NCERT Solutions, Question Answer and Mind Map for Class 12 Chemistry Chapter 12, “Aldehydes, Ketones, and Carboxylic Acids,” is a study material package designed to help students understand the structure, properties, and reactions of aldehydes, ketones, and carboxylic acids.

Page Contents

NCERT Solutions provide detailed explanations and answers to the questions presented in the chapter. The solutions cover all the topics in the chapter, including the preparation of aldehydes, ketones, and carboxylic acids, their physical and chemical properties, and their reactions with different reagents. They also provide tips on how to answer different types of questions, including short answer, long answer, and multiple-choice questions.

NCERT Solution / Notes Class 12 Chemistry Chapter 12 Aldehydes Ketones and Carboxylic Acids

The question-answer section of the chapter covers a wide range of topics, from the nomenclature of aldehydes, ketones, and carboxylic acids to their reactions with different reagents such as sodium bisulfite, Tollens’ reagent, and Fehling’s solution. It also includes questions on the different methods used for the preparation of aldehydes, ketones, and carboxylic acids.

The mind map provides a visual representation of the key topics covered in the chapter, allowing students to understand the connections between different concepts and ideas. The mind map covers the nomenclature, preparation, and properties of aldehydes, ketones, and carboxylic acids, as well as their reactions with different reagents.

NCERT Solution / Notes Class 12 Chemistry Chapter 12 Aldehydes Ketones and Carboxylic Acids with Mind Map PDF Download

ALDEHYDES, KETONES AND CARBOXYLIC ACIDS

Introduction

Carbonyl compounds are organic compounds containing carbon-oxygen double bond

NCERT Solution / Notes Class 12 Chemistry Chapter 12 Aldehydes Ketones and Carboxylic Acids – Class 12 Chemistry Chapter 12

Aldehydes have carbonyl group bonded to a carbon and hydrogen.

(Where R = H or alkyl or aryl group)

Ketones have carbonyl group bonded to two carbon atoms. O

(Where R and R’ may be same or different alkyl or aryl groups)

Nomenclature of Aldehydes and Ketones

ldehydes

Ketones

Structure and Nature of Carbonyl Group

Structure

The carbonyl carbon group is sp² hybridised and forms three sigma bonds.
The fourth electron in the p-orbital forms a n-bond by overlapping with p-orbital of oxygen.
The oxygen atom also has two non-bonding electron pairs.
So the carbonyl carbon with the three atoms linked to it lies in the same plane and the n-cloud lies above and below the plane.
The bond angle is 120° with expected trigonal coplanar structure.

Nature

The C-O double bond is polarised since oxygen is electronegative than carbon.
So the carbonyl carbon is an electrophilic centre and the carbonyl oxygen is a nucleophilic centre.
The carbonyl compounds have substantial dipole moments and are polar than ethers.
The high polarity of the carbonyl group can be explained on the basis of resonance involving a neutral

(A) and a dipolar (B) structures given below.

Preparation of Aldehydes

Rosenmund Reduction

In this reaction, acyl chloride on hydrogenation in the presence of palladium catalyst and barium sulphate gives aldehydes.

Stephen Reaction

Nitriles on reduction with stannous chloride in the presence of HCI give imine which on hydrolysis gives corresponding aldehyde.

An alternate method to reduce nitriles selectively is by diisobutylaluminium hydride to imines which on hydrolysis yields aldehydes.

Esters can also be reduced to aldehydes with DIBAL-H

From Aromatic Hydrocarbons

Aromatic aldehydes can be prepared using the following methods.

By Oxidation of Methylbenzene

Etard Reaction ( Use of Chromyl Chloride)

Chromyl chloride oxidises the methyl group to a chromium complex which on further hydrolysis gives corresponding benzaldehyde.

Use of Chromic oxide(CrO ₃)

Toluene when treated with chromic oxide in acetic anhydride gets converted into benzylidene diacetate which on hydrolysis with aqueous acid gives benzaldehyde.

