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 provides a brief overview of the chapter. These short notes are useful for CBSE Class 12 board examinations.

NCERT Class 12 notes of chapter 8 provide a structured approach to understanding the classification, properties, preparation, and reactions. Aldehydes, ketones, and carboxylic acids notes discuss the structure of the carbonyl group, Methods of preparation of Aldehydes and Ketones, physical properties of aldehydes and ketones and some of the important chemical reactions of Aldehydes and Ketones. We are also providing NCERT notes that cover all the topics and concepts presented in the NCERT textbook in a clear and comprehensive manner. These class 12 notes are also valuable resources for various competitive exams like JEE Main, NEET, and JEE Advanced. Also, check the NCERT Solutions for all the chapters.

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NCERT Notes for Class 12 Chapter 8: Download PDF

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NCERT Notes for Class 12 Chapter 8

Nomenclature and Structure of 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.

(a) Common Names: Aldehydes are often named based on the corresponding carboxylic acids by replacing the suffix –ic acid with –aldehyde. The names usually reflect the Latin or Greek origin of the compound. Substituent positions are indicated using Greek letters: α (alpha) for the carbon next to the aldehyde group, β (beta) for the next, and so on. Ketones are commonly named by listing the two alkyl or aryl groups attached to the carbonyl carbon. Substituent positions are shown as α, α′; β, β′, etc., starting from the carbon atoms next to the carbonyl group. Some ketones have traditional names (e.g., acetone). Alkyl phenyl ketones are often named by adding the acyl group as a prefix to phenone.

(b) IUPAC Names:

• Aldehydes: Replace –e of the alkane with –al. The chain is numbered starting from the aldehyde carbon.
• Ketones: Replace –e with –one. Numbering starts from the end closest to the carbonyl group.

Substituents are named alphabetically with position numbers. In cyclic ketones, the carbonyl carbon is position 1. For ring-attached aldehydes, add the suffix –carbaldehyde to the ring name, starting numbering from the aldehyde carbon. The simplest aromatic aldehyde, benzenecarbaldehyde, is commonly known (and IUPAC accepted) as benzaldehyde. Other aromatic aldehydes are named as substituted benzaldehydes.

Structure of the Carbonyl Group:

The carbonyl carbon is sp²-hybridised, forming three sigma (σ) bonds in a trigonal planar arrangement with bond angles close to 120°. Its unhybridised p-orbital overlaps with the p-orbital of oxygen to form a π-bond, while oxygen also carries two lone pairs. This results in a planar structure with the π-electron cloud above and below the plane., as shown in the figure.

The C=O bond is polar due to oxygen's higher electronegativity, making the carbonyl carbon electrophilic and the oxygen nucleophilic. Carbonyl compounds have significant dipole moments, making them more polar than ethers. The high polarity of the carbonyl group is explained on the basis of resonance involving a neutral (A) and a dipolar (B) structures as shown.

Preparation of Aldehydes and Ketones

a) 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

b) By the dehydrogenation of alcohols

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

c) From Hydrocarbons

• By ozonolysis of alkenes: As we know, ozonolysis of alkenes followed by reaction with zinc dust and water gives aldehydes,ketones or a mixture of both depending on the substitution pattern of the alkene.
• By hydration of alkynes: Addition of water to ethyne in the presence of H₂SO₄ and HgSO₄ gives acetaldehyde.

Preparation of Aldehydes

a) From acyl chloride (acid chloride):

Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate. This reaction is called Rosenmund reduction.

b) From nitriles and esters

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

$\mathrm{RCN}+{\mathrm{SnCl}}_2+\mathrm{HCl}\to \mathrm{RCH}=\mathrm{NH}+{\mathrm{H}}_3{\mathrm{O}}^{+}\to \mathrm{RCHO}$

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

Nitriles are reduced to aldehydes using DIBAL-H at low temperatures $(-{78}^{\circ }\mathrm{C})$.

$\mathrm{R}-\mathrm{C}\equiv \mathrm{N}\underset{-{78}^{\circ }\mathrm{C}}{\overset{\text{ DIBAL-H }}{\to }}\mathrm{R}-\mathrm{CHO}$

Esters can also be selectively reduced to aldehydes using DIBAL-H.

$\mathrm{R}-\mathrm{COOR}$ ' $\underset{-{78}^{\circ }\mathrm{C}}{\overset{\text{ DIBAL-H }}{\to }}\mathrm{R}-\mathrm{CHO}$

e) From Hydrocarbons: Benzaldehyde and its derivatives are syntehsized from hydrocarbons using the following methods:

By oxidation of methylbenzene: Strong oxidising agents oxidise toluene and its derivatives to benzoic acids. However, it is possible to stop the oxidation at the aldehyde stage with suitable reagents that convert the methyl group to an intermediate that is difficult to oxidise further. The following methods are used for this purpose.

a) Use of chromyl chloride (CrO₂Cl₂): Chromyl chloride converts the methyl group to a chromium complex, which when hydrolyzed, yields the benzaldehyde. This is known as Etard Reaction.

b)With chromic oxide: In the presence of chromic oxide, toluene or substituted toluene is converted to benzaldehyde in acetic anhydride.

