Mechanisms, IUPAC

This process has been called the SeC20 or Se2 (co-ord)21 mechanism (IUPAC designation An + cyclo-DEAED ). 

The reaction of the same compound with acetic acid-perchloric acid seems to proceed by an Se2 mechanism (IUPAC designation l/3/DEAE) 31... 

For aryl halides and sulfonates, even active ones, a unimolecular SnI mechanism (IUPAC Dn + An) is very rare it has only been observed for aryl triflates in which both ortho positions contain bulky groups (f-butyl or SiR-,).17 It is in reactions with diazonium salts that this mechanism is important w... 

IUPAC recommendations suggest that a copolymer structure, in this case poly(methyl methacrylate-co-styrene) or copoly(methyl methacrylate/slyrene), should be represented as 1. The most substituted carbon of the configurational repeat unit should appear first. This same rule would apply to the copolymer segments shown in Section 7.1. However, as was mentioned in Chapter I, in this book, because of the focus on mechanism, we have adopted the more traditional depiction 2 which follows more readily from the polymerization mechanism. 

Dediazoniation refers to all those reactions of diazo and diazonium compounds in which an N2 molecule is one of the products. The designation of the entering group precedes the term dediazoniation, e. g., azido-de-diazoniation for the substitution of the diazonio group by an azido group, or aryl-de-diazoniation for a Gomberg-Bachmann reaction. The IUPAC system says nothing about the mechanism of a reaction (see Sec. 1.2). For example, the first of the two dediazoniations mentioned is a heterolytic substitution, whereas the second is a homolytic substitution. 

In the former (Ingold) nomenclature this was called the SRN1 mechanism (see IUPAC, 1989a). This scission may consist of two steps, forming first an aryldiazenyl radical and the phenylthiolate radical (see Sec. 8.6). 

IUPAC (1989 a) System for Symbolic Representation of Reaction Mechanisms. Guthrie, R. D. (ed.). 

Figure 1. Schematic of an FFF channel with the separation mechanism for normal FFF shown in detail. Reprinted from [7] Beckett, R. and Hart, B. T. Use of field flow fractionation techniques to characterize aquatic particles, colloids and macromolecules . In Environmental Particles. Vol. 2, IUPAC Series on Analytical and Physical Chemistry of Environmental Systems. Series eds. Buffle, J. and van Leeuwen, H. P., pp. 165-205. Copyright 1993 IUPAC. Reproduced with permission...

The Mechanism of the Polymerisation of Cyclic Formals, P.H. Plesch, IUPAC International Symposium on Macromolecular Chemistry, 1969, Plenary and Main Lectures, Budapest, 1971, 213-221. 

By heterolytic dissociation into carbocations and dinitrogen followed by the addition of a nucleophile (called Dn + An mechanism in the new IUPAC nomenclature103, Sjyl in the former Ingold nomenclature). 

IUPAC Linear Representation of Reaction Mechanisms, (Ed. J. S. Littler), Pure Appl. Chem., 55, 3846 (1990). 

The average pore size of PS structures covers four orders of magnitude, from nanometers to tens of micrometers. The pore size, or more precisely the pore width d, is defined as the distance between two opposite walls of the pore. It so happens that the different size regimes of PS characterized by different pore morphologies and different formation mechanisms closely match the classification of porous media, as laid down in the IUPAC convention [Iu2]. Therefore the PS structures discussed in the next three chapters will be ordered according to the pore diameters as mostly microporous (d<2 nm), mostly mesoporous (2 nm50 rim). Note that the term nanoporous is sometimes used in the literature for the microporous size regime. 

Sn2 stands for substitution nucleophilic bimolecular. The IUPAC designation (p. 290) is AnDn. In this mechanism there is backside attack the nucleophile approaches the substrate from a position 180° away from the leaving group. The reaction is a one-step process with no intermediate (see, however, pp. 297-298 and 305). The C—Y bond is formed as the C—X bond is broken ... 

In the IUPAC system the SnI mechanism is DN + AN or DN + An (where t denotes the rate-determining step). The IUPAC designations for the SnI and Sn2 mechanisms thus clearly show the essential differences between them AnDn indicates that bond breaking is concurrent with bond formation DN + AN shows that the former happens first. 

The SeI mechanism (substitution electrophilic unimolecular) is rare, being found only in certain cases in which carbon is the leaving atom (see 1-38, 1-39) or when a very strong base is present (see 1-1, 1-11, and 1-42).29 It consists of two steps with an intermediate carbanion. The IUPAC designation is DE + AE. 

Both the Se2 (front) and Se2 (back) mechanisms are designated DFAE in the IUPAC system. With substrates in which we can distinguish the possibility, the former mechanism should result in retention of configuration and the latter in inversion. When the electrophile attacks from the front, there is a third possibility. A portion of the electrophile may assist in the removal of the leaving group, forming a bond with it at the same time that the new C—Y bond is formed ... 

This mechanism, which we call the SEi mechanism3 (IUPAC designation cyclo-DtAEDnAn), also results in retention of configuration.4 Plainly, where a second-order mechanism involves this kind of internal assistance, backside attack is impossible. 

The IUPAC designation is De + AE. First-order kinetics are predicted and many such examples have been found. Other evidence for the SeI mechanism was obtained in a study of base-catalyzed tautomerization. In the reaction... 

This is called the SrnI mechanism,38 and many other examples are known (see 3-4, 3-5, 3-7,3-14). The IUPAC designation is T + DN + AN.39 Note that the last step of the mechanism produces Arl radical ions, so the process is a chain mechanism40 (see p. 678). An electron donor is required to initiate the reaction. In the case above it was solvated electrons from KNH) in NH,. Evidence was that the addition of potassium metal (a good producer of solvated electrons in ammonia) completely suppresssed the cine substitution. Further evidence for the SrnI mechanism was that addition of radical scavengers (which would suppress a free-radical mechanism) led to 8 9 ratios much closer to 1.46 1. Numerous other observations of SrnI mechanisms that were stimulated by solvated electrons and inhibited by radical scavengers have also been recorded.41 Further evidence for the SrnI mechanism in the case above was that some 1,2,4-trimethylbenzene was found among the products. This could easily be formed by abstraction by Ar of H from the solvent NH3. Besides initiation... 

In a reaction with a moderately long chain, much more of the product will be produced by abstraction (4) than by coupling (5). Cleavage steps like (2) have been called SHl (H for homolytic), and abstraction steps like (3) and (4) have been called Sh2 reactions can be classified as ShI or Sh2 on the basis of whether RX is converted to R by (2) or (3).9 Most chain substitution mechanisms follow the pattern (3), (4), (3), (4). . . . Chains are long and reactions go well where both (3) and (4) are energetically favored (no worse that slightly endothermic, see pp. 683, 693). The IUPAC designation of a chain reaction that follows the pattern (3), (4). . . is ArDR + ARDr (R stands for radical). 

This mechanism, called the AdE3 mechanism (termolecular addition, IUPAC ANAE),4 has the disadvantage that three molecules must come together in the transition state. However, it is the reverse of the E2 mechanism for elimination, for which the transition state is known to possess this geometry (p. 983). 

This mechanism is the same as the simple electrophilic one shown on p. 734 except that the charges are reversed (IUPAC AN + AE or AN + AH). When the olefin contains a good leaving group (as defined for nucleophilic substitution), substitution is a side reaction (this is nucleophilic substitution at a vinylic substrate, see p. 335). 

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