Halo group, 131 Table

Alkyl- and Aryl-Halides. Because the halo-groups of organic molecules have large electronegativities and electron affinities, all halo-carbon molecules are electrophilic. Their electrochemical reduction potential is a measure of their electrophilicity (and electron affinity), which is illustrated in Figure 12.1 for hexacWorobenzene (C6C16), 1,2,3,4-tetrachlorobenzene (1,2,3,4-C6H2C14), and n-butyl iodide (n-BuI).8,9 Table 12.1 summarizes the reduction potentials for several alkyl-halides and ary 1-chlorides.810... 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

To derive the maximum amount of information about intranuclear and intemuclear activation for nucleophilic substitution of bicyclo-aromatics, the kinetic studies on quinolines and isoquinolines are related herein to those on halo-1- and -2-nitro-naphthalenes, and data on polyazanaphthalenes are compared with those on poly-nitronaphthalenes. The reactivity rules thereby deduced are based on such limited data, however, that they should be regarded as tentative and subject to confirmation or modification on the basis of further experimental study. In many cases, only a single reaction has been investigated. From the data in Tables IX to XVI, one can derive certain conclusions about the effects of the nucleophile, leaving group, other substituents, solvent, and comparison temperature, all of which are summarized at the end of this section. 

More recently, the same group has used a simpler and more easily prepared chiral ammonium phase-transfer catalyst 99 derived from BINOL in asymmetric Darzens reactions with a-halo amides 97 to generate glycidic tertiary amides 98 (Table 1.13). Unfortunately the selectivities were only moderate to low [48]. As mentioned in Section 1.2.3.1, tertiary amides can be converted to ketones. 

A great number of Kolbe dimerizations have been tabulated in refs. [9, 17-19]. Here no comprehensive coverage is intended, but to demonstrate with selected examples the range and limitations of Kolbe dimerization. In the following discussion and in Table 2 the carboxylates are arranged according to their functional groups in the order alkyl-, ester-, keto-, halo- and olefinic substituents. 

It is possible to synthesize alkylcobalamins containing halogenated alkyl groups. Of these, the compounds studied most extensively are the halo-methylcobalamins (745). For the various fluorine containing derivatives which have been prepared, 19F NMR spectra have been obtained and the results are shown in Table 6. Chemical shifts of a number of fluorome-thanes are included for comparison. 

Table 5 includes compounds with electronegative atoms singly bonded to Sn, such as halo, hydroxy, alkoxy, acyloxy, amino and their analogues. Acyloxy and analogous groups may appear as bidentate functions due to additional coordinative bonds established with a second oxygen atom, but this takes place with expansion of the coordination number of the tin atom to values higher than 4. 

Figure 3.38 shows a schematic cross-sectional view of the Galaxy, indicating the main stellar population groups (disk, bulge, halo and solar cylinder) that will figure in subsequent discussions and Table 7.9 gives some relevant statistics. 

Alkyl carbon atoms attached in a position a, / , or y to a carboxy function give rise to tx, [1. and y effects which shift the carboxy carbon resonances by 10, 4 and — 1 ppm, respectively. This trend is illustrated by the 13C shift values collected for formic, acetic, propionic, and butyric acids among others in Table 4.34 [305-309], Further, carboxy carbons of x halo acids and dicarboxylic acids with closely spaced carboxy groups arc shielded relative to those of parent alkanoic acids (Table 4.34). On the other hand, the x, j3 and y carboxy increments Z, = hi(RCOon)- — krh) f°r the carbon shifts of an alkyl chain are... 

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