How Are Aldehydes and Ketones Named

Aldehyde A compound containing a carbonyl group bonded to hydrogen (a CHO group). 

For unsaturated aldehydes, the presence of a carbon-carbon double bond is indicated by the infix -en-. As with other molecules with both an infix and a suffix, the location of the suffix determines the numbering pattern. 

For cyclic molecules in which —CHO is bonded directly to the ring, we name the molecule by adding the suffix -carbaldshyds to the name of the ring. We number the atom of the ring bearing the aldehyde group as number 1  

Among the aldehydes for which the lUPAC system retains common names are benzalde-hyde and cinnamaldehyde. Note here the alternative ways of writing the phenyl group. 

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Aldehydes and ketones, and other carbonyl compounds having hydrogen atoms on the a-carbon, exist in solution as equilibrium mixtures of two or more isomeric forms. These isomers are termed the keto form, which is how we normally represent a carbonyl compound, and the enol form, which takes its name from the combination of double bond and alcohol. 

In addition to using numerical prefixes in the general names of sugars to indicate how many carbon atoms are present, we often use the prefixes keto- and aldo- to indicate whether the sugar is a ketone or an aldehyde. For example, the monosaccharide fructose is frequently called a ketohexose to emphasize that it contains six carbons as well as the ketone functional group. For each of the monosaccharides shown in Table 22.8 classify the sugars as aldohexoses, aldopentoses, ketohexoses, or ketopentoses. 

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