Side Chain Chlorination

Toluene on side chlorination gives benzal chloride which on hydrolysis gives benzaldehyde.

Gatterman — Koch Reaction

Benzene or toluene on treatment with CO and HCI in the presence of AIC!3 or CuCI gives benzaldehyde or p-tolualdehyde.

Preparation of Ketones

From Acid chlorides or Acyl chlorides

Acyl chloride on treatment with dialkylcadmium obtained by reaction of cadium chloride with Grignard reagent gives ketones.

From Nitriles

Nitriles on treatment with Grignard reagent followed by hydrolysis yields a ketone

From Benzenes or Substituted Benzenes

Benzene or substituted benzene on treatment with acid chloride in the presence of anhydrous AICI3 gives the corresponding ketone and this reaction is known as Friedel-Crafts acylation reaction.

Chemical Reactions

Nucleophilic Addition Reactions

Aldehydes and Ketones undergo nucleophilic addition reactions.

Mechanism for Nucleophilic Addition Reactions
- A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group perpendicularly to the sp³ hybridised orbitals of carbonyl carbon.
- The hybridisation changes from sp² to sp³ and a tetrahedral alkoxide intermediate is formed.
- The intermediate grabs a proton from the reaction medium to give an electrically neutral product.
- The net result is addition of Nu- and H’ across the C-O double bond.

Reactivity
- Aldehydes are more reactive than ketones in nucleophilic reactions because of two reasons:
- Sterically, it is the presence of two relatively large groups in ketones that hinder the approach of nucleophile to carbonyl carbon than in aldehydes which have only one such substituent.
- Electronically, aldehydes are more reactive than ketones because the two alkyl groups in ketones decrease the electrophilicity of the carbonyl carbon more effectively than in aldehydes.

Important Examples of Nucleophilic Addition and Nucleophilic Addition- Elimination Reactions

Addition of Hydrogen cyanide (HCN)
- On addition of HCN to aldehydes and ketones they yield cyanohydrins.
- Since the reaction is very slow with pure HCN, it is catalysed with the help of a base and the cyanide ion (CN-) generated as a strong nucleophile adds to carbonyl compounds to give cyanohydrins.

Addition of Sodium Hydrogensulphite

Sodium hydrogen sulphite when added to aldehydes and ketones yield addition products.

Addition of Grignard Reagents

Grignard reagents on reacting with aldehydes and ketones yield alcohols.

Addition of Alcohols
- Aldehydes on treatment with one equivalent of monohydric alcohol in the presence of dry HCI give hemiacetal which on further treatment with one more molecule of alcohol gives acetal.

Ketones also react with ethylene glycols under similar conditions to give ethylene glycol ketals.

Addition of Ammonia and its Derivatives

Ammonia and its derivative add to the carbonyl group of an aldehydes and ketone

Reduction

Reduction to Alcohols

Aldehydes and ketones get reduced to primary and secondary alcohols by NaBH4 or LiAIH4.

Reduction to Hydrocarbons

Aldehydes and ketones reduce to —CH₂ group on treatment with zinc-amalgam and conc. HCI [CIemmenson reduction] or with hydrazine which on heating with sodium or potassium hydroxide in ethylene glycol[Wolff-Kishner reduction]

Oxidation
Aldehydes get oxidised to carboxylic acids with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate etc.

Ketones undergo oxidation with strong oxidising agents and elevated temperatures. The reaction involves carbon-carbon bond cleavage to give a mixture of carboxylic acids with lesser number of carbon atoms than the parent ketones.

Test to distinguish Aldehydes from Ketones

Tollens test

Aldehydes on warming with freshly prepared ammoniacal silver nitrate solution (Tollens reagent) produce a bright silver mirror due to the formation of silver metal.
The aldehydes are oxidised to corresponding carboxylate anion in alkaline medium.

