Aldehydes, Ketones and Carboxylic Acids Class 12th Notes- Free NCERT Class 12 Chemistry Chapter 12 Notes - Download PDF

What is the first thing that comes to mind when you smell something delicious or try a new scent? Perhaps it's the aroma of freshly baked bread or the sweet scent of a flower? Did you know that aldehydes, ketones, and carboxylic acids are often responsible for these smells? These are found in many industries, biological systems, and household items. This article will provide a quick overview of the chapter. These short notes are useful for CBSE Class 12 board examinations and strictly follow the most recent CBSE Syllabus.

NCERT Class 12 Chemistry chapter 8 notes discuss the structure of the carbonyl group, Methods Of Preparation Of Aldehydes And Ketones. CBSE Class 12 Chemistry chapter 8 notes also include physical properties of aldehydes and ketones, and discuss some of the important chemical Reaction Of Aldehydes And Ketones Aldehydes, ketones and Carboxylic Acids Class 12 notes give an idea about the structure of the carboxyl group, preparations of carboxylic acid. Physical properties and chemical reactions of carboxylic acid are also explained in notes for class 12 chemistry chapter 8, etc.

Students can refer to the Class 12 Chemistry revision notes solution chapter 8 for more details on these concepts. This chapter is part of organic chemistry, which can be confusing and daunting at first. Students can learn more about these subjects by reading the CBSE Class 12 Chemistry revision notes, Chapter 8.

Also, students can refer,

NCERT Class 12 Chemistry Aldehydes, ketones, and carboxylic acids Notes

Carbonyl Group

Aldehydes, ketones, carboxylic acids, and their derivatives are organic compounds with the carbonyl group (CO) as the functional group. These are referred to as carbonyl compounds together. The carbonyl group's oxygen atom is significantly more electronegative than the carbon atom. As a result, the oxygen atom attracts the electron cloud of the - bond towards itself, resulting in an unsymmetrical electron cloud of >C = O. As a result, carbonyl carbon has a positive charge while carbonyl oxygen has a negative charge. Thus, the carbonyl group is polar. In aldehydes, one hydrogen and an alkyl/aryl group are attached to carbonyl group. But in ketone, two alkyl/aryl groups are attached to the carbonyl group.

Preparation of Aldehydes

According to CBSE Class 12 Chemistry chapter 12 notes, the preparation of aldehydes is given by:

• By oxidation of alcohols

Aldehydes are formed when primary alcohols are oxidized in the presence of an oxidizing agent such as K₂Cr₂O₇ / H₂SO₄, KMnO₄, or CrO₃

• By the dehydrogenation of alcohols

Aldehyde is formed when primary alcohol vapours pass over heated copper or silver at 573 K.

• By Rosenmund reduction

Aldehyde is produced by hydrogenating acyl chloride over palladium on barium sulphate.

• By Stephen’s reaction

When nitriles are reduced in the presence of stannous chloride and HCl, an imine is formed, which when hydrolyzed yields the corresponding aldehyde.

RCN + SnCl₂ + HCl → RCH=NH + H₃O⁺→ RCHO

• From nitriles and esters

DIBAL-H (Diisobutylaluminium Hydride) preferentially reduces nitriles and esters to aldehydes.

RCN AlH(i-Bu)₂,H₂O→ RCHO

CH₃(CH₂)₉-CO-OC₂H₅ + DIBAL-H→ CH₃(CH₂)₉-CHO

• By oxidation of methylbenzene (Etard reaction)

Chromyl chloride converts the methyl group to a chromium complex, which when hydrolyzed, yields the benzaldehyde.

• With chromic oxide

n the presence of chromic oxide, toluene or substituted toluene is converted to benzaldehyde in acetic anhydride.

• By sidechain chlorination followed by hydrolysis

• By the Gattermann-Koch reaction

Benzaldehyde or substituted benzaldehydes are produced by treating benzene or its derivatives with carbon monoxide and HCl in the presence of anhydrous aluminium chloride or cuprous chloride (CuCl).

Preparation of Ketones

According to Aldehydes, ketones and carboxylic acid Class 12 notes and class 12 Aldehydes, ketones and carboxylic acid notes, the preparation of ketones is given by:

From acyl chlorides

Acyl chloride is converted to a ketone when it is treated with dialkyl cadmium (made by reacting cadmium chloride with a Grignard reagent).

