Aldehydes, Ketones and Carboxylic Acids

After studying this Unit, you will be able to • write the common and IUPAC names of aldehydes, ketones and carboxylic acids; • write the structures of the compounds containing functional groups namely carbonyl and carboxyl groups; • describe the important methods of preparation and reactions of these classes of compounds; • correlate physical properties and chemical reactions of aldehydes, ketones and carboxylic acids, with their structures; • explain the mechanism of a few selected reactions of aldehydes and ketones; • understand various factors affecting the acidity of carboxylic acids and their reactions; • describe the uses of aldehydes, ketones and carboxylic acids.

Carbonyl compounds are of utmost importance to organic chemistry. They are constituents of fabrics, flavourings, plastics and drugs.

In the previous Unit, you have studied organic compounds with functional groups containing carbon-oxygen single bond. In this Unit, we will study about the organic compounds containing carbon-oxygen double bond ($\text{ce}>C=O$) 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 ($\text{ce}-OH$) are known as carboxylic acids, while in compounds where carbon is attached to carbon or hydrogen and nitrogen of $\text{ce}-NH2$ 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.

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 8.6.1] 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 $\alpha$, $\beta$, $\gamma$, $\delta$, etc. The $\alpha$-carbon being the one directly linked to the aldehyde group, $\beta$-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, $\alpha {\alpha }^{\prime }$, $\beta {\beta }^{\prime }$ and so on beginning with the carbon atoms next to the carbonyl group, indicated as $\alpha {\alpha }^{\prime }$. 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 common and IUPAC names of some aldehydes and ketones are given in Table 8.1.

Structure Common name IUPAC name

Aldehydes

$\text{ce}HCHO$ Formaldehyde Methanal $\text{ce}CH3CHO$ Acetaldehyde Ethanal

$\text{ce}(CH3)2CHCHO$ Isobutyraldehyde 2-Methylpropanal

$\gamma$-Methylcyclohexanecarbaldehyde 3-Methylcyclohexanecarbaldehyde

$\alpha$-Methoxypropionaldehyde $\text{ce}CH3CH(OCH3)CHO$ 2-Methoxypropanal

$\text{ce}CH3CH2CH2CH2CHO$ Valeraldehyde Pentanal

$\text{ce}CH2=CHCHO$ Acrolein Prop-2-enal

Phthaldehyde Benzene-1,2-dicarbaldehyde

3-Bromobenzenecarbaldehyde or

m-Bromobenzaldehyde 3-Bromobenzaldehyde

Ketones

Methyl n-propyl ketone $\text{ce}CH3COCH2CH2CH3$ Pentan-2-one

$\text{ce}(CH3)2CHCOCH(CH3)2$ Diisopropyl ketone 2,4-Dimethylpentan-3-one

$\alpha$-Methylcyclohexanone 2-Methylcyclohexanone

$\text{ce}(CH3)2C=CHCOCH3$ Mesityl oxide 4-Methylpent-3-en-2-one

The carbonyl carbon atom is $s{p}^2$-hybridised and forms three sigma ($\sigma$) bonds. The fourth valence electron of carbon remains in its p-orbital and forms a $\pi$-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 $\pi$-electron cloud is above and below this plane. The bond angles are approximately $120^{\circ }$ as expected of a trigonal coplanar structure (Figure 8.1).

$\pi$