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Aldehydes, Ketones and Carboxylic Acids - Nomenclature and nature of carbonyl group

Grade 12ICSEChemistry

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). In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom (R−CHOR-CHO), while in ketones, it is bonded to two carbon atoms (R−CO−R′R-CO-R').

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Nature of Carbonyl Group: The carbonyl carbon is sp2sp^2 hybridized and forms three σ\sigma bonds. The fourth valence electron remains in a pp-orbital and forms a π\pi bond with oxygen by sideways overlap. The bond angle is approximately 120∘120^\circ, giving it a trigonal planar structure.

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Polarity: 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 results in a partial positive charge (δ+\delta^+) on carbon (electrophilic center) and a partial negative charge (δ−\delta^-) on oxygen (nucleophilic center).

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IUPAC Nomenclature of Aldehydes: The suffix '-al' is added to the parent alkane. The carbonyl carbon is always assigned position 1. For example, HCHOHCHO is methanal and CH3CHOCH_3CHO is ethanal.

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IUPAC Nomenclature of Ketones: The suffix '-one' is added to the parent alkane. The chain is numbered from the end that gives the carbonyl carbon the lowest possible number. For example, CH3COCH3CH_3COCH_3 is propan-2-one.

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Carboxylic Acids: These contain the carboxyl group (−COOH-COOH), which consists of a carbonyl group attached to a hydroxyl group. The IUPAC suffix is '-oic acid'. HCOOHHCOOH is methanoic acid and CH3COOHCH_3COOH is ethanoic acid.

📐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)}

Resonance Structure: [>C=O⟷>C+−O−]\text{Resonance Structure: } [ >C=O \longleftrightarrow >C^+ - O^- ]

μ>0 (Dipole moment of carbonyl compounds is significant)\mu > 0 \text{ (Dipole moment of carbonyl compounds is significant)}

💡Examples

Problem 1:

Give the IUPAC name for CH3−CH(CH3)−CH2−CHOCH_3-CH(CH_3)-CH_2-CHO.

Solution:

3-methylbutanal

Explanation:

The longest carbon chain containing the −CHO-CHO group has 4 carbons (butanal). Numbering starts from the aldehyde carbon, placing the methyl substituent at position 3.

Problem 2:

Write the IUPAC name for CH3−CO−CH2−CH2−CH3CH_3-CO-CH_2-CH_2-CH_3.

Solution:

Pentan-2-one

Explanation:

The parent chain has 5 carbons (pentane). The ketone group (>C=O>C=O) is at the second carbon when numbered from the left to give the functional group the lowest locant.

Problem 3:

Name the compound HOOC−CH2−COOHHOOC-CH_2-COOH according to IUPAC nomenclature.

Solution:

Propanedioic acid

Explanation:

The chain consists of 3 carbon atoms with two carboxylic acid groups at the terminals. The common name is Malonic acid.

Problem 4:

Explain why the boiling point of CH3CHOCH_3CHO (294K294 K) is higher than CH3CH2CH3CH_3CH_2CH_3 (231K231 K).

Solution:

Due to strong dipole-dipole interactions.

Explanation:

The carbonyl group in CH3CHOCH_3CHO is polar, leading to stronger intermolecular dipole-dipole attractions compared to the weak van der Waals forces in non-polar propane.

Nomenclature and nature of carbonyl group Revision - Class 12 Chemistry ICSE