Fehling’s test

Fehling reagent comprises of Fehling solution A and Fehling B. Fehling solution A = aqueous copper sulphate

Fehling solution B = Alkaline sodium potassium tartarate(Rochelle salt)

On heating Fehling’s reagent with an aldehyde, a reddish brown precipitate is obtained.
Aldehydes are oxidised to corresponding carboxylate anion while aromatic aldehydes do not respond to this test.

Haloform reaction

Aldehydes and ketones with at least one methyl group attached to the carbonyl carbon atom on oxidation with sodium hypohalite turn to sodium salts of corresponding acids with one carbon atom less than that of carbonyl compound. In this reaction, the methyl group is converted to haloform.

Reactions due to a-hydrogen

The a-hydrogen of aldehydes and ketones is acidic in nature hence they undergo a number of reactions.
The a-hydrogen atoms of carbonyl group is acidic due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.

Aldol Condensation

Aldehydes and ketones with at least one a-hydrogen undergo reaction in the presence of dilute alkali as catalyst to form §-hydroxy aldehydes (aldol) or §-hydroxy ketones (ketol) respectively. This is known as Aldol reaction.

Cross Aldol Condensation

In this reaction, two different aldehydes and/or ketones with a-hydrogen atoms reaction in the presence of dilute alkali as catalyst give a mixture of four products.

Ketones can be taken as one the component in the cross aldol reactions.

Other Reactions

Cannizzaro reaction
- Aldehydes with no a-hydrogen undergo self oxidation and reduction on heating with concentrated alkali.
- In this reaction, one molecule of the aldehyde is reduced to alcohol and another is oxidised to carboxylic acid salt.

Electrophilic Substitution Reaction

Aromatic aldehydes and ketones undergo electrophilic substitution reaction at the ring in which the carbonyl group acts as a deactivating and meta-directing group.

Carboxylic Acids

The carbonyl compounds in which carbonyl group is bonded to oxygen are known as carboxylic acids.

The derivative compounds of carboxylic acid where carbon is attached to nitrogen and to halogens are called amides and acyl halides respectively.

Nomenclature of Carbonyl Group

Structure of Carbonyl Group

The bonds to the carboxyl carbon in carboxylic acids lie in one plane and are separated by about 120°. Due to possible resonance structure given below, the carboxylic carbon is less electrophilic than carbonyl carbon.

Preparation of Carboxylic Acids

From Primary Alcohols and Aldehydes
Primary alcohols undergo oxidation with the help of oxidising agents like potassium permanganate in neutral, acidic or alkaline media or by potassium dichromate and chromium trioxide in acidic media to give carboxylic acid.

Carboxylic acids can also be prepared by treating aldehydes with mild oxidising agents.

From Alkylbenzenes

In this method, alkyl benzenes on vigorous oxidation with chromic acid or acidic or alkaline potassium permanganate yield aromatic carboxylic acids.
Primary and secondary alkyl groups are oxidised in this way while tertiary groups remain unaffected.

Substituted alkenes can also be oxidised using the same oxidising agents to yield carboxylic acids.

From Nitriles and Amides

Nitriles on hydrolysis give amides and then they are converted to acids in the presence of H+ or OH- catalyst.
Use of mild reaction conditions is done to stop the reaction at the amide stage.

Diagram, box and whisker chart

Description automatically generated

From Grignard Reagents

Grignard reagents on treating with carbon dioxide form salts of carboxylic acids which on acidification with mineral acid give corresponding carboxylic acids.

From Acyl Halides and Anhydrides

Acid chlorides on hydrolysis with water give carboxylic acids or readily undergo hydrolysis with aqueous base to give carboxylate ions which on acidification give corresponding carboxylic acids.
Anhydrides on the other hand undergo hydrolysis with water to give corresponding acids.

From Esters

Esters on hydrolysis with acid give acids directly while basic hydrolysis give carboxylates which on acidification give corresponding acids.

