All about aldehyes and ketones

MODULE - 7 Chemistry

Chemistry of Organic
Compounds


29
Notes
ALDEHYDES, KETONES AND
CARBOXYLIC ACIDS

In the last lesson, you studied about organic compounds containing functional groups
having carbon-oxygen single bond. There are other classes of organic compounds in which
the functional group contains the carbon- oxygen double bond. The examples of these
classes of compounds being carbonyl compounds such as aldehydes and ketones as well
as carboxylic acids and their derivatives. These organic compounds are very important
both in the industry and in the synthesis of other organic compounds. Therefore, their
study forms an important part of the organic chemistry. Let us study the chemistry of
these classes of compounds in detail.


Objectives
After reading this lesson, you should be able to
 give IUPAC names of aldehydes and ketones;
 describe the general methods of preparation of aldehydes and ketones;
 discuss the trends in physical properties of the aldehydes and ketones in the light of
the polar nature of the carbonyl group;
 explain important reactions exhibited by aldehydes and ketones;
 distinguish between aldehydes and ketones on the basis of certain reactions and tests
based on them;
 give IUPAC names of carboxylic acids;
 explain general methods of preparation of carboxylic acids;
 discuss the physical properties and their trends for simple monocarboxylic acids;
 describe important reactions exhibited by carboxylic acids;
 explain the preparation and some interconversion reactions of carboxylic acid
derivatives, and
 highlight the importance of aldehydes, ketones and carboxylic acids.

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, Aldehydes, Ketones and Carboxylic Acids MODULE - 7
29.1 Aldehydes and Ketones Chemistry of Organic
Compounds
You have some familiarity with these classes of compounds from previous lessons. These
O




—
compounds are also referred to as carbonyl compounds and have —C— functionality
present in them. These compounds exist widely in nature and are responsible for the
flavour and aroma of many foods. They are also important industrially both as reagents in Notes
synthesis and as solvents.

Aldehydes have at least one hydrogen atom bonded to the carbonyl group, the other group
may be either a hydrogen or an alkyl (or aryl) group. In ketones, the carbonyl group is
bonded to two alkyl or aryl groups. The two groups bonded to a ketone may be similar or
different resulting in a symmetrical or an unsymmatrical ketone, respectively.




You must be familiar with vanilin and camphor. Their structures are given below. You can
see that they contain an aldehyde and a keto functional group, respectively.




29.1.1 Nomenclature of Aldehydes and Ketones
In the IUPAC system of nomenclature, aliphatic aldehydes are named as alkanals. The
final -e in the name of the corresponding alkane is substituted by -al.
Some common examples of aldehydes and their names are given below :




Note that when the -CHO group is attached to a ring, then the compound is called a
carbaldehyde.

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, MODULE - 7 Chemistry

Chemistry of Organic Remember that the carbonyl carbon of the aldehydes is present at the end of the chain and
Compounds is assigned 1 position. Therefore, it is not necessory to specify its position in the name of
the aldehyde. The examples given below illustrate this point.




Notes

Ketones are named as alkanones in the IUPAC namenclature. Their names are obtained
by replacing final -e in the name of alkane by -one. The carbon chain is numbered in such
a way that the carbonyl group gets the lowest number. Some examples of ketones are
mentioned below :

O
O O
CH3C CH3 CH3CH 2 CCH3
4 3 2 1
Propanone
(Acetone) Butan-2-one Cyclohexanone
(Ethyl methyl ketone)


O O
CH3 CH 2 C CH 2 CH3 CH3 CCH 2 CH = CH 2
5 4 3 2 1
Pentan-3-one Pent-4-en-2-one
(Diethyl ketone)




29.1.2 Preparation of Aldehydes and Ketones
You have already studied most of the methods used in the synthesis of aldehydes and
ketones in the previous lesson. Let us now refresh them.
1. Oxidation of Primary and Secondary Alcohols
From the last lesson, you know that primary alcohols can be oxidised to aldehydes and
secondary alcohols can be oxidised to ketones.
2. Ozonolysis of Alkenes
This reaction has been discussed in lesson 26. The products obtained are aldehydes or
ketones depending upon the structure of the starting alkene.
3. Hydration of Alkynes
Hydration of alkynes can give an aldehyde or a ketone. Markovnikov’s hydration yields
ketones whereas anti-Markovnikov’s hydration gives aldehydes.
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