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Aldehydes, Ketones and Carboxylic Acids
In the previous Unit, you have studied organic compounds with functional groups containing carbonoxygen single bond. In this Unit, we will study about the organic compounds containing carbon-oxygen double bond (>C=O) called carbonyl group, which is one of the most important functional groups in organic chemistry.
In aldehydes, the carbonyl group is bonded to a carbon and hydrogen while in the ketones, it is bonded to two carbon atoms. The carbonyl compounds in which carbon of carbonyl group is bonded to carbon or hydrogen and oxygen of hydroxyl moiety (-OH) are known as carboxylic acids, while in compounds where carbon is attached to carbon or hydrogen and nitrogen of -NH2 moiety or to halogens are called amides and acyl halides respectively. Esters and anhydrides are derivatives of carboxylic acids. The general formulas of these classes of compounds are given below:
Aldehydes, ketones and carboxylic acids are widespread in plants and animal kingdom. They play an important role in biochemical processes of life. They add fragrance and flavour to nature, for example, vanillin (from vanilla beans), salicylaldehyde (from meadow sweet) and cinnamaldehyde (from cinnamon) have very pleasant fragrances.
They are used in many food products and pharmaceuticals to add flavours. Some of these families are manufactured for use as solvents (i.e., acetone) and for preparing materials like adhesives, paints, resins, perfumes, plastics, fabrics, etc.
Nomenclature and Structure of Carbonyl Group
Nomenclature
I. Aldehydes and ketones
Aldehydes and ketones are the simplest and most important carbonyl compounds. There are two systems of nomenclature of aldehydes and ketones.
(a) Common names
Aldehydes and ketones are often called by their common names instead of IUPAC names. The common names of most aldehydes are derived from the common names of the corresponding carboxylic acids [Section 12.6.1] by replacing the ending –ic of acid with aldehyde. At the same time, the names reflect the Latin or Greek term for the original source of the acid or aldehyde. The location of the substituent in the carbon chain is indicated by Greek letters a, b, g, d, etc. The a-carbon being the one directly linked to the aldehyde group, b- carbon the next, and so on. For example
The common names of ketones are derived by naming two alkyl or aryl groups bonded to the carbonyl group. The locations of substituents are indicated by Greek letters, a a¢, b b¢ and so on beginning with the carbon atoms next to the carbonyl group, indicated as aa¢. Some ketones have historical common names, the simplest dimethyl ketone is called acetone. Alkyl phenyl ketones are usually named by adding the name of acyl group as prefix to the word phenone. For example
(b) IUPAC names
The IUPAC names of open chain aliphatic aldehydes and ketones are derived from the names of the corresponding alkanes by replacing the ending –e with –al and –one respectively. In case of aldehydes the longest carbon chain is numbered starting from the carbon of the aldehyde group while in case of ketones the numbering begins from the end nearer to the carbonyl group. The substituents are prefixed in alphabetical order along with numerals indicating their positions in the carbon chain. The same applies to cyclic ketones, where the carbonyl carbon is numbered one. When the aldehyde group is attached to a ring, the suffix carbaldehyde is added after the full name of the cycloalkane. The numbering of the ring carbon atoms start from the carbon atom attached to the aldehyde group. The name of the simplest aromatic aldehyde carrying the aldehyde group on a benzene ring is benzenecarbaldehyde. However, the common name benzaldehyde is also accepted by IUPAC. Other aromatic aldehydes are hence named as substituted benzaldehydes.
The IUPAC names of open chain aliphatic aldehydes and ketones are derived from the names of the corresponding alkanes by replacing the ending –e with –al and –one respectively. In case of aldehydes the longest carbon chain is numbered starting from the carbon of the aldehyde group while in case of ketones the numbering begins from the end nearer to the carbonyl group. The substituents are prefixed in alphabetical order along with numerals indicating their positions in the carbon chain. The same applies to cyclic ketones, where the carbonyl carbon is numbered one. When the aldehyde group is attached to a ring, the suffix carbaldehyde is added after the full name of the cycloalkane. The numbering of the ring carbon atoms start from the carbon atom attached to the aldehyde group. The name of the simplest aromatic aldehyde carrying the aldehyde group on a benzene ring is benzenecarbaldehyde. However, the common name benzaldehyde is also accepted by IUPAC. Other aromatic aldehydes are hence named as substituted benzaldehydes.
The common and IUPAC names of some aldehydes and ketones are given in Table 12.1
Structure of the Carbonyl Group
The carbonyl carbon atom is sp2-hybridised and forms three sigma (s) bonds. The fourth valence electron of carbon remains in its p-orbital and forms a p-bond with oxygen by overlap with p-orbital of an oxygen. In addition, the oxygen atom also has two non bonding electron pairs. Thus, the carbonyl carbon and the three atoms attached to it lie in the same plane and the p-electron cloud is above and below this plane. The bond angles are approximately 120° as expected of a trigonal coplanar structure (Figure 12.1).
The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen relative to carbon. Hence, the carbonyl carbon is an electrophilic (Lewis acid), and carbonyl oxygen, a nucleophilic (Lewis base) centre. Carbonyl compounds have substantial dipole moments and are 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.
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NCERT Class 12 Books Chemistry Chapter 12- Aldehydes, Ketones and Carboxylic Acids- PDF Download
Unit 12: Aldehydes, Ketones and Carboxylic Acids
अध्याय 12: एल्डिहाइड, कीटोन एवं कार्बोक्सिलिक अम्ल
Safalta provides the latest NCERT course books for all the major subjects of Class 12. A team of proficient teachers drafts these matters in a precise and thorough manner. You can download the PDFs for all the subjects in a Chapter 12-wise format.Unit 12: Aldehydes, Ketones and Carboxylic Acids
अध्याय 12: एल्डिहाइड, कीटोन एवं कार्बोक्सिलिक अम्ल
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Unit 12: Aldehydes, Ketones and Carboxylic Acids
अध्याय 12: एल्डिहाइड, कीटोन एवं कार्बोक्सिलिक अम्ल
Unit 12: Aldehydes, Ketones and Carboxylic Acids
अध्याय 12: एल्डिहाइड, कीटोन एवं कार्बोक्सिलिक अम्ल
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The Chapter 12s in CBSE Books for Class 12 Chemistry are vital for board exams and higher classes. Students should read the Unit given in the CBSE books for Class 12 Science. These stories and practice questions can help gain excellent marks.
To get outstanding marks, we provide mock test papers that can help gear-up your preparations for exams. Additionally, you can also download e-books to get yourself prepared even in a better way.
To get outstanding marks, we provide mock test papers that can help gear-up your preparations for exams. Additionally, you can also download e-books to get yourself prepared even in a better way.