Introduction to NCERT Exemplar Solutions for Class 12 Chemistry Chapter 12: Aldehydes, Ketones, and Carboxylic Acids
Aldehydes, ketones, and carboxylic acids are highly important classes of carbonyl compounds in organic Chemistry. The carbonyl group, consisting of a carbon atom double-bonded to oxygen, is a defining feature that influences their chemical and physical properties. This chapter explains their nomenclature, preparation, properties, reactions, and applications. The Class 12 NCERT Exemplar Solutions provide detailed reasoning and step-by-step answers to advanced questions, helping students strengthen their understanding of these crucial compounds.
Introduction to Carbonyl Compounds
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Aldehydes contain a carbonyl group bonded to at least one hydrogen atom, for example, formaldehyde and acetaldehyde.
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Ketones have a carbonyl group bonded to two alkyl groups, such as acetone.
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Carboxylic acids contain a carbonyl group attached to a hydroxyl group, as seen in formic acid and acetic acid.
Exemplar questions often test the ability to classify compounds and recognize functional groups based on their structures.
Methods of Preparation
The chapter highlights several synthetic methods:
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Aldehydes and ketones can be prepared by oxidation of alcohols, ozonolysis of alkenes, hydration of alkynes, and by Rosenmund and Etard reactions.
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Carboxylic acids are obtained through oxidation of primary alcohols and aldehydes, hydrolysis of nitriles, and carbonation of Grignard reagents.
Exemplar solutions include step-by-step reaction schemes and mechanisms, making it easier for students to understand and memorize the methods.
Physical Properties
The physical properties of aldehydes, ketones, and carboxylic acids are influenced by the polar nature of the carbonyl group.
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Boiling points: Carboxylic acids have the highest boiling points due to strong hydrogen bonding, followed by aldehydes and ketones.
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Solubility: Lower members of all three classes are soluble in water because of hydrogen bonding with water molecules. Solubility decreases with increasing molecular mass.
Exemplar problems often involve comparing physical properties based on intermolecular forces.
Chemical Properties
The chemical reactivity of these compounds is mainly governed by the highly polarized carbonyl group:
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Aldehydes and ketones undergo nucleophilic addition reactions, such as addition of hydrogen cyanide, sodium bisulfite, and ammonia derivatives. They also undergo oxidation and reduction reactions. Aldehydes are more reactive than ketones due to the presence of a hydrogen atom directly attached to the carbonyl carbon.
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Carboxylic acids are acidic in nature due to resonance stabilization of the carboxylate ion. They undergo reactions such as esterification with alcohols, reduction to primary alcohols, and substitution of the hydroxyl group.
Exemplar questions encourage students to predict products of reactions and explain differences in reactivity.
Tests for Aldehydes and Ketones
This chapter also highlights important tests:
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Tollen’s test: Aldehydes give a silver mirror when treated with Tollen’s reagent.
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Fehling’s test: Aldehydes reduce Fehling’s solution to a red precipitate of copper(I) oxide.
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Iodoform test: Compounds containing a CH₃CO group, such as acetone, give a yellow precipitate of iodoform.
Exemplar problems often focus on identifying compounds using these characteristic tests.
Applications and Importance
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Aldehydes such as formaldehyde are used in plastics and disinfectants.
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Ketones like acetone are widely used as solvents in industry and laboratories.
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Carboxylic acids are key in food, pharmaceuticals, and industry. Acetic acid is used in vinegar, while benzoic acid is a preservative.
NCERT Exemplar questions often connect these applications to conceptual knowledge, making the subject more practical and relatable.