Substituents defined

Equation (7) also constitutes a basis for further empirical modeling of substituent effects on benzene radical cations since it gives the intercepts for the linear relationships between one-electron reduction potentials of 1,4-substituted benzene radical cations and of the 4-substituent given a specific 1-substituent. The 1-substituent defines the family, e.g., phenol, anisole, aniline, etc. Since the 1,4-substituents can belong to either the 1-substituent family or the 4-substituent family, the general empirical relationship must be symmetrical, i.e., the resulting one-electron reduction potential must be the same regardless of choice of family. 

Tris(pyrazolyl)borate (Tp) ligands have been employed to model the His3 facial motif in a variety of model complexes of nonheme oxygenase enzymes. Their use is widespread due to the relative ease of modifying the Tp hgand in order to vary steric and electronic effects imparted onto the Fe center. Fiedler and coworkers synthesized several such Fe(Tp)-NO derivatives of the general formula [Fe( Tp)(acac )(NO)] (R substituents defined in Figure 1 acac = substituted acetylacetonate X defined in... 

Ethers can be viewed as alkane or aromatic compounds containing the group RO— this group is known as the alkoxy group. The lUPAC system for naming ethers treats them as alkanes that bear an alkoxy substituent, that is, as alkoxyalkanes. The smaller substituent is considered part of the alkoxy group and the larger substituent defines the stem. Thus, for example, the lUPAC name for ethylmethyl ether is methoxyethane, and for anisole, it is methoxybenzene. 

In order to allow any multiple chlorination of the biphenyl skeleton, the user may define an atom list (eonsisting of hydrogen and chlorine atoms) and substitute all H-atoms by this list. One may click on the drop-down selection box behind the element icons, select the options Generics. .set the user-defined atom to A1 and quit by the OK button. As a result this atom selection is active for the subsequent drawing steps. After this atom list is drawn ten times as the ten substituents, its composition has to be defined by clicking the A, icon on the left-hand side of the structure editor and by selecting H and Cl in the periodic table (Figure 5-16). 

In such tables, typical chemical shifts are assigned to standard structure fragments (e.g., protons in a benzene ring). Substituents in these blocks (e.g., substituents in ortho, meta, or para positions) are assumed to make independent additive contributions to the chemical shift. These additive contributions are listed in a second series of tables. Once the tables are defined, the method is easy to implement, does not require databases, and is extremely fast. Predictions for a molecule with 50 atoms can be made in less than a second. On the other hand, it requires that the parent structure and the substituents are tabulated, and it considers no interaction... 

The breaking of a strategic bond and the generation of synthesis precursors defines a synthesis reaction. In the simplest case, the reaction is already known from literature. In most cases, however, the rcaaion step obtained has to be generalised in order to find any similar and successfully performed reactions with a similar substituent pattern or with a similar rearrangement of bonds. One way of generalizing a reaction is to identify the reaction center and the reaction substructure of the reaction. This defines a reaction type. 

Herein is the rate constant for a dienophile with substituent x ko is the corresponding rate constant for unsubstituted 2,4c Ox is the substituent constant for substituent x and p is the reaction constant, defined as the slope of the plot of log (k / ko) versus Ox. The parameter p is a measure of the sensitivity of the reactions towards introduction of substituents. Figure 2.3 and Table 2.4 show the results of correlating the kinetic data for the reaction of 2.4a-e with 2.5 with a. ... 

The more extensive problem of correlating substituent effects in electrophilic substitution by a two-parameter equation has been examined by Brown and his co-workers. In order to define a new set of substituent constants. Brown chose as a model reaction the solvolysis of substituted dimethylphenylcarbinyl chlorides in 90% aq. acetone. In the case ofp-substituted compounds, the transition state, represented by the following resonance structures, is stabilized by direct resonance interaction between the substituent and the site of reaction. 