By sidechain chlorination followed by hydrolysis: Toulene is chlorinated which gives benzal chloride, which on further hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde.

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 8 Aldehydes, ketones and carboxylic acid notes, the preparation of ketones is given by:

a) 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).

$2\mathrm{R}-\mathrm{Mg}-\mathrm{X}+{\mathrm{CdCl}}_2\to {\mathrm{R}}_2\mathrm{Cd}+2\mathrm{MgXCl}$

2 R'-CO-Cl $+{\mathrm{R}}_2\mathrm{Cd}\to 2$ R'-CO-R $+{\mathrm{CdCl}}_2$

b) From nitriles

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

c) 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

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.

a) Addition of hydrogen cyanide

b) Addition of sodium hydrogensulphite

c) Addition of alcohols

d) Addition of ammonia and its derivatives

2. Reduction

a) Reduction to alcohols

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

b) Reduction to 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.

a) 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

b) Fehling’s test

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

$\mathrm{R}-\mathrm{CHO}+2{\mathrm{Cu}}^{2+}+5{\mathrm{OH}}^{-}\to 2{\mathrm{ROO}}^{-}+{\mathrm{Cu}}_2\mathrm{O}+3{\mathrm{H}}_2\mathrm{O}$

c) Oxidation of methyl ketone by the 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, α-hydrogens in aldehydes and ketones are acidic in nature.

a) 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].

b) Cross Aldol condensation

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

5. Other reactions

a) 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.

6) 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.

Uses of Aldehyde and Ketones

Aldehydes and ketones serve as important solvents, reagents, and starting materials in the chemical industry.

• Formaldehyde (as 40% formalin) is used to preserve biological specimens and in the production of bakelite, urea-formaldehyde resins, and other polymers.
• Acetaldehyde is a key raw material for making acetic acid, ethyl acetate, vinyl acetate, polymers, and pharmaceuticals.
• Benzaldehyde finds applications in perfumery and the dye industry.
• Acetone and ethyl methyl ketone are widely used as industrial solvents.
• Several aldehydes and ketones like butyraldehyde, vanillin, acetophenone, and camphor are valued for their distinctive odours and flavours.

Carboxylic Acid

Carboxylic acids are organic compounds containing the –COOH group, which combines a carbonyl and a hydroxyl group. They are classified as aliphatic (RCOOH) or aromatic (ArCOOH) based on the attached group. Many carboxylic acids occur naturally, including fatty acids (C₁₂–C₁₈) found in fats as glycerol esters.

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°.

Methods of Preparation of Carboxylic Acids

a) From primary alcohols and aldehydes

b) From alkylbenzenes

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

c) 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.

d) From Grignard reagent

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

e) 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

f) 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. The hydrogen bonds are not broken completely even in the vapour phase. Most carboxylic acids exist as dimer in the vapour phase
or in the aprotic solvents.

Chemical Reactions of Carboxylic Acid

1. Reactions Involving Cleavage of O–H Bond (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-halo carboxylic 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.

Uses of Carboxylic Acid

Methanoic acid is utilised in the rubber, textile, dyeing, leather, and electroplating industries. Ethanoic acid serves as a solvent and is utilized as vinegar in the food sector. Hexanedioic acid is utilized in the production of nylon-6,6. Esters of benzoic acid. Acids are utilized in fragrance. Sodium benzoate serves as a food preservative. Long-chain fatty acids are utilized in the production of soaps and detergents.

Aldehydes, Ketones and Carboxylic Acids Previous year Question and Answer

Question: The number of molecules from below that cannot give an iodoform reaction is :
Ethanol, Isopropyl alcohol, bromoacetone, 2-Butanol, 2-Butanone, Butanal, 2-Pentanone, 3-Pentanone, Pentanal and 3-Pentanol

(1) 5

(2) 4

(3) 3

(4) 2

Answer:

A compound gives a positive iodoform test if it contains either:

• A methyl ketone group $\left({\mathrm{COCH}}_3\right)$
• A secondary alcohol that can be oxidized to a methyl ketone

The following will not give an iodoform reaction/test.

• Butanal- Butanal is an aldehyde; butanal does not have a ${\mathrm{COCH}}_3$ (methyl ketone) group.
• 3-Pentanone - This does have a $-{\mathrm{COCH}}_3$ group a methyl ketone in its structure.
• Pentanal - Pentanal is also an aldehyde like butanal, but does not have the ${\mathrm{CH}}_3-\mathrm{CO}-$ group.
• 3-Pentanol- This is a secondary alcohol, but it does not have the structure $-\mathrm{CH}(\mathrm{OH}){\mathrm{CH}}_3$, which is required for oxidation to a methyl ketone.