2 R-Mg-X + CdCl₂ → R₂Cd+2 MgXCl

2 R’-CO-Cl + R₂Cd → 2 R’-CO-R + CdCl₂

• From nitriles

Nitriles are converted to ketones after being treated with a Grignard reagent and then hydrolyzed.

• From benzene or substituted benzene

Ketone is formed when benzene or substituted benzene is treated with acid chloride in the presence of anhydrous aluminium chloride.

Physical Properties of Aldehydes and Ketones

• At room temperature, methane is a gas. Ethanol is a very flammable liquid. At room temperature, other aldehydes and ketones are liquid or solid.

• Boiling points are greater than hydrocarbons and ethers of comparable molecular weights. This is because of weak molecular association in aldehydes and ketones due to the dipole-dipole interactions. Due to the lack of H-bonding, their boiling points are lower than those of alcohols.

• Because they form H-bonds with water, lower aldehydes and ketones like methanal, ethanal, and propanone are miscible with water in all proportions.

• Lower aldehydes have a harsh, pungent odour. The odour gets less strong and more delicate as molecular mass increases.

Chemical Reactions of Aldehydes and Ketones

NCERT notes for Class 12 Chemistry chapter 12 explain the chemical reactions as:

1. Nucleophilic addition reactions

The hybridization of C shifts from sp² to sp³ when a nucleophile attacks the carbonyl carbon. Due to steric and electrical reasons, aldehydes are more sensitive to nucleophilic addition processes than ketones.

• Addition of hydrogen cyanide

• Addition of sodium hydrogensulphite

• Addition of alcohols

• Addition of ammonia and its derivatives

2. Reduction

• Reduction of alcohols

Sodium borohydride or lithium aluminium hydride, as well as catalytic hydrogenation, reduce aldehydes and ketones to primary and secondary alcohols, respectively.

• Reduction of hydrocarbons

When aldehydes and ketones are treated with zinc amalgam and strong hydrochloric acid, the carbonyl group is reduced to CH₂,(Clemmensen reduction)

On treating carbonyl compound with hydrazine followed by sodium or potassium hydroxide in a high boiling solvent such as ethylene glycol reduces the carbonyl group of aldehydes and ketones to CH₂ (Wolff-Kishner reaction)

3. Oxidation

Oxidation reactions of aldehyde and ketones are different

Aldehydes and ketones can be distinguished using the 2 reactions below because aldehydes are easier to oxidize than ketones.

• Tollen’s test

A bright silver mirror is formed by heating an aldehyde with freshly made ammonical AgNO₃ solution (Tollen's reagent).

RCHO + 2 [Ag(NH₃)₂]⁺ + 3 OH^- → RCOO^- + 2 Ag + 2 H₂O + 4 NH₃

• Fehling’s test

A reddish-brown precipitate is formed when an aldehyde is heated with Fehling reagent.

R-CHO + 2 Cu²⁺ + 5 OH^- → 2 ROO^-+ Cu₂O + 3 H₂O

• Oxidation of methyl ketone by haloform reaction

This test is positive for all carboxyl compounds with the – COCH₃ group.

4. Reactions due to α-hydrogen

Because of the strong electron-withdrawing impact of the carbonyl group and the resonance stabilization of the conjugate base, -hydrogens in aldehydes and ketones are acidic in nature.

• Aldol condensation

In the presence of dil. alkali, aldehydes and ketones with at least one -hydrogen undergo Aldol Condensation to create β-hydroxy aldehydes (aldol) or β-hydroxy ketones [ketol].

• Cross Aldol condensation

Cross aldol condensation occurs when aldol condensation occurs between two separate aldehydes and/or ketones.

5. Other reactions

• Cannizzaro reaction

When aldehydes without -hydrogen atom are treated with strong alkali, they undergo self-oxidation and reduction (disproportionation) reactions, resulting in alcohol and acid salt.

• Electrophilic substitution reaction

Electrophilic substitution reactions occur at the ring of aromatic aldehydes and ketones, while the carbonyl group serves as a deactivating and meta-directing group.

Structure of Carboxyl Group

The carboxylic carbon is less electrophilic due to the potential resonance configuration depicted below. The carbonyl carbon and the carboxylic carbon bonds are then aligned in the plane and separated by 120°.

