CBSE Class 12th Chemistry Notes: Aldehydes, Ketones and Carboxylic Acids (Part - I)

CBSE Class 12 Chemistry Notes on Chapter 12: Aldehydes, Ketones and Carboxylic Acids are provided in this article. With this article, you will get a quick glance of the chapter. These quick notes are based the latest CBSE syllabus for CBSE Class 12th Chemistry and are important for CBSE Class 12 board examinations.

The main topics covered in this article are:

o    Nature of Carbonyl Group

o    Aldehydes and ketones

     •    Introduction

     •    Nomenclature

     •    Preparation

     •    Physical properties

Nature of Carbonyl Group

Aldehydes, ketones and carboxylic acids are the compounds containing carbon-oxygen double bond (>C=O) called carbonyl group. The carbon and oxygen of the carbonyl group are sp² hybridised and the carbonyl double bond is comprised of one σ-bond and one π-bond. The π electron cloud is present above and below the plane and is present between carbonyl carbon and oxygen.

Due to high electronegativity of oxygen compared to carbon, the carbonyl group is polar and the dipole is formed.

Aldehydes

Aldehydes are the organic compounds in which carbonyl group is attached to one hydrogen atom and one alkyl or aryl group.

Where R can be an alkyl or aryl group.

Ketones

Ketone are the organic compounds in which carbonyl group is attached to two alkyl group or aryl group or both alkyl and aryl group.

Where R and R’ may be alkyl or aryl groups

CBSE Class 12th Chemistry Notes: Alcohols, Phenols and Ethers

Nomenclature of Aldehydes

IUPAC naming system:

•    In IUPAC system, the suffix ‘e’ of alkane is replaced by the ‘al’.

For example:

  HCHO                                   CH₃CHO

Methanal                                 Ethanal

Common naming system:

•    The common names of aldehydes are derived from the common names of the corresponding carboxylic acids in which the ending ‘-ic’  is replaced with ‘-aldehyde’of acid with aldehyde.

For example:

     HCHO                                           CH₃CHO

Formaldehyde                                  Acetaldehyde

Nomenclature of Ketones

IUPAC naming system:

•    In IUPAC system, the suffix ‘e’ of alkane is replaced by the ‘one’.

For example:

Common naming system:

•    The common names of ketones are derived by naming two alkyl or aryl groups bonded to the carbonyl group.

For example:

•    Alkyl phenyl ketones are usually named by adding the acyl group as prefix to phenone. For example:

Preparation of Aldehyde and Ketones

(a) By oxidation of alcohols:

Oxidation of primary and secondary alcohols in presence or oxidizing agent like K₂Cr₂O₇/H₂SO₄, KMnO₄. CrO₃ gives aldehydes and ketones respectively.

(b) By dehydrogenation of alcohols:

(c)  By ozonolysis of alkenes:

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.

(d) By hydration of alkynes:

This is a commercial method to prepare ethanal.

All other alkynes gives ketones.

(e) By Rosenmund reduction:
Hydrogenation or acyl chloride over palladium on barium sulphate gives aldehyde.

(f) From nitriles (RCN):

This reaction is called Stephen reaction.

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Preparation of Benzaldehyde and Acetophenone

(a) By oxidation of methylbenzene:

This reaction is called Etard reaction.

(b) By side chain chlorination of methybenzene followed by hydrolysis:

This is a commercial method of manufacture of benzaldehyde.

(iii) By Gatterman – Koch reaction

Physical Properties of Aldehydes and Ketones

•   The intermolecular forces of attraction in aldehydes and ketones are dipole-dipole interactions. These are stronger than van der Waals forces in alkanes but weaker than H- bonding in alcohols. Therefore, the boiling point order is:    Alkanes < Aldehyde / Ketones < Alcohols

•   Due to the polarity of the carhonyl group, lower aldehydes and ketones are capable of forming hydrogen bond with water and are soluble while solubility decreases down the homologous series as size of R group increases.

•   All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, chloroform, etc.

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