Nucleophilicity factors determining

Kf., while not quantified experimentally, was recently introduced by Kirby and coworkers on the basis of product formation from O-attack at electrophilic P and C centers, as well as MO calculations incorporating the novel species, ammonia oxide, NH3+-0 . In common with other ambident nucleophiles, factors such as electronic, steric, kinetic and thermodynamic effects will determine actual extant pathways in a given system. Af-substituted hydroxylamines (see Scheme 1) can in principle partake of the equilibria shown in Scheme 2. Again, actual outcomes will be influenced by the aforementioned criteria. 

What are the major factors determining the rates of nucleophilic substitution reactions Q 13.3... 

Factors determining nucleophilicity and leaving group ability... 

Detailed studies by Bruice, Jerina, and co-workers, referred to earlier, showed that three factors determine whether nucleophilic reaction of tissue materials with arene oxides occur directly. They are (a) the structure of the... 

Steric hindrance and the direction of nucleophilic attack are the other factors determining the rate of the reduction of a-thioiminium ions. Attack at the p face of the molecule by the reducing nucleophile is faster than at the a face which is more hindered (Scheme 4). 

According to this equation the rate constant k of a substitution reaction is determined by the polarizability (P) of the nucleophile and the pKa of its conjugate acid. We have developed (5) a similar interpretation of nucleophilic reactivity based on an elementary consideration of the energy factors determining the activation energy of a displacement reaction, and this procedure may be used for equilibria in solution. The reaction... 

Table 2 also indicates that the nucleophiles effective for vinyl ethers are relatively mild, when compared with those for isobutene (cf., Section V.B.2). In fact, stronger bases lead to inhibition or severe retardation of polymerization [36,64] ketones aldehydes, amides, acid anhydrides, dimethyl sulfoxide (retardation) alcohols, aliphatic amines, pyridine (inhibition). The choice of nucleophiles is determined by their Lewis basicity (as measured by pKb, etc. [64,103]), and this factor determines the effic-tive concentrations of the nucleophiles. For example, the required amounts of esters and ethers decrease in the order of increasing basicity (i.e., a stronger base is more effective and therefore less is needed) [101,103] tetrahydrofuran < 1,4-dioxane ethyl acetate < diethyl ether. On the other hand, for amines not only basicity but also steric factors play an important role [142] thus, unsubstituted pyridine is an inhibitor, while 2,5-dimethylpyridine is an effective nucleophile for controlled/living polymerization, although the latter is more Lewis basic. 

The equilibria can assume different configurations depending upon the four basic factors determining the character of a cationic polymerisation, viz. the strength of the original acid used (or the weakness of the conjugate base formed from it, B or MtX +i), the nucleophilicity of the monomer (or the acidity of R" ), the polarity of the solvent, and the temperature. Shifting of all the equilibria in favour of free... 

Generally, in bromine addition to carbon-carbon double bonds, bromine bridging, solvent dependent dissociation of the ionic intermediates, steric interactions between the counteranion and the first bonded halogen during the nucleophilic step, the possibility of carbon-carbon rotation in the carbenium ion intermediate, preassociation phenomena and nucleophilic assistance determine the stereochemical behavior of the reaction . Several of these factors have been invoked to explain the stereochemistry of bromine addition to dienes, although others have been completely ignored or neglected. Bromine addition to cyclopentadiene, 1,3-cyclohexadiene, 2,4-hexadienes and 1,3-pentadienes has been examined repeatedly by Heasley and coworkers and the product distribution has been... 

We give below the basic data obtained from a systematic study of the factors determining the mechanism and stoichiometry of the nucleophilic substitution reactions used for the synthesis of cellulose derivatives. 

In these systems therefore it may be concluded that the major factor determining the aptitude for pentacoordination is the capacity of the Si-X bond to be stretched under the influence of the donor atom. The stronger N -> Si coordination in chlorosilatrane compared with fluorosilatrane24, and the exceptional increase in length of some Si-Cl bonds trans to the donor atom in pentacoordinate systems, as revealed by X-ray analysis and discussed in Section III.B, are in agreement with this interpretation. The sequence above parallels the tendency to inversion in nucleophilic substitution of X at a chiral silicon centre, and the susceptibility to racemization in nucleophilic solvents. 

The purpose of this chapter is to give an introduction to the subject of nucleophilicity. The chapters of the present volume are collected into five groups (1) Marcus theory, methyl transfers, and gas-phase reactions (2) Br0nsted equation, hard-scft acid-base theory, and factors determining nucleophilicity (3) linear free-energy relationships for solvent nucleophilicity (4) complex nucleophilic reactions and (5) enhancement of nucleophilicity. The present chapter is divided in the same way, giving an introduction to each of the five topics followed by a description of key points in each chapter as they relate to current studies of nucleophilicity and the other chapters of the book. 

Br0nsted Equation, Hard-Soft Acid-Base (HSAB) Theory, and Factors Determining Nucleophilicity... 

One-step bond-forming reactions of an electrophile with a nucleophile, Lewis acid-base reactions, are among the most common elementary reactions in organic chemistry. An understanding of the factors determining the rates and equilibria of such reactions would constitute an understanding of much of the entire field of organic chemistry. 

The high positional and substrate selectivity of the homol5dic substitutions of protonated heteroaromatic bases with nucleophilic carbon free radicals is one of the main factors determining the S5mthetic success of these reactions. In this section it will be shown that the selectivity is mainly determined by the influence of polar effects. As the extent of these effects is much larger than that previously observed in all the other reactions of the same radicals, these reactions have provided very useful models for determining the structure nucleophilicity relationship of the most common carbon free radicals. 

The polarity of the free radicals is certainly one of the main factors determining the sensitivity to polar effects. Often the terms of polarity and electrophilicity and nucleophilicity of the free radicals are considered synonymous. 

In principle, any compound with a lone pair can act as either a base or a nucleophile toward C(sp )—X, causing either E2 elimination or S>j2 substitution to occur. It is possible to predict the course of such a reaction with moderate accuracy (Table 2.1). Two major factors determine the course of the reaction (1) the nu-cleophilicity and basicity of the lone-pair-bearing compound and (2) the identity of the substrate Me or Bn, 1°, 2°, or 3° halide. 

The ideal situation would be one in which any substrate could be made to react with any nucleophile by either mechanism, simply by altering the reaction conditions. As you will see later, modification of the reaction conditions can sometimes lead to a change in the mechanism, but the major factor determining which mechanism operates is the structure of the substrate. 

Nucleophilic strength for a given substituent can be measured in terms of the rate of the Sn2 reaction or in reactions with carbonyl derivatives. The relative rates of several nucleophiles were determined by reaction with iodomethane and are shown in Table 2.13. As mentioned previously, several factors contribute to nucleophilic strength. Electronic effects are important, as illustrated by the electron releasing methyl group, which should make methoxide more nucleophilic than hydroxide. The rate of the Sn2 reaction of sodium hydroxide with iodomethane is 1.3 x lO" M s whereas the rate with sodium methoxide with iodomethane is 2.51 x 1Q2 M- s-1.98... 

The inability of ionic liquids to interact strongly with neutral nucleophilic species has been considered recently to be the main factor determining the high rate of rearrangement of the Z-phenylhydrazone of 3-benzoyl-5-phenyl-l,2,4-oxadiazole in ionic liquids in comparison to molecular solvents (Scheme 5.1-13) [42]. 

L. Ya. Zakharova, F. G. Valeeva, L. A. Kudryavtseva, Yu. F. Zuev, Factors determining the micellar effect in nucleophilic substitution reactions, Russ. J. Phys. Chem., 2000, 74, 1825-1829. 

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