Hence, the correct answer is option (2).

Question: Both acetaldehyde and acetone (individually) undergo which of the following reactions?
A. Iodoform Reaction
B. Cannizaro Reaction
C. Aldol condensation
D. Tollen's Test
E. Clemmensen Reduction

Choose the correct answer from the options given below:

(1) A, B and D only

(2) A, C and E only

(3) C and E only

(4) B, C and D only

Answer:

A, C, and E only

Hence, the correct answer is option (2).

Question: Write the structures of $A,B$ and $C$ in the following sequence of reaction:

${\mathrm{CH}}_3\mathrm{COOH}\stackrel{{\mathrm{SOCl}}_2}{\to }\mathrm{A}\stackrel{{\mathrm{H}}_2,\mathrm{Pd}-{\mathrm{BaSO}}_4}{\to }\mathrm{B}\stackrel{{\mathrm{H}}_2\mathrm{N}-{\mathrm{NH}}_2}{\to }\mathrm{C}$

Answer:

Given:

${\mathrm{CH}}_3\mathrm{COOH}\stackrel{{\mathrm{SOCl}}_2}{\to }A\stackrel{{\mathrm{H}}_2,\mathrm{Pd}/{\mathrm{BaSO}}_4}{\to }B\stackrel{{\mathrm{NH}}_2{\mathrm{NH}}_2}{\to }C$

Acyl chloride formation

${\mathrm{CH}}_3\mathrm{COOH}\stackrel{{\mathrm{SOCl}}_2}{\to }{\mathrm{CH}}_3\mathrm{COCl}$

Structure of A: Acetyl chloride

$\mathrm{A}={\mathrm{CH}}_3\mathrm{COCl}$

Rosenmund reduction
Reduction of acid chloride to aldehyde using ${\mathrm{H}}_2,\mathrm{Pd}/{\mathrm{BaSO}}_4$ (poisoned catalyst)

${\mathrm{CH}}_3\mathrm{COCl}\stackrel{{\mathrm{H}}_2,\mathrm{Pd}/{\mathrm{BaSO}}_4}{\to }{\mathrm{CH}}_3\mathrm{CHO}$
Structure of B: Acetaldehyde

$\mathrm{B}={\mathrm{CH}}_3\mathrm{CHO}$

Wolff-Kishner Reduction
Aldehyde reacts with hydrazine (${\mathrm{NH}}_2{\mathrm{NH}}_2$) under basic conditions to give alkane:

${\mathrm{CH}}_3\mathrm{CHO}\stackrel{{\mathrm{NH}}_2{\mathrm{NH}}_2,\text{ base }}{\to }{\mathrm{CH}}_3{\mathrm{CH}}_3\text{ (C) }$

Structure of C: Ethane

$\mathrm{C}={\mathrm{CH}}_3{\mathrm{CH}}_3$

Approach to Solve Questions of Class 12 Chemistry Chapter 8 Aldehydes, Ketones, and Carboxylic Acids

The following are the points that can help you build a good approach to solve the questions effectively and efficiently:
1. Understand functional groups & nomenclature
It is easy yet crucial as there will be reactions on specific functional groups. To solve the question, you also need to draw the structures of the given compound.

2. Grasp the general preparation methods
Know how each class is prepared from alcohols (oxidation), nitriles/hydrolysis, acid chlorides, Grignard reagents, etc. Also learn to write balanced reactions with reagents and conditions.

3. Master Important reactions and mechanisms
For Aldehydes and Ketones-

• Nucleophilic addition reactions (e.g., HCN, NaHSO₃, Grignard reagents).
• Reduction and oxidation reactions.
• Aldol condensation, Cannizzaro reaction.

For Carboxylic Acids-

• Acid-base reactions, decarboxylation, esterification (Fischer esterification).
• Substitution on –COOH group, like Hell–Volhard–Zelinsky reaction.

4. Learn oxidation and reduction trends
Learn the specifity of the reagents used and their reactivity order.

• Oxidizing agents- KMnO₄, H₂CrO₄, Tollen’s, Fehling’s.
• Reducing agents- LiAlH₄, NaBH₄, Zn-HCl (Clemmensen), NH₂NH₂-KOH (Wolff–Kishner).

5. Pay special attention to Named Reactions
These are too important as most of the time they are asked directly in the exams.

• Aldol Condensation
• Cannizzaro Reaction
• Rosenmund Reduction
• Stephen Reduction
• Hell–Volhard–Zelinsky Reaction

7. Practice conversion & interconversion problems

8. Solve the NCERT intext and the NCERT exemplar questions for better understanding and concept clearence.

9. Questions from NCERT books are asked directly in NCERT boards and other competitive exams.

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. These notes definitely help students prepare for competitive exams like VITEEE, BITSAT, JEE Main, NEET, etc.

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