And leaving group

Table III-52 indicates the kinetic data for quatemization of 2- and 4-alkylthiazoles. The 8 value shows that the 4-positioiH more sensitive than the 2-position to steric effects, the bond angle HjCjN (123°6) being greater than that of HjQN (119 4). This result has been confirmed for all solvents and leaving groups (256).

FIGURE 5 11 Confer mations of as and trans 4 tert butylcyclohexyl bromide and their relationship to the preference for an anti copla nar arrangement of proton and leaving group... 

Effects that arise because one spatial arrangement of electrons (or orbitals or bonds) IS more stable than another are called stereoelectronic effects There is a stereoelec tromc preference for the anti coplanar arrangement of proton and leaving group in E2 reactions Although coplanarity of the p orbitals is the best geometry for the E2 process modest deviations from this ideal can be tolerated In such cases the terms used are syn periplanar and anti periplanar... 

The leaving group in the alkylating reagent has a major effect on whether C- or O-alkylation occurs. In the case of the lithium enolate of acetophenone, for example, C-alkylation is predominant with methyl iodide, but C- and O-alkylation occur to approximately equal extents with dimethyl sulfate. The C- versus O-alkylation ratio has also been studied for the potassium salt of ethyl acetoacetate as a function of both solvent and leaving group. ... 

Elimination of hydrogen halides from polyfluoroalkanes by bases also usually involves earbanion intermediates (ElcB mechanism) [<8/l, and orientation is there tore governed by relative C H acidities and leaving group mobility Some examples are shown in equations 16-18 [145]... 

The parameters Ci, t2 were postulated to be dependent only upon the substrate, and d, d2, upon the solvent. A large body of kinetic data, embodying many structural types and leaving groups, was subjected to a statistical analysis. In order to achieve a unique solution, these arbitrary conditions were imposed cj = 3.0 C2 for MeBr Cl = C2 = 1.0 for f-BuCl 3.0 Ci = C2 for PhsCF. Some remarkably successful correlations [calculated vs. experimental log (fc/fco)l were achieved, but the approach appeared to lack physical significance and was not much used. Many years later Peterson et al. - showed a correspondence between Eqs. (8-69) and (8-74) in particular, the very simple result di + d, = T was found. 

The catalytic effect of protons has been noted on many occasions (cf. Section II,D,2,c) and autocatalysis frequently occurs when the nucleophile is not a strong base. Acid catalysis of reactions with water, alcohols, mercaptans, amines, or halide ions has been observed for halogeno derivatives of pyridine, pyrimidine (92), s-triazine (93), quinoline, and phthalazine as well as for many other ring systems and leaving groups. An interesting displacement is that of a 4-oxo group in the reaction of quinolines with thiophenols, which is made possible by the acid catalysis. 

When the azine-nitrogen and leaving group are in the same ring,... 

Figure 11.7 Energy diagrams showing the effects of (a) substrate, (b) nucleophile, (c) leaving group, and (d) solvent on Sn2 reaction rates. Substrate and leaving group effects are felt primarily in the transition state. Nucleophile and solvent effects are felt primarily in the reactant ground state.

The reactions in this chapter are classified according to the attacking atom of the nucleophile in the order O, S, N, halogen, H, C. For a given nucleophile, reactions are classified by the substrate and leaving group, with alkyl substrates usually considered before acyl ones. Nucleophilic substitutions at a sulfur atom are treated at the end. 

The bioreductive alkylating agents developed in this laboratory did not afford observable quinone methide species upon quinone reduction and leaving group... 

EFFECT OF ENTERING AND LEAVING GROUPS 4.5.1 The entering group... 

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