Organic reaction mechanisms nucleophilic substitution reactions

Early attempts to establish mechanistic models for substitution at metal centers used the labels Sjp and SN2 (substitution, nucleophilic, unimolecular or bimolecular) inherited from Ingold s attempts26 to extend his classic studies in organic reaction mechanisms to substitution at elements other than carbon. Unfortunately, Ingold s attribution of the discontinuity in reaction rates in the replacement of one Cl in m-Co(en)2CI2 1 by various anions A in methanol in the sequence... 

The ideas of reaction mechanisms of substitution reactions came primarily from organic chemistry. The first notion was that the initial reaction step is the addition of the reactant to the substrate, yielding a new molecular compound from which the group to be replaced is eliminated. This idea was put forward in 1909 but the relevant publications appeared in 1911. The same year Le Bel suggested that the entry of the nucleophilic reagent and the departure of the leaving group were two independent events in a simultaneous process. 

In its simplest form, the phase-transfer catalysed nucleophilic substitution reaction, RX + Y -4 RY + X", in which the active nucleophile Y" is transferred from the aqueous into the organic phase, can be depicted by Scheme 1.5. The mechanism requires the extraction of the nucleophilic anion by the quaternary ammonium cation Q+ as the ion-pair [Q+Y ] into the organic phase, where the nucleophilic reaction can take place. Subsequent to the reaction the spent catalyst forms an ion pair with the released anion X- and equilibration of [Q+X-] between the two phases establishes a... 

However, the major factor stimulating the rapid development of static and dynamic sulfur stereochemistry was the interest in the mechanism and steric course of nucleophilic substitution reactions at chiral sulfur. Very recently, chiral organic sulfur compounds have attracted much attention as useful and efficient reagents in asymmetric synthesis. 

Organic chemistry and instrumental analysis Synthesis 3 Mechanisms of nucleophilic substitution reactions... 

The basic concepts of nucleophilic substitution reactions appeared in the first semester of organic chemistry. These reactions follow or Sp 2 mechanisms. (In aromatic nucleophilic substitution mechanism, we use the designation Sp Ar.) In Sfjl and Sp 2 mechanisms, a nucleophile attacks the organic species and substitutes for a leaving group. In aromatic systems, the same concepts remain applicable, but with some differences that result from the inherent stability of aromatic systems. 

As noted in Section 4.2.1, the gas phase has proven to be a useful medium for probing the physical properties of carbanions, specifically, their basicity. In addition, the gas phase allows chemists to study organic reaction mechanisms in the absence of solvation and ion-pairing effects. This environment provides valuable data on the intrinsic, or baseline, reactivity of these systems and gives useful clues as to the roles that solvent and counterions play in the mechanisms. Although a variety of carbanion reactions have been explored in the gas phase, two will be considered here (1) Sn2 substitutions and (2) nucleophilic acyl substitutions. Both of these reactions highlight some of the characteristic features of gas-phase carbanion chemistry. 

Short-lived organic radicals, electron spin resonance studies of, 5, 53 Small-ring hydrocarbons, gas-phase pyrolysis of, 4, 147 Solid state, tautomerism in the, 32, 129 Solid-state chemistry, topochemical phenomena in, 15, 63 Solids, organic, electrical conduction in, 16, 159 Solutions, reactions in, entropies of activation and mechanisms, 1, 1 Solvation and protonation in strong aqueous acids, 13, 83 Solvent effects, reaction coordinates, and reorganization energies on nucleophilic substitution reactions in aqueous solution, 38, 161 Solvent, protic and dipolar aprotic, rates of bimolecular substitution-reactions in,... 

An understanding of electron transfer and proton transfer is of great importance to the chemist, the first because it is concerned with a change in oxidation state and the second because to transform an organic compound it is usually necessary to disrupt the skin of hydrogen atoms that protect the compound. Equally familiar to the well-educated chemist are nucleophilic substitution reactions together with their SN1 and SN2 mechanisms. In recent 1 For the present addresses see p. v. 

Nucleophilic substitution reactions are among the first synthetic transformations introduced to beginners learning organic chemistry. The mechanisms for these... 