Preparation of Carboxylic Acid

• From primary alcohols and aldehydes

• From alkylbenzenes

Aromatic carboxylic acids are made by oxidizing alkylbenzenes with chromic acid or acidic or alkaline potassium permanganate at high temperatures.

• From nitriles and amides

When nitriles are hydrolyzed in the presence of dilute acids or bases, an amide is formed, which can then be further hydrolyzed to produce carboxylic acid.

• From Grignard reagent

Grignard reagents react with carbon dioxide (dry ice) to produce carboxylic acid salts, which are then hydrolyzed to produce carboxylic acids.

• From acyl halides and anhydrides

When acid chlorides are hydrolyzed with water, carboxylic acids are formed. Carboxylate ions are generated during basic hydrolysis and are then acidified to form carboxylic acids. When anhydrides are hydrolyzed, the corresponding acid is produced

• From esters

Basic hydrolysis of esters produces carboxylates, which are acidified to produce corresponding carboxylic acids. Acidic hydrolysis of esters directly produces corresponding carboxylic acids.

Physical Properties of Carboxylic Acid

Because the non-polar component of the acid increases as the size of the alkyl group increases, the solubility of carboxylic acid decreases. Carboxylic acids boil at a higher temperature than aldehydes, ketones, or even alcohols with similar molecular weights. This is due to significant intermolecular hydrogen bonding between carboxylic acid molecules.

Chemical Reactions of Carboxylic Acid

1. Acidity

• Reaction with metals

2 RCOOH + 2 Na → 2 R-COO^-Na⁺ + H₂

R-COOH + NaOH → 2 R-COO^-Na⁺ + H₂O

R-COOH + NaHCO₃ → 2 R-COO^-Na⁺ + H₂O + CO₂

Carboxylic acid dissociates in water to yield the carboxylate anion and the hydronium ion, which are both resonance-stabilized.

Effects of substituents on carboxylic acid acidity: Electron-withdrawing groups increase carboxylic acid acidity by stabilizing the conjugate base by delocalization of the negative charge via inductive and/or resonance effects. Electron-donating groups, on the other hand, reduce acidity by destabilizing the conjugate base.

2. Reactions involving the cleavage of the C-OH bond

• Formation of anhydride

• Esterification

In the presence of a mineral acid such as concentrated H₂SO₄ or HCl gas as a catalyst, carboxylic acids are esterified with alcohols.

RCOOH + R’OH (acidic medium) ↔ RCOOR’ + H₂O

• Reactions with PCl₃, PCl₅ and SOCl₂

RCOOH + PCl₅ → RCOCl + POCl₃ + HCl

3 RCOOH + PCl₃ → 3 RCOCl + H₃PO₃

RCOOH + SOCl₂ → RCOCl +SO₂ + HCl

• Reaction with ammonia

3. Reactions involving -COOH group

• Reduction

In the presence of LiAlH₄ or B₂H₆, carboxylic acids are reduced to alcohols.

• Decarboxylation

Heating sodium or potassium salts of carboxylic acids with soda lime (NaOH + CaO in a 3:1 ratio) produces hydrocarbons with one less carbon than the parent acid.

4. Substitution reactions in the hydrocarbon part

• Halogenation (Hell-Volhard-Zelinsky reaction)

Carboxylic acids containing α-hydrogen are halogenated at the α-position with chlorine or bromine in the presence of a small amount of red phosphorus to produce α-halocarboxylic acids).

• Ring substitution

Electrophilic substitution reactions occur in aromatic carboxylic acids. In benzoic acid, the carboxyl group is an electron-withdrawing and meta-directing group.

Significance of NCERT Class 12 Chemistry Chapter 8 Notes

To attempt and answer problems in the chapter "Aldehydes, Ketones, and Carboxylic Acids," a good understanding of concepts is required. The key topics in Aldehydes, Ketones, and carboxylic acid Class 12 notes have been fractured to help students better study and comprehend the subjects. The chapter is adorned with concepts, equations, and the answers are written under expert supervision.

NCERT notes for Class 12 Chemistry chapter 8 provide an overview of the main topics of the Class 12 CBSE Chemistry Syllabus. NCERT notes for Class 12 Chemistry Chapter 8 definitely help students for the preparation of competitive exams like VITEEE, BITSAT, JEE Main, NEET etc.

NCERT Class 12 Notes Chapter-Wise

Subject-Wise NCERT Exemplar Solutions

Subject-Wise NCERT Solutions

NCERT Books and Syllabus