Substituent groups order

The quatemization of 16 2,4-dialkylthiazoles with methyl tosylate in nitrobenzene at 25 C has been studied (256) in order to examine the simultaneous influence of two substituents grouped around a reaction center (257). 

Table 7 lists some of the commonest substituent groups and the corresponding prefixes and/or suffixes, in order of priority for selection as principal group. Some examples of their use are given in (148)-(152). 

Another factor which strongly affects the reactivity of these carboxylic acid derivatives is the leaving-group abihty of the substituents. The order is Cl > OAr > OR > NR2 > 0 so that not only does the ease of forming the tetrahedral intermediate decrease in the order Cl>0Ar>0R>NR2>0 , but the tendency for subsequent elimination to occur is also in the same order. Because the two factors work together, there are large differences in reactivity toward the nucleophiles. 

The nature of the substituent group X plays an important role in determining the relative stability of the various tautomeric forms. In aliphatic systems the tendency of C—XH to become CH— C 1=X increases markedly in the order X = CHz < NH < O, and certainly hydroxy compounds show less inclination to exist as such than do amino compounds. The position of S in this series is not completely... 

The Free-Wilson analysis provides more site-specific information than a Hansch analysis. It is recommended to carry out a Free-Wilson analysis first in order to obtain an idea of the importance of the substituent groups and of the sensitivity of the substitution sites. This type of analysis can be regarded as being qualitative, as it points to the important pharmacophores in the molecule. The information thus obtained may guide the selection of the appropriate physicochemical, topological... 

Like other spiropyrans, the colored form of spirooxazines generated by UV irradiation, reconverts to the colorless form. However, it is possible to measure the thermal decay rates and activation energies at ambient temperature, since this fading reaction obeys first-order kinetics in solution. The thermal decay rate constant for spiroindolinonaphthooxazine has been found to be 0.02-0.15s 1 in ethanol and 0.1-1.4s 1 in toluene, although this may vary according to the substituent groups.72,77 However, these values are smaller than those of the spironaphthopyran series. 

Relative values of the rate constants are useful in themselves by measuring such values for the reactions of a series of compounds with the same reactant, one is able to determine the rank order of reactivity within the series. Such determinations are useful in the development of correlations of the effects of substituent groups on the... 

French workers have studied the 1H- and 13C-NMR parameters of disubstituted selenophenes.37 38 The proton chemical shifts are discussed in terms of magnetic anisotropy and electric field effects of the substituents in order to study the conformational equilibrium of the carbonyl group. The relationship between the H- and 13C-chemical shifts and 7t-electron distribution calculated by the PPP method are examined. Shifts and coupling constants are discussed in additivity terms. 

In order to describe derivatives of B12 or B12H122 having icosahedral structures, it is necessary to have a way to designate positions of atoms or substituent groups. In order to do that, the positions are identified by a numbering system that is illustrated as shown in Figure 13.5. 

Since none of the lattice models is now clearly superior, the choice for interpretation of spin relaxation in polymers is arbitrary. Familiarity leads us to select the Jones and Stockmayer model so we will now consider application of this model to several well studied polymer systems in order to compare dynamics from polymer to polymer. Also the equations required to consider anisotropic Internal rotation of substituent groups and overall molecular tumbling as independent motions in addition to backbone rearrangements caused by the three-bond jump are available for the Jones and Stockmayer model (13). 

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