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Class 12 CBSE Chemistry - Aldehydes , Ketones and Carboxylic Acids - Full Revision |Xylem CBSE 11&12
Xylem Class 12 CBSE
Overview
This video provides a comprehensive revision of Aldehydes, Ketones, and Carboxylic Acids for Class 12 CBSE Chemistry. It covers the fundamental structure of these compounds, their preparation methods including oxidation of alcohols, ozonolysis of alkenes, hydration of alkynes, and various named reactions like Rosenmund reduction, Stephen reaction, Etard reaction, and Gattermann-Koch reaction. The video also touches upon the physical properties such as boiling points and solubility, and delves into key chemical reactions including nucleophilic addition, reduction to alcohols, reduction to alkanes (Clemmensen and Wolff-Kishner), oxidation, and reactions with ammonia derivatives. The aim is to equip students to answer a wide range of questions from this chapter.
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Chapters
- Aldehydes, ketones, and carboxylic acids are grouped together due to the presence of the carbonyl group (C=O).
- Aldehydes have a carbonyl group bonded to at least one hydrogen atom (RCHO).
- Ketones have a carbonyl group bonded to two alkyl or aryl groups (RCOR').
- Carboxylic acids have a carbonyl group bonded to a hydroxyl group (RCOOH).
- Other related compounds like acid halides, esters, and anhydrides also contain the carbonyl group but are distinct.
Understanding the basic structure and the common carbonyl group is crucial for grasping their reactivity and classification.
RCHO for aldehyde, RCOR' for ketone, RCOOH for carboxylic acid.
- Primary alcohols can be oxidized to aldehydes using mild oxidizing agents like CrO3.
- Primary alcohols can be directly oxidized to carboxylic acids using strong oxidizing agents like K2Cr2O7 or KMnO4.
- Secondary alcohols are oxidized to ketones using similar oxidizing agents.
- Ozonolysis of alkenes, followed by reductive workup (e.g., with Zn/H2O), cleaves the double bond to form aldehydes and/or ketones.
These methods show how to synthesize aldehydes and ketones from readily available alcohols and alkenes, forming the basis for many organic syntheses.
Oxidation of pent-2-ene by ozonolysis yields ethanal and propanal.
- Hydration of ethyne (acetylene) in the presence of HgSO4/H2SO4 yields ethanal (an aldehyde).
- Hydration of other alkynes yields ketones.
- Rosenmund reduction converts acid chlorides (RCOCl) to aldehydes using H2 and a poisoned catalyst (Pd/BaSO4).
- Stephen reaction reduces nitriles (RCN) to aldehydes using SnCl2/HCl followed by hydrolysis.
- Etard reaction oxidizes methyl groups on benzene rings (e.g., toluene) to aldehydes using chromyl chloride (CrO2Cl2).
These reactions provide specific routes to aldehydes and ketones, often involving unique reagents and conditions, which are important for targeted synthesis.
Toluene reacts with chromyl chloride to form benzaldehyde.
- Gattermann-Koch reaction formylates benzene or substituted benzenes using CO, HCl, and a Lewis acid catalyst (e.g., AlCl3/CuCl) to produce aldehydes.
- Friedel-Crafts acylation can be used to synthesize ketones from benzene derivatives using acyl chlorides and Lewis acids.
- Grignard reagents react with nitriles (RCN) to form ketones after hydrolysis.
- Grignard reagents react with acid chlorides to form tertiary alcohols, not aldehydes or ketones directly.
These reactions offer alternative pathways to synthesize aromatic aldehydes and ketones, expanding the synthetic toolkit.
Benzene reacts with CO and HCl in the presence of AlCl3/CuCl to form benzaldehyde.
- Aldehydes and ketones have higher boiling points than hydrocarbons and ethers of comparable molecular mass due to dipole-dipole interactions.
- However, they have lower boiling points than alcohols of comparable molecular mass because they cannot form hydrogen bonds with themselves.
- Lower molecular weight aldehydes and ketones are soluble in water due to their ability to form hydrogen bonds with water molecules.
