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Aldehydes, Ketones and Carboxylic Acids - Nomenclature and Structure of Carbonyl Group

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The carbonyl group consists of a carbon atom double-bonded to an oxygen atom: >C=O>C=O.

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In aldehydes, the carbonyl group is bonded to at least one hydrogen atom, represented as R−CHOR-CHO. In ketones, the carbonyl group is bonded to two carbon atoms, represented as R−CO−R′R-CO-R'.

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The carbonyl carbon atom is sp2sp^2 hybridized and forms three σ\sigma bonds. The fourth valence electron of carbon remains in a pp-orbital and forms a π\pi bond with oxygen by overlap with a pp-orbital of oxygen.

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The structure of the carbonyl group is trigonal planar with bond angles of approximately 120∘120^\circ.

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The C=OC=O bond is highly polar due to the higher electronegativity of oxygen (3.53.5) compared to carbon (2.52.5). This makes the carbonyl carbon an electrophilic (Lewis acid) center and the carbonyl oxygen a nucleophilic (Lewis base) center.

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IUPAC Nomenclature: Aldehydes are named by replacing the terminal '-e' of the corresponding alkane with '-al'. Ketones are named by replacing '-e' with '-one'.

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Carboxylic acids contain a carboxyl functional group (−COOH-COOH) and are named by replacing the terminal '-e' of the alkane with '-oic acid'.

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The resonance structures of the carbonyl group can be represented as: [>C=O⟷>C+−O−][ >C=O \longleftrightarrow >\overset{+}{C}-\overset{-}{O} ].

📐Formulae

CnH2nO (General formula for Aliphatic Aldehydes and Ketones)C_nH_{2n}O \text{ (General formula for Aliphatic Aldehydes and Ketones)}

CnH2nO2 (General formula for Aliphatic Carboxylic Acids)C_nH_{2n}O_2 \text{ (General formula for Aliphatic Carboxylic Acids)}

Bond Angle≈120∘\text{Bond Angle} \approx 120^\circ

Dipole Moment (μ)>0 (Polar nature)\text{Dipole Moment } (\mu) > 0 \text{ (Polar nature)}

💡Examples

Problem 1:

Write the IUPAC name of the following compound: CH3−CH(CH3)−CH2−CH2−CHOCH_3-CH(CH_3)-CH_2-CH_2-CHO.

Solution:

4-methylpentanal

Explanation:

The longest chain containing the −CHO-CHO group has 5 carbon atoms (pentane). Numbering starts from the carbonyl carbon (C1C1). A methyl substituent is present at C4C4.

Problem 2:

Draw the structure of 3-Methylcyclopentanone.

Solution:

A five-membered carbon ring with a double-bonded oxygen (=O=O) on one carbon and a methyl group (−CH3-CH_3) on the third carbon relative to the carbonyl group.

Explanation:

The parent suffix '-one' indicates a ketone in a cyclic structure (cyclopentane). The carbonyl carbon is assigned position 1, and the methyl group is placed at position 3.

Problem 3:

Explain why the carbon atom in the carbonyl group is electrophilic.

Solution:

Due to the electronegativity difference (O>CO > C), the π\pi electrons are pulled towards oxygen, creating a partial positive charge (δ+\delta^+) on carbon.

Explanation:

The resonance contributor >C+−O−>\overset{+}{C}-\overset{-}{O} highlights the electron deficiency at the carbon atom, making it susceptible to attack by nucleophiles.

Nomenclature and Structure of Carbonyl Group Revision - Class 12 Chemistry CBSE