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Overview: Aldehydes, Ketones and Carboxylic Acids

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Estimated time: 27 minutes
CBSE: Class 12

Definition: Acyl halides

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

CBSE: Class 12

Definition: Esters

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

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Definition: Acid anhydrides

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

CBSE: Class 12

Definition: Amides

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

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Definition: Carbonyl group

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

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Formula: Acyl halide

R–COX

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Formula: Ester

R–COOR′

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Formula: Acid anhydride

(R–CO)₂O

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Formula: Amide

R–CONH₂

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Key Points: Structure of Carbonyl Group

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.

CBSE: Class 12

Key Points: Reactivity of Aldehydes vs Ketones

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.

CBSE: Class 12

Key Points: Reduction of Aldehydes and Ketones

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.

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Aldol Reaction

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

On heating:

CH3CH = CHCHO + H2O

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Cannizzaro Reaction

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Esterification

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

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Reduction

\[RCHO\xrightarrow{NaBH_4}RCH_2OH\]

\[RCOOH\xrightarrow{LiAlH_4}RCH_2OH\]

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Decarboxylation

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

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