Silicon compounds with coordination number larger than four are the object of many studies first with respect to their application as catalysts in organic and inorganic syntheses and second as starting materials for the preparation of a broad variety of organosilicon compounds [1]. Additionally, hypervalent silicon hydride compounds can successfully be used as model compounds to study, for instance, the mechanism of nucleophilic substitution reactions, which is of great interest since the silicon atom is able to easily extend its coordination number [1]. Moreover, hypervalent silanes are suitable as starting materials for the synthesis and stabilization of low-valent silanediyl transition metal complexes [2-5]. 

The dissociative, or D, mechanism is also sometimes called an SnI reaction, a form of nomenclature used in organic reaction mechanisms. In the representation SN1, S refers to the process (substitution), subscript N to the character of the leaving and entering group (a nucleophile, equated in this case to a Lewis base with a lone pair of electrons), and 1 to the number of molecules involved in the rate-determining step (unimolecular). For the D mechanism, the reaction rate can be expressed by Equation 5.37 ... 

Nucleophilic substitution reactions play a large role in the study of organic chemistry, and they are excellent illustrations of the importance of kinetic measurements in determining the mechanism of a reaction. A nucleophile is an... 

Common organic reaction mechanisms, such as nucleophilic substitution and general acid-base catalysis, are known to play roles in enzymatic catalysis. 

Classes of organic pollutants that hydrolyze via nucleophilic substitution reactions include the halogenated hydrocarbons, epoxides, and phosphorus esters. Further discussion of the factors affecting the reactivity of nucleophilic substitution reactions will be made as the hydrolysis mechanisms of these chemicals are examined in greater detail. 

A typical LLPTC cycle involves a nucleophilic substitution reaction, as shown in Eq. (8). A difficult problem in the kinetics of PT-catalyzed reactions is to sort out the rate effects due to equilibrium anion-transfer mechanism for transfer of anions from the aqueous to the organic phase. The reactivity of the reaction by PTC is controlled by the rate of reaction in the organic phase, the rate of reaction in the aqueous phase, and the mass transfer steps between the organic and aqueous phases [27-29]. In general, one assumes that the resistances of mass transfer and of chemical reaction in the aqueous phase can be neglected for a slow reaction in the organic phase by LLPTC. 

Nucleophilic substitution reactions are important in organic synthesis because the halogen atom on halogenoalkanes can be replaced by other functional groups. The reaction with potassium cyanide is a good illustration of this. The cyanide ion reacts to form a nitrile. For example, bromoethane reacts by an Sj42 mechanism with a solution of potassium cyanide in ethanol to form propanenitrile ... 

Adrenaline (or epinephrine), a hormone secreted by the adrenal gland, increases blood pressure and heart rate, and dilates lung passages. Individuals often speak of the "rush of adrenaline" when undertaking a particularly strenuous or challenging activity. Adrenaline is made in the body by a simple organic reaction called nudeophilic substitution. In Chapter 7 we learn about the mechanism of nucleophilic substitution and how adrenaline is synthesized in organisms. 

In the nucleophilic substitution reactions of alkylsulfonyl chlorides and ring-substituted benzylsulfonyl chlorides, the operation of two competing reaction pathways has been established (a) E-A (sulfene) mechanism and (b) direct nucleophilic substitution at sulfur (general base catalysis). The relative significance of these competing mechanisms depends on the nature of the substrate and the precise reaction conditions. In the hydrolysis of cyclopropane-sulfonyl chloride 11 with tertiary amines in organic media, the sulfene 12 appears to be the crucial intermediate in the formation of cyclopropanesulfonic acid 13 (Equation 12). ... 

Sj l mechanism the steps in a nucleophilic substitution reaction in which the rate of the reaction (which is determined by the slow step in the mechanism) involves only the organic reactant, e.g. in the hydrolysis of a tertiary halogenoalkane. 

Nucleophilic substitution reactions are one of the most important classes of reactions in organic chemistry. In particular, 8 2 reactions are among the most extensively stndied chemical processes in solution and in the gas phase, both theoretically and experimentally. The history of the study of these reactions closely parallels (and is sometimes responsible for) the development of concepts such as structure-reactivity relationships, linear free-energy relationships, steric inhibition, kinetics as a probe of mechanism, stereochemistry as a probe of mechanism and solvent effects. 

Since the publication of the last Report, a further volume in the series Organic Reaction Mechanisms has been published, with a chapter on nucleophilic aromatic substitution. 

---