The Nomenclature of Aldehydes and Ketones

The systematic (lUPAC) name of an aldehyde is obtained by replacing the final e on the name of the parent hydrocarbon with al. For example, a one-carbon aldehyde is called methana/, and a two-carbon aldehyde is called ethana/. The position of the carbonyl carbon does not have to be designated because it is always at the end of the parent hydrocarbon (or else the compound would not be an aldehyde), so it always has the 1-position. 

The common name of an aldehyde is the same as the common name of the corresponding carboxylic acid, except that aldehyde is substituted for oic acid (or ic acid ). Recall that the position of a substituent is designated by a lowercase Greek letter when common names are used. The carbonyl carbon is not given a designation, so the carbon adjacent to the carbonyl carbon is the a-carbon (Section 16.1). 

Notice that the terminal e of the parent hydrocarbon is not removed in hexanedial. (The e is removed only to avoid two successive vowels.) 

If the aldehyde group is attached to a ring, then the aldehyde is named by adding carbaldehyde to the name of the cyclic compound. 

In Section 7.2, we saw that a carbonyl group has a higher nomenclature priority than an alcohol or an amino group. However, all carbonyl compounds do not have the same priority. Nomenclature priorities of the various functional groups, including carbonyl groups, are listed in Table 17.1. 

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Let s take a moment to review the nomenclature of aldehydes and ketones, before we look at their reactions. The suffix for the aldehyde group, CHO, is -al, and the carbonyl carbon atom is always numbered as C-1. The suffix for ketones is -one, and the chain is numbered so that the carbonyl group has the lowest number. So 14.1 is 5-chloro-4-ethyl-2-methyheptanal, and 14.2 is 4-chloro-6,8-dimethyl-3-nonanone. Remember that aldehydes and ketones are described as oxo substituents when there is an acid derivative in the molecule and that the acid derivative will take precedence in the numbering scheme. 

Nomenclature of Aldehydes and Ketones.—The aldehydes are usually named from the acids into which they pass on oxidation, thus CH3.CHO is acetic aldehyde or acetaldehyde, as it is converted by oxidizing agents into acetic acid. In another system of nomenclature the name of an aldehyde is formed by adding the syllable al to the name of the hydrocarbon which contains the same number of carbon atoms as the aldehyde. According to this system acetaldehyde is called ethanal. 

An apparently similar reaction to the Arndt-Eistert process is the homologization of aldehydes and ketones by diazomethane. In the lUPAC nomenclature of transformations (1989 c), both are methylene insertions, but the homologization of aldehydes and ketones does not involve a carbene or ketene intermediate. We discussed it therefore in Section 7.7 in the context of dediazoniations via diazonium ions and carbocations. It is worthwhile to draw attention to the early work of Wolff, as mentioned at the beginning of this section. He discussed in one of his early papers the striking fact that, in boiling water, diazoacetophenone yields the expected (at least at that time) product of a hydroxy-de-diazoniation, but, in the presence of silver... 

The basic nomenclature for aldehydes and ketones follows that of other organic compounds. The following steps cire the keys ... 

The rules of nomenclature for aldehydes and ketones were described in Chapter 16 (Section 16.2). How are aldehydes and ketones prepared In many cases, the answer is via oxidation of an alcohol. Indeed, one of the most common and most important functional group transformations converts an alcohol to a ketone, as in the oxidation of 2-propanol (1) to acetone (2). This section will discuss several reagents that accomplish this transformation. 

The carbonyl group in glucose and ribose is an aldehyde such compounds are termed aldoses. Fructose, by contrast, has a ketone group and is therefore classified as a ketose. Glucose could also be termed an aldohexose and fructose a ketohexose, whereas ribose would be an aldopentose, names which indicate both the number of carbons and the nature of the carbonyl group. Another aspeet of nomenclature is the use of the suffix -ulose to indicate a ketose. Fructose could thus be referred to as a hexulose, though we are more likely to see this suffix in the names of specific sugars, e.g. ribulose is a ketose isomer of the aldose ribose. 

The structure and 2-ethyl-4-methylpentanal nomenclature of some aldehydes and ketones. 

In Chapter 21 we continue the study of carbonyl compounds with a detailed look at aldehydes and ketones. We will first learn about the nomenclature, physical properties, and spectroscopic absorptions that characterize aldehydes and ketones. The remainder of Chapter 21 is devoted to nucleophilic addition reactions. Although we have already learned two examples of this reaction in Chapter 20, nucleophilic addition to aldehydes and ketones is a general reaction that occurs with many nucleophiles, forming a wide variety of products. 

At one time it was customary to designate the products of addition of alcohols to ketones as ketals. This term has been dropped from the lUPAC system of nomenclature, and the term acetal is now applied to the adducts of both aldehydes and ketones. 

