With carbonyl group defined

Several of the functional groups listed in Table 25.4 contain a C=O double bond. This particular group of atoms is called a carbonyl group. The carbonyl group, together with the atoms that are attached to the carbon of the carbonyl group, defines several important functional groups that we consider in this section. 

This section deals with reactions that correspond to Pathway C, defined earlier (p. 64), that lead to formation of alkenes. The reactions discussed include those of phosphorus-stabilized nucleophiles (Wittig and related reactions), a a-silyl (Peterson reaction) and a-sulfonyl (Julia olefination) with aldehydes and ketones. These important rections can be used to convert a carbonyl group to an alkene by reaction with a carbon nucleophile. In each case, the addition step is followed by an elimination. 

A second reason why AI is of value to scientists is that it offers powerful tools to cope with complexity. In favorable circumstances, the solutions to problems can be expressed by rules or by a well-defined, possibly trivial, model. If we want to know whether a compound contains a carbonyl group, we could record its infrared spectrum and check for a peak near 1760 cm1. The spectrum, paired with the rule that ketones generally show an absorption in this region, is all that we need. But other correlations are more difficult to express by rules or parametrically. What makes a good wine We may (or may not) be able to recognize a superior wine by its taste, but would have considerable difficulty in determining whether a wine is good, or even if it is palatable, if all we had to go on was a list of the chemicals of which it is comprised. 

The [2+2]-photocycloaddition of carbonyl groups with olefins (Paterno-Buchi reaction) is one of the oldest known photochemical reactions and has become increasingly important for the synthesis of complex molecules. Existing reviews have summarized the mechanistic considerations and defined the scope and limitations of this photocycloaddition73. Although this reaction likely proceeds via initial excitation of the carbonyl compound and not the excited state of the diene, the many examples of this reaction in natural product synthesis justify inclusion in this chapter. 

For phorone dioxide (55) H-NMR spectra (66DOK(167)575) show only one conformation, but its structure appears poorly defined dipole moments suggest as more probable the anti orientation of the two heterocyclic oxygen atoms, which appear to be s-cis oriented with respect to the carbonyl group. 

SCHEME 8. Schematic representation for the chelate-controlled addition of an organometaUic reagent (M—R) to the carbonyl group of a chiral a-alkoxy carbonyl compound (17). Two diastere-omers 18 with different orientation of R with respect to CH2R can be obtained. The syn diastereomer is obtained when the nucleophihc attack of R takes place on the same face of the plane, defined by the carbonyl group and the R-substituted carbon atom, where CH2R is located in the chelate complex... 

The term Molecular Clip has been coined for molecules of type 2. That these molecules do indeed possess the geometric features of a clip is apparent from the X-ray structure of the tetramethoxy derivative 3a (Fig. 2) [lla,b]. The o-xylylene moieties of this molecule define a tapering cavity, the walls of which are at an angle of 39.5 with the centers of the benzene rings 6.67 A apart. The carbonyl groups of the glycoluril moiety, which are hydrogen-bond acceptor sites, are separated by 5.52 A. It was also possible to obtain a crystal structure of the chiral dibromo-derivative 4 of clip 3 (Fig. 3). This compound was separated into its enantiomers by HPLC on a chiral stationary phase [12]. 

Polarity Parameter. Despite their apparent simplicity, these parameters, ( ), show a good correlation with plasticizer activity for nonpolymeric plasticizers (10). The parameter is defined as = [M(Apf P0)]/1000 where M = molar mass of plasticizer, Ap = number of carbon atoms in the plasticizer excluding aromatic and carboxylic acid carbon atoms, and P0 = number of polar (eg, carbonyl) groups present. The 1000 factor is used to produce values of convenient magnitude. Polarity parameters provide useful predictions of the activity of monomeric plasticizers, but are not able to compare activity of plasticizers from different families. 

Other important tests are for acid and alkalinity number and for water content (266), because water content and alkalinity of the polyether glycol can influence the reaction with isocyanates. The standard ASTM test for acid and alkalinity number, ASTM D4662 (267), is not sensitive enough for the low acidity and alkalinity numbers of PTMEG, and special methods have been developed. A useful alkalinity number (AN) has been defined as milliequivalents KOH per 30 kg of PTMEG, as titrated in methanol solution with 0.005 N HC1 (268). Other useful nonstandard tests are for heavy metals, sulfated ash, and peroxide. The peroxides formed initially in oxidations are quickly transformed into carbonyl groups, which are detectable by infrared spectroscopy. On oxidation, a small C—O peak develops at 1726 cm-1 and can be detected in thick (0.5-mm) films. A relative ratio of this peak against an internal standard peak at 2075 C—O is sometimes defined as the carbonyl ratio. 

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