethanol.
• Acetone: Solvent (nail polish remover), used in the manufacture of chloroform.
• Benzaldehyde: Used in perfumes and dyes.

• Solubility: Decreases with an increase in the size of the hydrocarbon part.
• Miscibility: Lower carboxylic acids (up to 4 C atoms) are miscible with water due to H-bonding.
• Boiling point: Carboxylic acids have higher B.P. than ketones, aldehydes, and alcohols of comparable molecular mass due to intermolecular H-bonding.
• Order of B.P. (carboxylic acids & aldehydes): Valeric > Butyric > Propionic > Acetic > Formic acid; Hexanal > Pentanal > Butanal > Propanal.
• Order of B.P. (ketones): Hexan-2-one > Pentan-2-one > Butan-2-one > Propanone.

• Acid strength order: Greater halogen electronegativity → stronger acid, smaller pKₐ; F–CH₂COOH > Cl–CH₂COOH > Br–CH₂COOH > I–CH₂COOH > CH₃COOH.
• Substituent effect: EWG (–Cl, –CN, –NO₂) ↑ acidity; EDG (–CH₃, –OH, –OCH₃) ↓ acidity; 4-nitrobenzoic acid > Benzoic acid > 4-methyl benzoic acid.
• Tests for –COOH: Litmus (blue → red), NaHCO₃ (CO₂↑), Ester test (with C₂H₅OH/H⁺ gives ester).
• Key conversions: SOCl₂/PCl₃/PCl₅ → acyl chloride; NH₃ (Δ) → amide; P₂O₅ (Δ) → acid anhydride.
• Decarboxylation & reduction: RCOONa + NaOH/CaO (Δ) → R–H + Na₂CO₃; RCOOH + LiAlH₄/dry ether → RCH₂OH.