Aldehydes IUPAC nomenclature

In substitutive IUPAC nomenclature, cyanohydrins are named as hydroxy derivatives of nitriles. Because nitrile nomenclature will not be discussed until Section 20.1, we will refer to cyanohydrins as derivatives of the parent aldehyde or ketone as shown in the examples. This conforms to the practice of most chemists. 

Aldehydes, acids, and esters have roots for one and two carbons that are usually form- and acet-, rather than meth- and eth-, because these prefixes had been used so long they were grandfathered into the naming system (formaldehyde and acetic acid, rather than methanal and ethanoic acid). Departures from IUPAC nomenclature often occur for very common substances and, fortunately, they rarely can be misunderstood (ethyl alcohol instead of ethanol). 

A word about nomenclature. Once upon a time, chemists made a useful distinction between acetals (derived from aldehydes) and ketals (derived from ketones) which has since perished. The International Union of Pure and Applied Chemistry (IUPAC) decided (Rule C-331.1) that the term ketal is redundant and that the term acetal should now apply to all 1,1-to-ethers whether derived from aldehydes or ketones. Nevertheless, in A Guide to IUPAC Nomenclature of Organic Compounds, Recommendations 1993 (R. Panico, W.H. Powel, I-C Richter, Eds. Blackwell Science Oxford, 1993) the IUPAC graciously reinstated the term ketal , by popular demand, as a subclass of the generic term acetals (Rule R-5.6,4),... 

In IUPAC nomenclature, the suffix for aldehydes is -al and for ketones, -one. The prefix for both is oxo. 

IUPAC Nomenclature of Aldehydes Section ll.2A (a) decanal (b) 4-methylpentanal (c) 5-ethyl-3-methylheptanal ... 

Reaction of an a-substituted enolate with an aldehyde or ketone can give two pairs of aldol dia-stereomers, which are conveniently designated as the syn form (17) and the anti form (18), where R2 is part of the parent chain in IUPAC nomenclature (equation 29). For simplicity, only one enantiomer of each pair will usually be shown throughout this section. The synlanti notation for aldol diastereomers has been described in detail by Heathcock.8... 

It is hard to find a class of compounds in which the common names of its members have influenced organic nomenclature more than carboxylic acids. Not only are the common names of carboxylic acids themselves abundant and widely used, but the names of many other compounds are derived from them. Benzene took its name from benzoic acid and propane from propionic acid, not the other way around. The name butane comes from butyric acid, present in rancid butter. The common names of most aldehydes are derived from the common names of carboxylic acids—valeraldehyde from valeric acid, for example. Many carboxylic acids are better known by common names than by their systematic ones, and the framers of the IUPAC rules have taken a liberal view toward accepting these common names as permissible alternatives to the systematic ones. Table 19.1 lists both common and systematic names for a number of important carboxylic acids. 

The five IUPAC rules previously outlined in the Nomenclature section for aldehydes also apply to ketones, with one exception After dropping the -e ending of the alkane name, you add -one for ketones (rather than -al, which designates aldehydes). 

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. 

The IUPAC rules for naming aldehydes append the suffix -al to the parent name. The aldehyde carbon is always the first carbon in the chain, so a locant is not necessary. The simplest aldehyde, methanal, is commonly known as formaldehyde, a highly toxic gas used to preserve biological specimens. Ethanal is the next aldehyde, although most organic chemists call it acetaldehyde. 2,2,2-Tri-chloroethanal, more commonly known as chloral, reacts with water to form the sedative chloral hydrate. Phenylmethanal, more commonly known as benzalde-hyde, is used as artificial cherry or almond flavoring. Nomenclature examples of aldehydes are shown in Figure 11.34. 

The common method of naming aldehydes corresponds very closely to that of the related acids (see Carboxylic acids), in that the term aldehyde is added to the base name of the acid. For example, formaldehyde (qv) comes from formic acid, acetaldehyde (qv) from acetic acid, and butyraldehyde (qv) from butyric acid. If the compound contains more than two aldehyde groups, or is cyclic, the name is formed using carbaldehyde to indicate the functionality. The IUPAC system of aldehyde nomenclature drops the final e from the name of the parent acyclic hydrocarbon and adds al. If two aldehyde functional groups are present, the suffix - dial is used The prefix formyl is used with polyfunctional compounds. Examples of nomenclature types are shown in Table 1. 

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