CBSE Class 12 Chemistry Revision Notes Chapter 8 Aldehydes Ketones and Carboxylic Acids

Aldehydes, ketones and carboxylic acids are important organic compounds containing carbonyl or carboxyl groups.

In CBSE Class 12 Chemistry, this chapter explains their structure, preparation, reactions, tests and acidic behaviour.

Class 12 Chemistry Chapter 8 Aldehydes, Ketones and Carboxylic Acids is an important organic chemistry chapter. It deals with compounds that contain the carbonyl group (>C=O) and the carboxyl group (-COOH).

Aldehydes, ketones and carboxylic acids are found in many natural and industrial products. Their reactions are also important for understanding many conversions in organic chemistry. In this chapter, students learn nomenclature, structure, preparation methods, physical properties, nucleophilic addition reactions, oxidation, reduction, aldol condensation, Cannizzaro reaction and acidity of carboxylic acids.

Key Takeaways

• Carbonyl Group: Aldehydes and ketones contain the >C=O group, where carbonyl carbon is electrophilic.
• Aldehydes vs Ketones: Aldehydes are generally more reactive than ketones towards nucleophilic addition.
• Distinguishing Tests: Aldehydes give Tollen’s test and Fehling’s test, while ketones usually do not.
• Carboxylic Acids: Carboxylic acids are acidic because the carboxylate ion is resonance stabilised.

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Access 30 Minutes Class 12 Chemistry Chapter 8 Aldehydes Ketones and Carboxylic Acids Notes

Students can use these CBSE Class 12 Chemistry Revision Notes Chapter 8 for quick revision before tests and board exams. The chapter has many named reactions and reagent-based conversions, so it is better to revise it in small parts.

First revise the functional groups. Then move to preparation methods, reactions and tests. After that, revise carboxylic acids separately because their reactions and acidic nature are different from aldehydes and ketones.

Aldehydes Ketones and Carboxylic Acids Class 12 Chemistry Chapter 8 Overview

Aldehydes, ketones and carboxylic acids are organic compounds that contain oxygen as an important part of their functional group.

Aldehydes and ketones are called carbonyl compounds because they contain the carbonyl group. Carboxylic acids contain the carboxyl group, which is made of a carbonyl group and a hydroxyl group.

Carbonyl Group in Class 12 Chemistry Chapter 8

The carbonyl group is represented as >C=O. It is present in aldehydes and ketones.

In the carbonyl group, carbon is double bonded to oxygen. Oxygen is more electronegative than carbon, so it pulls the electron density towards itself. Because of this, carbonyl carbon becomes electron-deficient and acts as an electrophilic centre.

This is why aldehydes and ketones easily undergo nucleophilic addition reactions.

Difference Between Aldehydes Ketones and Carboxylic Acids

Nomenclature of Aldehydes Ketones and Carboxylic Acids

In IUPAC naming, aldehydes, ketones and carboxylic acids have different suffixes.

For aldehydes, the carbon of -CHO is counted as carbon number 1. For ketones, the position of the carbonyl group is mentioned by using the lowest possible number. For carboxylic acids, the carbon of -COOH is counted as carbon number 1.

Structure of the Carbonyl Group

The carbonyl carbon is sp² hybridised. It forms three sigma bonds and one pi bond with oxygen.

The atoms around the carbonyl carbon lie in the same plane. The bond angle is close to 120°. Due to the polarity of the C=O bond, the carbonyl carbon behaves as an electrophile and the oxygen behaves as a nucleophile.

This structure explains most reactions of aldehydes and ketones.

Preparation of Aldehydes and Ketones

Aldehydes and ketones can be prepared by oxidation, reduction and other organic reactions.

By Oxidation of Alcohols

Primary alcohols give aldehydes on controlled oxidation.

RCH₂OH → RCHO

Secondary alcohols give ketones on oxidation.

R₂CHOH → R₂CO

By Dehydrogenation of Alcohols

Alcohols can also be converted into aldehydes or ketones by removing hydrogen.

Primary alcohols form aldehydes, while secondary alcohols form ketones.

Rosenmund Reduction

Acyl chlorides are reduced to aldehydes using hydrogen in the presence of palladium catalyst supported on barium sulphate.

RCOCl + H₂ → RCHO + HCl

This reaction is useful because the reaction stops at the aldehyde stage.

