Aldehydes and Ketones

Unit 12 Objectives 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. Aldehydes Aldehydes,, Ke tones and Carboxylic A cids 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 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. 12.1 Nomenclature and Structure of Carbonyl Group 12.1.1 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 α, β, γ, δ, etc. The α-carbon being the one directly linked to the aldehyde group, βcarbon 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, α α′, β β′ and so on beginning with the carbon atoms next to the carbonyl group, indicated as αα′. 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 common and IUPAC names of some aldehydes and ketones are given in Table 12.1. Table 12.1: Common and IUPAC Names of Some Aldehydes and Ketones Structure Common name IUPAC name Aldehydes HCHO CH3CHO Formaldehyde Acetaldehyde Methanal Ethanal (CH3)2CHCHO Isobutyraldehyde 2-Methylpropanal γ-Methylcyclohexanecarbaldehyde 3-Methylcyclohexanecarbaldehyde CH3CH(OCH3)CHO α-Methoxypropionaldehyde 2-Methoxypropanal CH3CH2CH2CH2CHO Valeraldehyde Pentanal CH2=CHCHO Acrolein Prop-2-enal Phthaldehyde Benzene-1,2-dicarbaldehyde m-Bromobenzaldehyde 3-Bromobenzenecarbaldehyde or 3-Bromobenzaldehyde Ketones CH3COCH2CH2CH3 Methyl n-propyl ketone Pentan-2-one (CH3)2CHCOCH(CH3)2 Diisopropyl ketone 2,4-Dimethylpentan-3-one α-Methylcyclohexanone 2-Methylcyclohexanone Mesityl oxide 4-Methylpent-3-en-2-one (CH3)2C=CHCOCH3