Revision: Aldehydes, Ketones and Carboxylic Acids Chemistry Science (English Medium) Class 12 CBSE

Organic compounds containing carbon-oxygen double bond, i.e. \[\mathrm{>C=O}\] group, are known as carbonyl compounds.

The organic compounds in which the –OH group of a carboxylic acid is replaced by a halogen atom are called acyl halides.

The organic compounds in which the –OH group of a carboxylic acid is replaced by an –OR group are called esters.

The organic compounds formed by removal of one molecule of water from two molecules of carboxylic acid are called acid anhydrides.

The organic compounds in which the –OH group of a carboxylic acid is replaced by –NH₂ or substituted amino group are called amides.

The carbon–oxygen double bond (>C=O) functional group present in aldehydes, ketones and acids is called carbonyl group.

R–COX

R–COOR′

(R–CO)₂O

R–CONH₂

\[2CH_3CHO\xrightarrow{dil.NaOH}CH_3CH(OH)CH_2CHO\]

On heating:

→CH₃​CH = CHCHO + H₂​O

2RCHO + OH⁻ → RCOO⁻ + RCH₂​OH

\[RCOOH+R^{\prime}OH\xrightarrow{H^+}RCOOR^{\prime}+H_2O\]

\[RCHO\xrightarrow{NaBH_4}RCH_2OH\]

\[RCOOH\xrightarrow{LiAlH_4}RCH_2OH\]

\[RCOONa+NaOH\xrightarrow{CaO}RH+Na_2CO_3\]

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).

Addition of HCN:

\[\text{HCN} + \text{OH}^- \rightleftharpoons :\text{CN}^- + \text{H}_2\text{O}\]

Addition of NaHSO₃:

Addition of Grignard reagent:

Addition of alcohols:

\[\text{R}-\text{CHO} + 2[\text{H}] \xrightarrow[\text{or BH}_3]{\text{LiAlH}_4} \text{RCH}_2\text{OH}\]

\[\begin{array}{r@{\;}c@{\;}l r@{\;}c@{\;}l} \ce{R} & & & \ce{R} & & \\ & \backslash & & & \backslash & \\ & & \ce{C=O + H2 ->[Ni or Pt]} & & & \ce{CH2OH} \\ & / & & & / & \\ \ce{R} & & & \ce{R} & & \end{array}\]

Clemmensen reduction:

\begin{array}{r@{\;}c@{\;}l c r@{\;}c@{\;}l} & & & & & & \\ & \backslash & & & & \backslash & \\ & & \ce{C=O} & \xrightarrow[\text{HCl}]{\text{Zn-Hg}} & & & \ce{CH2 + H2O} \\ & / & & & & / & \\ & & & & & & \end{array}

Wolff-Kishner reduction:

\begin{array}{r@{\;}c@{\;}l c r@{\;}c@{\;}l c r@{\;}c@{\;}l} & & & & & & & & & & \\ & \backslash & & & & \backslash & & & & \backslash & \\ & & \ce{C=O} & \xrightarrow[-\ce{H2O}]{\ce{NH2NH2}} & & & \ce{C=NNH2} & \xrightarrow{\text{KOH ethylene Glycol, }\Delta} & & & \ce{CH2 + N2} \\ & / & & & & / & & & & / & \\ & & & & & & & & & & \end{array}

\[\underset{\text{Aldehyde}}{\ce{R-CHO}} + \ce{[O]} \xrightarrow[\text{or }\ce{KMnO4/H2SO4}]{\ce{K2Cr2O7/H2SO4}} \underset{\text{Carboxylic acid}}{\ce{R-COOH}}\]

Aldol condensation:

\[\ce{CH3 - \overset{\displaystyle O}{\overset{||}{C}} - H + H - CH2 - \overset{\displaystyle O}{\overset{||}{C}} - H} \xrightarrow{\text{dil. NaOH}} \ce{H3C - \underset{\underset{\displaystyle OH}{|}}{CH} - \underset{\underset{\displaystyle H}{|}}{CH} - \overset{\displaystyle O}{\overset{||}{C}} - H} \\ \xrightarrow{\text{dil. }\ce{H2SO4}\Delta, \ce{-H2O}} \ce{CH3 - CH = CH - \overset{\displaystyle O}{\overset{||}{C}} - H}\]

Mechanism:

Cross aldol condensation:

\[\begin{array}{ccccccccccccc} & \ce{O} & & \ce{O} & & & & \ce{OH} & & \ce{O} & & & \\ & || & & || & & & & | & & || & & & \\ \ce{C6H5 -} & \ce{C} & \ce{+ HCH2 -} & \ce{C} & \ce{- H} & \overset{\text{dil. NaOH}}{\rightleftharpoons} & \ce{C6H5 -} & \ce{C} & \ce{- CH2 -} & \ce{C} & \ce{- H} & \xrightarrow[\Delta]{\ce{H2O}} & \ce{C6H5CH=CHCHO} \\ & | & & & & & & | & & & & & \text{Cinnamaldehyde} \\ & \ce{H} & & & & & & \ce{H} & & & & & \end{array}\]