Stephen Reaction

Nitriles are reduced to imines using stannous chloride and hydrochloric acid. On hydrolysis, they give aldehydes.

RCN → RCHO

This is another important method for preparing aldehydes.

Friedel-Crafts Acylation

Aromatic ketones can be prepared by treating benzene or substituted benzene with acyl chloride in the presence of anhydrous aluminium chloride.

This is used for preparing compounds such as acetophenone.

Physical Properties of Aldehydes Ketones and Carboxylic Acids

Aldehydes, ketones and carboxylic acids are polar compounds. Their boiling points are higher than hydrocarbons and ethers of similar molecular mass.

Lower aldehydes and ketones are soluble in water because they can form hydrogen bonds with water. Their solubility decreases as the size of the alkyl group increases.

Carboxylic acids have higher boiling points than aldehydes and ketones because they form strong intermolecular hydrogen bonds.

Nucleophilic Addition Reactions of Aldehydes and Ketones

Aldehydes and ketones undergo nucleophilic addition reactions because the carbonyl carbon is electron-deficient.

In these reactions, a nucleophile attacks the carbonyl carbon. Then the oxygen gets protonated to give the final addition product.

General idea:

Nu⁻ attacks C=O → alkoxide intermediate → protonation → addition product

Why Aldehydes Are More Reactive Than Ketones

Aldehydes are generally more reactive than ketones towards nucleophilic addition.

There are two main reasons:

1. Aldehydes have one alkyl group and one hydrogen atom, so there is less steric hindrance.
2. Ketones have two alkyl groups, which reduce the positive character of carbonyl carbon by the +I effect.

So the carbonyl carbon of aldehydes is more easily attacked by nucleophiles.

Important Nucleophilic Addition Reactions

These reactions are important for both conversions and compound identification.

Oxidation of Aldehydes and Ketones

Aldehydes are easily oxidised to carboxylic acids.

RCHO → RCOOH

Common oxidising agents include potassium permanganate, potassium dichromate and nitric acid.

Ketones do not oxidise easily. They need strong oxidising conditions. On strong oxidation, ketones undergo carbon-carbon bond cleavage and form smaller carboxylic acids.

Tollen’s Test

Tollen’s reagent is ammoniacal silver nitrate solution.

Aldehydes reduce Tollen’s reagent to metallic silver. This gives a bright silver mirror on the wall of the test tube.

Aldehyde → Carboxylate ion
Ag⁺ → Ag

Ketones usually do not give this test. So Tollen’s test is used to distinguish aldehydes from ketones.

Fehling’s Test

Fehling’s solution is prepared by mixing Fehling solution A and Fehling solution B.

Aliphatic aldehydes give a reddish-brown precipitate of cuprous oxide with Fehling’s reagent.

Aldehyde → Carboxylate ion
Cu²⁺ → Cu₂O

Ketones do not give Fehling’s test. Aromatic aldehydes also generally do not respond to this test.

Reduction of Aldehydes and Ketones

Aldehydes and ketones can be reduced to alcohols.

Common reducing agents are sodium borohydride and lithium aluminium hydride.

Clemmensen Reduction

In Clemmensen reduction, aldehydes and ketones are reduced to hydrocarbons using zinc amalgam and concentrated hydrochloric acid.

C=O → -CH₂-

This reaction is done in acidic medium.

Wolff-Kishner Reduction

In Wolff-Kishner reduction, aldehydes and ketones are reduced to hydrocarbons using hydrazine and strong base in a high-boiling solvent.

This reaction is done in basic medium.

Haloform Reaction

Methyl ketones undergo haloform reaction.

Aldehydes or ketones having the CH₃CO- group react with sodium hypohalite to form carboxylate salt and haloform.

In the iodoform reaction, yellow precipitate of iodoform is formed.

This test is used to detect the CH₃CO- group or compounds that can form this group on oxidation.

Reactions Due to Alpha Hydrogen

Alpha hydrogen means hydrogen attached to the carbon next to the carbonyl carbon.

Aldehydes and ketones having alpha hydrogen show special reactions because alpha hydrogen is acidic. This acidity is due to the electron-withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.

Aldol Condensation

Aldehydes and ketones having at least one alpha hydrogen undergo aldol reaction in the presence of dilute alkali.

They form beta-hydroxy aldehydes or beta-hydroxy ketones.