The name carbohydrates comes from their composition which is represented by the general formula C (H20) which resembles the combination of carbon and water. This composition implies that a molecule of monosaccharide consists of a hydrocarbon chain with attached hydroxyl groups. More detailed analysis shows that most of the monosaccharide molecules have hydrocarbon chains which are from five to seven carbon atoms long. From the study of the chemical behavior of different monosaccharides it follows that some of them exhibit reactions typical for aldehydes and others show reactions typical for ketones. The monosaccharides can therefore be classified into two groups aldoses and ketoses. In their nomenclature, the names of carbohydrates are formed from the root based on the number of C-atoms, the fundamental functional group (ketone or aldehyde) and the suffix -ose. While the aldehyde group always contains the first carbon atom of the chain, keto-group in all known carbohydrates appears at the second carbon atom of the chain. Some monosaccharides named in accordance with these rules are represented in the scheme below. 

The lUPAC nomenclature is based on the longest continuous chain (alkane, alkene, or alkyne based) that contains the carboxyl carbon (the -COOH unit). As with aldehydes, the carbon of the COOH group is always at one terminus of the chain and must receive the lowest possible number (1), which is omitted from the name. The suffix for carboxylic acids is oic acid, with the word acid separated from the first part of the name. As with aldehydes and ketones, the carboxyl unit is higher in priority than an alkene or alkyne. Compound 38 is an eight-carbon acid with an alkane backbone, so it is octanoic acid (note the short notation for the carboxyl group). Substituents are numbered relative to the carbonyl carbon of the COOH unit, so 39 is 2,2-dimethyl-5-phenyloctanoic acid. In a similar manner, 40 is named 16-chloro-4-ethylheptadecanoic acid. 

In Chapter 5 (Section 5.5.3), it was apparent that thiols are similar to alcohols, at least in terms of nomenclature and structure. Alcohols react with aldehydes and ketones, and it is known that thiols react in a similar manner. If butanal (20) reacts with ethanethiol (CH3CH2SH) in the presence of an acid catalyst, the reaction mechanism is identical to that for the reaction with ethanol (Section 18.6) except that the sulfur of the thiol is the nucleophile. Reaction with the thiol (rather than an alcohol) leads to 65, a thioacetal. The term thio is used to show the presence of a sulfur atom rather than an oxygen atom. 

Chapter 15 provides a brief introduction to commonly used organic molecules that also have a carbon-metal bond—organometallics. Chapter 16 introduces the fundamental characteristics of molecules that contain the carbon functional group, along with a review of the nomenclature of carbonyl-containing molecules. Carbonyl compounds are often prepared by oxidation reactions, and several key oxidation reactions are discussed in Chapter 17. Oxidation reactions of a few other functional groups are included. Chapter 18 elaborates the chemical reactions of the carbonyl-containing molecules known as aldehydes and ketones. This chemistry is dominated by the acyl addition reaction introduced in Chapter 16. 

In this chapter, we will be discussing the chemistry of carboxylic acids, esters, acyl halides, anhydrides, and amides. This is dominated by substitution, where one group is exchanged with another. Substitution is NOT possible for aldehydes and ketones, as you can t displace H or — they are hopeless leaving groups. First, let s review some nomenclature. The suffix for carboxylic acids is -oic acid and the carbonyl of the acid is always numbered as C-1. The acid takes precedence over most other functional groups. Some examples are shown in Figure 15.1. Notice that when we have both a ketone and an acid in the molecule, it is named as a carboxylic acid, and the ketone is described as oxo. ... 

Aldehydes and ketones The carbonyl group has a carbon atom attached by a double bond to an oxygen atom. If one of the two substituents attached to the carbonyl group is an H atom, the compound is an aldehyde, otherwise it is a ketone. In lUPAC nomenclature, the suffix -al is used for aldehydes and -one for ketones. 

NADH (reduced nicotinamide adenine dinucleotide) is utilized in biological reductions to deliver hydride to an aldehyde or ketone carbonyl group (see Box 7.6). A proton from water is used to complete the process, and the product is thus an alcohol. The reaction is catalysed by an enzyme called a dehydrogenase. The reverse reaction may also be catalysed by the enzyme, namely the oxidation of an alcohol to an aldehyde or ketone. It is this reverse reaction that provides the dehydrogenase nomenclature. 

The carbonyl group, C=0, is present in both aldehydes (RCH=0) and ketones (R2C=0). The IUPAC ending for naming aldehydes is -a/, and numbering begins with the carbonyl carbon. The ending for the names of ketones is -one, and the longest chain is numbered as usual. Common names are also widely used. Nomenclature is outlined in Sec. 9.1. 

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