Sulfur-based functional groups

First, the carbon atom is much closer to the substituent dian the proton. In the compounds in Table 15.2, the methyl carbon atom is directly bonded to the substituent, while the protons are separated from it by the carbon atom of the methyl group. If the functional group is based on a large electron-withdrawing atom like sulfur, the protons will experience a simple inductive electron withdrawal and have a proportional downfield shift. The carbon atom is close enough to the sulfur atom to be shielded as well by the lone-pair electrons in the large 3sp3 orbitals. The proton shift... 

One class of aromatic polyethers consists of polymers with only aromatic rings and ether linkages ia the backbone poly(phenylene oxide)s are examples and are the principal emphasis of this article. A second type contains a wide variety of other functional groups ia the backbone, ia addition to the aromatic units and ether linkages. Many of these polymers are covered ia other articles, based on the other fiinctionahty (see Polymers containing sulfur, POLYSULFONES). 

Another difference between diese catalysts is found in dieir functional group tolerance. Catalysts such as 12 are more robust to most functionalities (except sulfur and phosphorus), moisture, oxygen, and impurities, enabling them to easily polymerize dienes containing functional groups such as esters, alcohols, and ketones.9 On die other hand, catalyst 14 is more tolerant of sulfur-based functionalities.7 The researcher must choose die appropriate catalyst by considering the chemical interactions between monomer and catalyst as well as the reaction conditions needed. 

Alkanesulfonates are the petrochemically derived sulfur analogs of soaps, which are alkane carboxylates based on renewable resources. The main difference between alkanesulfonates and soaps is, however, that alkanesulfonates consist of a rather complex mixture of homologs with different carbon chain lengths and isomers with an almost statistical distribution of the functional group along the hydrophobic carbon chain (Fig. 1), whereas soap is a mixture of homologs of alkane 1-carboxylates with an even number of carbon atoms. 

Several trends have emerged in the extensive carbon-13 NMR spectroscopy data that have been accumulated for sulfones and sulfoxides. Based on many studies of cyclic systems—particularly five- and six-membered ring sulfur compounds—these trends were shown to generally apply equally to both the cyclic and acyclic systems . Thus (a) oxidation of a sulfide to a sulfone results in a 20-25 ppm downfield chemical shift for sp -hybridized a-carbon atoms and 4-9 ppm upfield shift for / -carbons , and (b) there is very little difference between the chemical shifts of a-carbon atoms of sulfones and sulfoxides despite the difference in the inductive effects of these two functional groups . A difference is observed, however, in the H chemical shift of related cyclic sulfoxides and sulfones . 

Representative data illustrating the influence of Lewis base functional groups in the ADMET reaction are shown in Table 1. When molybdenum catalysts are used to polymerize ether or thioether dienes, little change in reaction rate is observed as compared with the standard, 1,9-decadiene, which possesses no heteroatoms in its structure. When a sulfur atom is three carbons atoms away from the alkene site, the reaction rate is reduced approximately one order of magnitude otherwise, the kinetics are all essentially unaffected [20a]. 

Cimetidine contains an imidazole ring comparable to histamine, a sulfur atom (thioether group) in the side-chain, and a terminal functional group based upon a guanidine (see Section 4.5.4). Ranitidine bears considerable similarity to cimetidine, but there are some important differences. The heterocycle is now furan rather than imidazole, and the guanidine has been modified to an amidine (see Section 4.5.4). A newer drug, nizatidine, is a variant on ranitidine with a thiazole heterocyclic ring system. 

Cyclopropanations are known for several other carbanionic intermediates of the general type (7), in which the substituent G is ultimately lost as an anionic leaving group in the last step of the ring-forming pathway (see Scheme 3 above). The substituent G is most often a functional group based upon sulfur, selenium or nitrogen. Halide-substituted derivatives probably react via the a-elimination pathway in most cases (see Section 4.6.3.1), but in some reactions with electron deficient alkenes as substrates, the normal order of steps may be altered (e.g. Table 10, ref. 162). 

Among the various sulfur nucleophiles which are encountered in natural waters, most are Bronsted acids or bases, i.e., they can accept or lose a proton. The sulfhydryl functional group is a monoprotic Bronsted acid, while H2S and H2SO3 are diprotic Bronsted acids. The rate of displacement of halide from a given substrate in aqueous solution containing a nucleophilic Bronsted acid depends upon the relative concentrations among the different protonated forms of the acid (of which there are two for a monoprotic acid, three for a diprotic acid), as well as the respective rates of reaction of the different protonated... 

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