On heating, these products lose water and form alpha, beta-unsaturated carbonyl compounds.

This reaction is known as aldol condensation.

Cross Aldol Condensation

When aldol condensation takes place between two different aldehydes or ketones, it is called cross aldol condensation.

If both compounds have alpha hydrogen, a mixture of products is formed. This is why cross aldol condensation needs careful selection of reactants.

Cannizzaro Reaction

Aldehydes that do not have alpha hydrogen undergo Cannizzaro reaction.

In this reaction, one molecule of aldehyde is oxidised to carboxylic acid salt, while another molecule is reduced to alcohol.

This reaction takes place in the presence of concentrated alkali.

Example:

Benzaldehyde gives benzyl alcohol and sodium benzoate.

Preparation of Carboxylic Acids

Carboxylic acids can be prepared by several methods.

By Oxidation of Primary Alcohols and Aldehydes

Primary alcohols and aldehydes are oxidised to carboxylic acids.

RCH₂OH → RCHO → RCOOH

By Oxidation of Alkylbenzenes

Alkylbenzenes are oxidised to aromatic carboxylic acids using strong oxidising agents.

For example, toluene gives benzoic acid.

By Hydrolysis of Nitriles and Amides

Nitriles and amides can be hydrolysed to carboxylic acids under acidic or basic conditions.

RCN → RCOOH

RCONH₂ → RCOOH

By Grignard Reagent

Grignard reagents react with carbon dioxide and then undergo hydrolysis to form carboxylic acids.

RMgX + CO₂ → RCOOH

Physical Properties of Carboxylic Acids

Carboxylic acids are polar compounds. Lower carboxylic acids are soluble in water because they form hydrogen bonds with water.

As the carbon chain becomes longer, solubility decreases because the hydrocarbon part becomes larger.

Carboxylic acids have high boiling points because they form hydrogen-bonded dimers.

Acidity of Carboxylic Acids

Carboxylic acids are acidic because they can donate a proton.

RCOOH → RCOO⁻ + H⁺

The carboxylate ion formed after losing H⁺ is resonance stabilised. This makes carboxylic acids more acidic than alcohols and phenols in many comparisons.

Effect of Substituents on Acidity

Electron-withdrawing groups increase the acidity of carboxylic acids. They stabilise the carboxylate ion.

Electron-donating groups decrease acidity because they destabilise the carboxylate ion.

Reactions of Carboxylic Acids

Carboxylic acids show reactions due to the -COOH group.

Reaction with Bases

Carboxylic acids react with bases to form salts.

RCOOH + NaOH → RCOONa + H₂O

Esterification

Carboxylic acids react with alcohols in the presence of acid to form esters.

RCOOH + R’OH → RCOOR’ + H₂O

This reaction is reversible.

Reaction with PCl₅, PCl₃ and SOCl₂

Carboxylic acids react with these reagents to form acyl chlorides.

SOCl₂ is preferred because the by-products are gases and escape from the reaction mixture.

Reaction with Ammonia

Carboxylic acids react with ammonia to form ammonium salts. On heating, these salts form amides.

Decarboxylation

Carboxylic acids lose carbon dioxide when their sodium salts are heated with soda lime.

RCOONa + NaOH → RH + Na₂CO₃

This reaction is called decarboxylation.

Hell-Volhard-Zelinsky Reaction

Carboxylic acids having alpha hydrogen react with chlorine or bromine in the presence of red phosphorus.

The halogen replaces alpha hydrogen and forms alpha-halo carboxylic acids.

This reaction is called Hell-Volhard-Zelinsky reaction.

Important Named Reactions in Aldehydes Ketones and Carboxylic Acids

Quick Revision Table for Chapter 8

Common Mistakes in Class 12 Chemistry Chapter 8

Students often mix up the conditions of Clemmensen and Wolff-Kishner reductions. Clemmensen reduction is done in acidic medium, while Wolff-Kishner reduction is done in basic medium.

Another common mistake is assuming all aldehydes give Fehling’s test. Aliphatic aldehydes give Fehling’s test, but aromatic aldehydes generally do not.

Students also confuse aldol and Cannizzaro reactions. Aldol condensation needs alpha hydrogen. Cannizzaro reaction occurs in aldehydes that do not have alpha hydrogen.

Useful Links for Class 12 Chemistry