Substitution mechanisms nomenclature

This nuance of the original Sr I mechanism may thus occur in quite a number of cases. Nomenclature purists may consider it necessary to find other symbols to name this mechanism and, presumably, to question the adequacy of the 1 in this case. Beyond symbols, if the Sr I mechanism is viewed as an outer sphere electron-transfer-induced nucleophilic substitution , a possible designation of the mechanism under discussion might be dissociative electron-transfer-induced nucleophilic substitution . The original designation of these reactions as nucleophilic reactions proceeding via anion radical intermediates (Komblum, 1975) would still apply to both nuances of the mechanism since, in the present case, RNu is an essential intermediate in the reaction, even if RX is not. 

We begin by bringing you up to speed on mechanisms and reminding you how to push electrons around with those curved arrows. We jog your memory with a discussion of substitution and elimination reactions and their mechanisms, in addition to free radical reactions. Next you review the structure, nomenclature, synthesis, and reactions of alcohols and ethers, and then you get to tackle conjugated unsaturated systems. Finally, we remind you of spectroscopic techniques, from the IR fingerprints to NMR shifts. The review in this part moves at a pretty fast pace, but we re sure you can keep up. 

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

In discussing the molecularity of substitution reactions the Langford—Gray nomenclature will be used throughout.6 This defines a stoichiometric mechanism, which distinguishes processes... 

Hydrolysis and condensation reactions of silanes may be considered in the broad category of nucleophilic substitutions at silicon. The common nomenclature for these reactions is SN.V-Si, where A represents the kinetic order or molecularity, Si indicates that silicon is the reaction center, and SN indicates that the reaction is a nucleophilic substitution. Nucleophilic reactions at silicon have been reviewed thoroughly and have been the subject of fundamental studies by several laboratories over the last three decades [33]. The literature is not as voluminous as the literature on the corresponding reactions at carbon. A general mechanistic view of these reactions has, however, emerged. There are many parallels to carbon-centered reaction mechanisms. One distinction from carbon-centered reactions is clearly apparent. Silicon is able to form relatively stable higher coordinated (pentavalent) intermediates carbon is not [33]. 

In this Chapter are described the possible mechanisms of electrophilic substitution at saturated carbon, as a preliminary to the discussion of the kinetics of substitution. Additionally, there is a description of the nomenclature that has been used to date. There has been no general agreement on the nomenclature of the mechanisms of electrophilic substitution at saturated carbon, and the notation used in subsequent chapters in the present work can thus usefully be enumerated here. We deal first of all with the fundamental mechanisms, that is with mechanisms that do not involve rearrangement or nucleophilic (anionic) catalysis. 

The nomenclature used to describe this mechanism is not a subject of controversy, and all workers3-6 have used the symbol SE1 that is substitution, electrophilic, unimolecular. The unimolecular process referred to is the elementary reaction (1). 

The nomenclature used in describing bimolecular electrophilic substitutions involving cyclic transition states reflects, in part, the above-mentioned difficulty. Ingold3 has adopted the nomenclature of Winstein et al.1 and refers to such substitutions as SEi, but to the present author this is not a particularly appropriate choice since it does not indicate the bimolecular nature of the substitution. Dessy et al.8 have used the term SF2 to describe a mechanism, such as that in reaction (5), in which a four-centred transition state is formed, but not only is such a term too restricted, it also provides no indication that the mechanism is one of electrophilic substitution. The view of Reutov4 is that the cyclic, synchronous mechanism is very close to the open mechanism and that both can be described as SE2 mechanisms. Dessy and Paulik9 used the term nucleophilic assisted mechanisms to describe these cyclic, synchronous mechanisms and Reutov4,10 has recently referred to them in terms of internal nucleophilic catalysis , internal nucleophilic assistance , and nucleophilic promotion . Abraham, et al,6 have attempted to reconcile these various descriptions and have denoted such mechanisms as SE2(cyclic). 

Even now, there are two further limiting cases, for if k equilibrium constant for complex formation. Experimentally it will be a matter of extreme difficulty to distinguish either of these possibilities from mechanism SE2(open) or SE2 (cyclic), since all of these mechanisms require the reaction to follow second-order kinetics. Indeed, Reutov4 appears to include a situation such as (7), if the complex is present but in very low concentration, under the mechanistic title of SE2. This is also the nomenclature used by Traylor and co-workers11, but Abraham and Hill5 refer to such a situation as SEC (substitution, electrophilic, via co-ordination). 

NOMENCLATURE USED TO DESCRIBE MECHANISMS OF ELECTROPHILIC SUBSTITUTION... 

One of the most useful features of the recommended notation is that the unmodified term SE2 may be used to describe an electrophilic substitution in which the kinetic form is second-order (first-order in substrate, first-order in electrophile) even though it may not be apparent which of the various possible SE2 mechanisms is in force. The nomenclature of catalysed substitutions, and of allylic substitutions follows quite simply. 

This section deals, in general, with electrophilic substitution in allylic systems. For more complicated molecular rearrangements which may accompany electrophilic substitution, there is no simple method of description other than by a full statement of the mechanism. The recommended nomenclature is given in square brackets, [ ]. 

The reaction order with respect to the solvent is, however, not determinable and thus there can be no direct kinetic evidence as to the number of solvent molecules in the transition state. In view of this, it is not reasonable to attempt to include nucleophilic catalysis by the solvent in any formal nomenclature of the mechanism, although it is clear that the role of the solvent is of great importance in studies of electrophilic substitutions. 

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