Chemical Reactions of Carboxylic Acids

Reactions involving cleavage of O-H bond

Acidity
- Carboxylic acids react with metals to form salts with the evolution of hydrogen gas.
- They also react with weaker bases such as carbonates and hydrogen carbonates
- This reaction is used to detect the presence of carboxyl group in an organic compound.
- Carboxylic acids dissociate in water to give resonance stabilised carboxylate anions and hydronium ion.

Acidity of carboxylic acids: Carboxylic acids are more acidic than phenols. The strength of acid depends on the extent of ionisation, which in turn depends on the stability of anion formed.

Effect of electron-donating substituents on the acidity of carboxylic acids: Electron-donating substituent decreases the stability of carboxylate ion by intensifying the negative charge and hence decreases the acidity of carboxylic acids.
Effect of electron-withdrawing substituent on the acidity of carboxylic acids: The electron-withdrawing group increases the stability of carboxylate ion by delocalising negative charge and hence increases acidity of carboxylic acid.

The effect of the following groups in increasing acidity order is:

Ph < I < Br < CI < F < CN < NO₂ < CF₃

Effect of the number of electron-withdrawing groups: As the number of electron-withdrawing groups increases, the —I effect increases, increasing the acid strength.
Effect of position of electron-withdrawing group: As the distance between electron-withdrawing group and carboxylic group increases, electron-withdrawing influence decreases.

Reactions involving cleavage of C-OH bond

Formation of Anhydride

Anhydrides are obtained on treating carboxylic acids with mineral acids such as H2SO4 or with

P 2O 5.

Esterification

Esters are formed on treating alcohols or phenols with carboxylic acids in the presence of conc. H₂SO₄ or HCI gas as a catalyst.

Reactions with PCI₅, PCl₃ and SOCl₂

On treating with PCI₅, PCl₃ or SOCl₂, the hydroxyl group of alcohols is replaced by chlorine atom. SOCI₂ is preferred since the two products formed are volatile and escape easily making the purification of the products easier.

RCOOH + PCl₅ → RCOCI + POCI₃ + HCI 3RCOOH + PCI₃ → 3RCOCI + H₃PO₃ RCOOH + SOCl₂ → RCOCI + SO₂+ HCI

Reaction with Ammonia

When carboxylic acids are allowed to react with ammonia, ammonium salt is formed which on further heating at high temperature gives amides.

Reactions involving -COOH group

Reduction

On reduction with reducing agents like LiAIH₄ or diborane, carboxylic acids are reduced to primary alcohols.

Decarboxylation

Sodium salts of carboxylic acids on heating with sodalime lose carbon dioxide to form hydrocarbons. The reaction is known as decarboxylation.

Substitution reactions in the Hydrocarbon

Halogenation

Carboxylic acids with a-hydrogen atom undergo halogenation at the a-position on treatment with small amount of red phosphorus to give a-halocarboxylic acids. The reaction is known as HeII- Volhard-Zelinsky reaction.

Ring Substitution

Aromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group acts as a deactivating and meta-directing group.

They however do not undergo Friedel-Crafts reaction because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group.

Comparison of acidic strength of alcohol , phenols and carboxylic acid

If we compare Alcohol and carboxylic acid, we see that :

It’s seen that more the ion is stablized , more the reaction will be favoured in found direction.Below you can see carboxylic acid is resonance stabilized. So, it is more stable and on the other hand the R group attached to O- in alcohol intensifies its charge. Hence, the stability is lowered in mparison to carboxylic acid.

Phenols and carboxylic acid : Acidic character

Similarly, if we look for phenoxide ion and carboxylate ion that is formed by phenol and carboxylic acid after loosing hydrogen ion, we can easily make out that carboxylate ion is more stabilized. In it negative charge resides on electronegative ion (resonating structure ) whereas in case of phenoxide ion the negative charge is on carbon.let’s see. the structures below :