- Solubility decreases with increasing carbon chain length as the hydrophobic alkyl part becomes dominant.
- Methanal is a gas at room temperature, while ethanal and acetone are volatile liquids.
Understanding physical properties helps predict behavior in reactions and separation techniques.
Ethanol has a higher boiling point than propanal, which has a higher boiling point than diethyl ether, all with similar molecular weights.
- The carbonyl carbon is electrophilic due to the polar C=O bond, making it susceptible to nucleophilic attack.
- Common nucleophiles include cyanide ions (CN-), bisulfite ions (HSO3-), Grignard reagents (RMgX), and ammonia derivatives.
- Addition of HCN forms cyanohydrins.
- Addition of sodium bisulfite forms bisulfite addition compounds.
- Addition of Grignard reagents leads to alcohols after hydrolysis.
Nucleophilic addition is a fundamental reaction type for carbonyl compounds, leading to the formation of new carbon-carbon and carbon-heteroatom bonds.
Reaction of propanal with HCN forms 2-hydroxybutanenitrile.
- Reduction of aldehydes using agents like LiAlH4 or NaBH4 yields primary alcohols.
- Reduction of ketones yields secondary alcohols.
- Clemmensen reduction (Zn(Hg)/conc. HCl) reduces the carbonyl group to a methylene group (-CH2-), converting aldehydes and ketones to alkanes.
- Wolff-Kishner reduction (H2NNH2/KOH, heat) also converts the carbonyl group to a methylene group, yielding alkanes.
Reduction reactions are vital for converting carbonyls to alcohols or even alkanes, offering pathways to saturated hydrocarbons.
Acetone (a ketone) is reduced to propan-2-ol using NaBH4.
- Aldehydes are easily oxidized to carboxylic acids by mild oxidizing agents (e.g., Tollens' reagent, Fehling's solution, CrO3).
- Ketones are generally resistant to oxidation by mild agents but can be oxidized by strong agents, often leading to a mixture of carboxylic acids (cleavage of C-C bond).
- Aldehydes and ketones react with ammonia and its derivatives (like hydroxylamine, hydrazine, phenylhydrazine, semicarbazide) to form imines, oximes, hydrazones, phenylhydrazones, and semicarbazones, respectively, with the elimination of water.
These reactions highlight the difference in reactivity between aldehydes and ketones and show how to form characteristic derivatives used for identification.
Reaction of benzaldehyde with phenylhydrazine forms benzaldehyde phenylhydrazone.
Key takeaways
- The carbonyl group (C=O) is the central functional group dictating the chemistry of aldehydes, ketones, and carboxylic acids.
- Oxidation and reduction reactions are interconvertible pathways between alcohols, aldehydes, ketones, and carboxylic acids.
- Named reactions provide specific and often high-yield methods for synthesizing these compounds, especially aromatic ones.
- Nucleophilic addition is a characteristic reaction of aldehydes and ketones due to the electrophilic nature of the carbonyl carbon.
- The difference in reactivity between aldehydes and ketones (e.g., ease of oxidation) is a key concept for synthesis and identification.
- Physical properties like boiling point and solubility are influenced by intermolecular forces and molecular structure.
- Carbonyl compounds can be converted to alkanes via Clemmensen or Wolff-Kishner reduction, removing the oxygen atom entirely.
Key terms
Carbonyl groupAldehydeKetoneCarboxylic acidOxidationReductionNucleophilic additionOzonolysisRosenmund reductionStephen reactionEtard reactionGattermann-Koch reactionClemmensen reductionWolff-Kishner reductionTollens' reagentFehling's solutionGrignard reagentHydration of alkynesTotomerism
Test your understanding
- How does the presence of the carbonyl group influence the reactivity of aldehydes and ketones compared to alcohols?
- What are the key differences in the preparation of aldehydes versus ketones from secondary alcohols?
- Explain the mechanism and significance of the Rosenmund reduction in synthesizing aldehydes.
- How can you differentiate between an aldehyde and a ketone based on their oxidation reactions?
- Describe the process of nucleophilic addition to a carbonyl group and provide an example of a reaction that proceeds via this mechanism.