A plot against Hammett s cr-constants of the logarithms of the rate constants for the solvolysis of a series of Mz-substituted dimethylphenylcarbinyl chlorides, in which compounds direct resonance interaction with the substituent is not possible, yielded a reasonably straight line and gave a value for the reaction constant (p) of — 4 54. Using this value of the reaction constant, and with the data for the rates of solvolysis, a new set of substituent parameters (cr+) was defined. The procedure described above for the definition of cr+, was adopted for... 

These and other studies of the relative substituent effects of X and CH X in nitration were considered in terms of the transmission factor a of the methylene group. To avoid complications from conjugative interactions, attention was focussed mainly on substitution at the meta-position, and ct was defined in terms of partial rate factors by the equation ... 

Dithiolane (132) derivatives also possess non-planar skeletons the most important conformation is probably of symmetry C2 (half-chair). The dithiolane ring may be quite flexible and a minimum energy. conformation is only well defined if there is a bulky substituent at the 2-position. 

Most parent structures consist essentially of an assembly of rings and/or chains, the degree of hydrogenation of which is defined (usually completely saturated or containing the maximum number of non-cumulative double bonds in cyclic portions), and having no attached functional substituents (the carbohydrates are a notable exception to this). The stereochemistry at all (or most) chiral centres is defined thus such parent structures are sometimes referred to as stereoparents . Some examples are shown (77)-(83). 

Sometimes the terms erythro and threo are used to specify fee relative configuration of two adjacent stereogenic centers. The terms are derived fom fee sugars erythrose and threose. The terms were originally defined such feat a Fischer projection formula in which two adjacent substituents were on the same side was fee erythro isomer and feat in whidi the substituents were on opposite sides was the threo isomer. 

The numerical values of the terms a and p are defined by specifying the ionization of benzoic acids as the standard reaction to which the reaction constant p = 1 is assigned. The substituent constant, a, can then be determined for a series of substituent groups by measurement of the acid dissociation constant of the substituted benzoic acids. The a values so defined are used in the correlation of other reaction series, and the p values of the reactions are thus determined. The relationship between Eqs. (4.12) and (4.14) is evident when the Hammett equation is expressed in terms of fiee energy. For the standard reaction, o%K/Kq = ap. Thus,... 

The reference compound is benzoic acid. The substituent parameter, which is called the Hammett substituent constant cr, is defined by... 

We take the view of McDaniel and Brown that the Hammett substituent constants should be defined by Eq. (7-22). Table 7-1 lists many of these constants based on the ionization of meta- and para-substituted benzoic acids. 

Reactions that occur with the development of an electron deficiency, such as aromatic electrophilic substitutions, are best correlated by substituent constants based on a more appropriate defining reaction than the ionization of benzoic acids. Brown and Okamoto adopted the rates of solvolysis of substituted phenyldimeth-ylcarbinyl chlorides (r-cumyl chlorides) in 90% aqueous acetone at 25°C to define electrophilic substituent constants symbolized o-. Their procedure was to establish a conventional Hammett plot of log (.k/k°) against (t for 16 /wcra-substituted r-cumyl chlorides, because meta substituents cannot undergo significant direct resonance interaction with the reaction site. The resulting p value of —4.54 was then used in a modified Hammett equation. 

The (T and scales of substituent effects result from changes in the standard reaction that defines the cr scale. An alternative approach to dealing with substituents that possess more than one mechanism of electronic interaction with the reaction site is to make use of more than one substituent constant. Yukawa and Tsuno ... 

The constants oi were taken equal to a scaled value of the aliphatic polar substituent constants a (which are defined in Section 7.3), and a was set at 3 (or a = in for substituents capable of through resonance). The resulting plots of Eq. (7-32) gave good LEER, which was interpreted to justify the approach. Refinements, - of this treatment showed that a depends upon the reaction, although most values fell ... 

Another approach to evaluating cri was taken by Roberts and Moreland,who defined inductive substituent constants in terms of the acid dissociation constants of 4-substituted bicyclof2.2.2]octane-l-carboxylic acids, 3. 

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