Nature of the Carbonyl Group

The structural features especially the very polar nature of the carbonyl group point clearly to the kind of chemistry we will see for aldehydes and ketones in this chapter The partially positive carbon of C=0 has carbocation character and is electrophilic The planar arrangement of its bonds make this carbon relatively uncrowded and susceptible to attack by nucleophiles Oxygen is partially negative and weakly basic... 

Hydrogens on the carbon next to a carbonyl group are slightly deshielded and absorb near 2 8 in the Mi NMR spectrum. T he exact nature of the carbonyl group can t be determined by lH NMR, however, because the a hydrogens of all add derivatives absorb in the same range. Figure 21.10 shows the lHNMR spectrum of ethyl acetate. 

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 resulting ketone (21-4) is then condensed with methylmagnesium bromide to give the corresponding methyl-carbinol the highly unreactive nature of the carbonyl group at position 11 is further illustrated by the selective reaction at the 6 position. 

Bhupendra K. Mazumdar Have you any views on the nature of the carbonyl groups—quinonoid or nonaromatic ketonic ... 

Dr. Murchison I am afraid that I can offer no help on this point now. We have not investigated the nature of the carbonyl groups of the resinites. 

The acidity of the carboxyl group arises, at least in part, from the polar nature of the carbonyl group, the polarity of which can be ascribed to contribu-... 

Monosaccharides, also called simple sugars, are classified by the number of carbon atoms (triose, tetrose, pentose, and so on) and by the nature of the carbonyl group (aldose or ketose). 

This observation is not related to traces of base or acid from the silver salts used since control experiments mled out this possibility. It was known from the literature that the 5-exo-dig versus 6-endo-dig cyclization mode could depend on the nature of the carbonyl group,56 57 of the alkyne substituent,58 59 and of the nature60 61 and oxidation state62 of the metallic source used. Also, work from Yamamoto25 demonstrated the importance of both a- and Jt-Lewis acidity properties of silver(I) complexes. Therefore, depending on the silver salt used, two mechanistic pathways were proposed (pathways A and B, Scheme 5.15). 

The nature of the carbonyl group in the a,(3-unsaturated electrophile is also important as the B more electrophilic carbonyl groups give more direct addition and the less electrophilie carbonyl... 

The precise nature of the carbonyl groups determines what happens next. If R is a leaving group (OR, Cl, etc.), the tetrahedral intermediate collapses to form a ketone and the product is a 1,3-di-ketone. The synthesis of dimedone (later in this chapter) is an example of this process where an alkoxy group is the leaving group. Alternatively, if R is an alkyl or aryl group, loss of R is not an option and the cyclization is an intramolecular aldol reaction, Dehydration produces an a,P-unsaturated ketone, which is a stable final product. 

The exact position of the C-O absorption is highly diagnostic of the exact nature of the carbonyl group. As the data in Table 19.2 indicate. 

In this chapter wc shall continue with our study of carbanion chemistry, with emphasis on the attachment of alkyl groups to the a-carbons of carbonyl and acyl compounds. Such alkylation reactions owe their great importance to the special nature of the carbonyl group, and in two ways. First, the carbonyl group makes a-hydrogens acidic, so that alkylation can take place. Next, the products... 

Chemoselectivity is affected by electronic factors as well. Cross experiments show that small changes in the electrophilic nature of the carbonyl group are responsible for marked differences in the nucleophilic reactivity of Ti and Zr reagents (equations 22 and 23). The electronic nature of the nucleophilic moiety of the metal reagent also affects the chemoselectivity of the nucleophilic addition. This is the case for some special titanium(IV) triisopropoxide derivatives [e.g. (53)-<56)] which are selective towards aldehydes and give high addition yields, due to their resonance-stabilized residues acting as nucleophiles. ... 

Although C NMR is useful for determining the presence or absen of a carbonyl group in a molecule of unknown structure, precise infer tion about the nature of the carbonyl group is difficult to obtain. Aldehyd and ketones absorb near 200 S, while the carbonyl carbon atoms of variou acid derivatives absorb in the range 160-180 S (Table 21.4). 

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