Cannizzaro reaction: It is a self-oxidation reduction reaction.

\[\begin{array}{r@{\;}c@{\;}l} \ce{H} & & \\ & \backslash & \\ & & \ce{C=O} \\ & / & \\ \ce{H} & & \end{array} + \begin{array}{r@{\;}c@{\;}l} \ce{H} & & \\ & \backslash & \\ & & \ce{C=O} \\ & / & \\ \ce{H} & & \end{array} + \text{conc. KOH} \xrightarrow{\Delta} \ce{H - \underset{\underset{\displaystyle H}{|}}{\overset{\overset{\displaystyle H}{|}}{C}} - OH} + \ce{H - \overset{\displaystyle O}{\overset{||}{C}} - OK}\]

Haloform Reaction:

The reaction can be used to transform acetyl groups into carboxyl groups or to produce chloroform or iodoform. This reaction has been used in qualitative analysis to indicate the presence of a methyl ketone in which excess iodine is used to halogenate the compound. The product iodoform precipitates as a yellow-coloured substance and has a characteristic odour.

\[\underset{\substack{\text{Acetone} \\ \text{Iodoform Test}}}{\ce{H3C - \overset{\displaystyle O}{\overset{||}{C}} - CH3}} \xrightarrow[4\ce{NaOH}]{3\ce{Cl2}} \underset{\text{Sodium acetate}}{\ce{H3C - \overset{\displaystyle O}{\overset{||}{C}} - O^\ominus Na^\oplus}} + \underset{\text{Chloroform}}{\ce{CHCl3}}\]

• 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.

1. Methanoic acid (Formic acid): Leather tanning, dyeing and finishing in textiles.
2. Ethanoic acid (Acetic acid): Manufacturing of rayon and plastic; used as vinegar in cooking.
3. Benzoic acid: Used as a food preservative and in perfumery.
4. Salicylic acid: Used in the preparation of Aspirin (analgesic/antipyretic), Salol, and Oil of Wintergreen (methyl salicylate).

• Aspirin = acetylsalicylic acid; anti-pyretic and pain killer.
• Methyl salicylate = Oil of wintergreen (from methanol + salicylic acid).

Statement:

Carbonyl compounds undergo reduction to alcohols or hydrocarbons depending on reagents used.

Reduction to Alcohols

Reagents:

• NaBH₄
• LiAlH₄
• Catalytic hydrogenation (H₂/Ni)

Reactions:

Aldehyde → Primary alcohol
R–CHO → R–CH₂OH

Ketone → Secondary alcohol
R–CO–R′ → R–CHOH–R′

Reduction to Hydrocarbons

1. Clemmensen Reduction:
Zn(Hg)/HCl

R–CO–R′ → R–CH₂–R′

2. Wolff–Kishner Reduction:
NH₂NH₂/KOH

R–CO–R′ → R–CH₂–R′

Key Points:

• Carbonyl group converted to CH₂ group.
• Choice of reagent depends on acidic/basic conditions.

Conclusion:

Carbonyl compounds can be selectively reduced to alcohols or completely reduced to hydrocarbons.

Statement:

Aldehydes are more reactive than ketones towards nucleophilic addition reactions.

Reasons:

1. Steric Effect

• Aldehydes have: One alkyl group + one hydrogen.
• Ketones have: Two alkyl groups.
• More alkyl groups → more steric hindrance → less reactivity.

2. Electronic Effect (+I Effect)

• Alkyl groups show +I effect.
• They donate electron density to carbonyl carbon.
• This reduces partial positive charge on carbon.

Ketones (two alkyl groups) are less electrophilic.

Order of Reactivity:

Formaldehyde > Other aldehydes > Ketones

Example:

HCHO > CH₃CHO > CH₃COCH₃

Conclusion:

Due to lower steric hindrance and higher electrophilicity, aldehydes react faster than ketones.

Statement:

The carbonyl carbon atom is sp² hybridised, forms a trigonal planar structure, and possesses a polar C=O bond.

Explanation:

1. Hybridisation:

• Carbonyl carbon is sp² hybridised.
• It forms:

1. Three σ (sigma) bonds.
2. One π (pi) bond with oxygen.

• The π-bond is formed by sideways overlap of p-orbitals.

2. Geometry:

• The carbonyl carbon and three attached atoms lie in the same plane.
• Bond angle ≈ 120°.
• Geometry is trigonal planar.

3. Polarity:

• Oxygen is more electronegative than carbon.
• Hence, electron density shifts towards oxygen.
• Carbon acquires partial positive charge (δ⁺).
• Oxygen acquires partial negative charge (δ⁻).

4. Resonance:
Two contributing structures:

R–C=O ↔ R–C⁺–O⁻

This explains:

• High dipole moment.
• Electrophilic nature of carbonyl carbon.

Conclusion:

The planar structure, polarity and resonance make the carbonyl carbon highly reactive towards nucleophiles.