The Extended Selectivity Treatment

In contrast to thiophene, benzo[6]thiophene is preferentially substituted at the /3-position. The /3 a reactivity ratios and partial rate factors for the electrophilic substitutions of benzo[6]thiophene have been summarized. The reactivity ratio varies over a wide range, depending on the nature of the electrophile and the temperature of the reaction in the case of acetylation, the percentage of the a-substituted product increases with temperature. Also in contrast to thiophene, the extended selectivity treatment applied to the reactions at the a- and /3-positions of benzo[6]thiophene gives a non-linear plot. The effect of fusion of a benzene ring to thiophene is to decrease the reactivity of the a-position and increase the reactivity of the /3-position. 

This approach, with 8C as a substitute for p, is designated as the Selectivity Treatment. The alternative procedure for the examination of the substitution data, employing p as the principal reference parameter, is termed the Extended Selectivity Treatment. The Selectivity Treatment was first employed for the evaluation of linear relationships and c-g-constants. As more data have become available and an independent technique for the assessment of p has evolved, the Extended Selectivity Treatment has become more useful. 

The procedure adopted to portray the scope and utility of a linear free-energy relationship for aromatic substitution involves first a determination of the p-values for the reactions. These parameters are evaluated by plotting the values of log (k/ka) for a series of substituted benzenes against the values based on the solvolysis studies (Section IV, B). The resultant slope of the line is p, the reaction constant. The procedure is then reversed to assess the reliability and validity of the this approach the log ( K/ H) observations for a single substituent are plotted against p for a variety of reactions. This method assays the linear or non-linear response of each substituent to variations in the selectivity of the reagents and conditions. Unfortunately, insufficient data are available to allow the assignment of p for many reactions. It is more practical in these cases to adopt the Selectivity Factor S as a substitute for p and revert to the more empirical Selectivity Treatment for an examination of the behavior of the substituents. 

Figure 29 presents an analysis of the data for p-phenyl groups in the Extended Selectivity Treatment. The reactivity of the para position increases significantly with an increase in the electron demand of the substitution reaction. This result is confirmed by an analysis of the data through the Selectivity Relationship in which a linear relationship is predicted for a diagram of log pfh against S (Fig. 30). Again, curvature is evident. It must be concluded that the substitution reactions of biphenyl do not adhere to a linear free-energy relationship (Eabom and Taylor, 1961b Stock and Brown, 1962a).

The Selectivity Relationship was shown to be applicable for substitution in the meta and para positions of toluene (Section II). The fine adherence of the -methyl group to a linear free-energy relationship (Fig. 37) is apparently typical of the behavior of the other alkyl substituents, as illustrated for the p-ethyl, p-i-propyl, and p-t-butyl groups (Figs. 38-40). Indeed, the data for electrophilic substitution in toluene are better correlated by a linear relationship than are the data for ordinary side-chain reactions of p-tolyl derivatives (Stock and Brown, 1959a). In the Extended Selectivity Treatment (Fig. 25) the side-chain reactions show a slightly greater scatter from the correlation line than the aromatic substitution reactions. 

The application of the Extended Selectivity Treatment to the partial rate factors for the wi-fluoro, ra-chloro, and ra-bromo substituents is shown in Figs. 43-45. Too few data are available for ra-iodo. Inspection of these diagrams reveals that substitution meta to the halogens conforms to a linear relationship. No serious deviations are detected. The log mf values for non-catalytic chlorination are displaced from the correlation line, but this discrepancy is almost certainly the consequence of the failure of the additivity principle on which the results are based (Stock and Baker, 1962). 

Too few results are presented to permit a thorough examination of the application of the Extended Selectivity Treatment. A preliminary examination of the data is most readily accomplished by the Selectivity Treatment (34). 

The Extended Selectivity Treatment is the third procedure for the assignment of the a+-constants. The constants derived by this approach are compared with those elucidated by the other two procedures in Table 27. 

Following the procedure suggested by Brown and Stock190 (termed the extended selectivity treatment ) logaf and log/3f have been plotted against the reaction constants p for benzene derivatives (Figs. 2 and 3). 

Those for reactions at the a position are numerous enough for testing the applicability of the extended selectivity treatment (Fig. 4). The points nicely fit a straight line with the theoretical origin (r = 0.995, s = 0.79). From the slope a substituent constant relative to the structural modification caused by the substitution of an oxygen atom for a CH=CH in the benzene ring may be calculated. The value obtained —0.93, is noticeably more negative than for thiophene, —0.79. 

Electrophilic replacement constants crXr have been obtained for all the positions of benzo[6]thiophene from the solvolysis of isomeric l-(benzo[ >]thienyl)ethyl chlorides in 80% ethanol-water. These constants signify replacement of the entire benzene ring by another aromatic system (74JOC2828). The positional order of reactivity was determined to be 3>2>6>5>4>7, all positions being more reactive than benzene. The same order was also derived from the kinetic data for pyrolysis of the isomeric l-(benzo[6]thienyl)ethyl acetates (78JCS(P2)1053). A modified extended selectivity treatment has been developed to correlate electrophilic substitution data in benzo[Z> ]thiophene, which assumes a dual activation mechanism (79JOC724). 

The reaction constants obtained in the previous section for numerous substitution reactions permit the examination of the applicability of a linear free-energy relationship by the Extended Selectivity Procedure. The utility of this approach is demonstrated by application to a series of data for side-chain reactions which are correlated with good precision by the Hammett equation. The variations as detected by the procedure serve as a convenient frame of reference for the behavior to be anticipated in other treatments. 

An examination of the data for substitution in the 2-position of fluorene (Fig. 32) reveals a reasonable correlation. Some scatter is observed but no more than is generally encountered in an application of the Hammett treatment to data covering a wide range of reactivity. Certainly, there is no evidence for a pronounced curvature of the kind found in the related treatment of the data for biphenyl (Figs. 29 and 30). In contrast to substitution in the para position of biphenyl, substitution in the structurally equivalent 2-position of fluorene conforms to the Extended Selectivity Relationship (see, however, p. 147). 

The partial rate factors for many substituents in many substitution reactions have been explored in the previous sections. Analyses of these data by the Selectivity and Extended Selectivity Treatments indicate the adherence of the data to the predictions of a linear free-energy relationship, and only two groups, p-phenyl andp-fluoro, deviate significantly. Several comparisons of the applicability of a linear relationship for substitution and for Hammett side-chain reactions reveal the... 

In addition to the careful selection of structural metals, the cathodic protection of water-wetted parts may also be specified. For most boiler plant systems, however, because of the tortuous and extended waterside surfaces involved, the use of cathodic protection is only a partial solution to controlling corrosion and should never be the sole secondary protocol. Rather, cathodic protection functions well when employed as part of a more comprehensive program that includes appropriate internal chemical treatments. 

There are two principal chemical concepts we will cover that are important for studying the natural environment. The first is thermodynamics, which describes whether a system is at equilibrium or if it can spontaneously change by undergoing chemical reaction. We review the main first principles and extend the discussion to electrochemistry. The second main concept is how fast chemical reactions take place if they start. This study of the rate of chemical change is called chemical kinetics. We examine selected natural systems in which the rate of change helps determine the state of the system. Finally, we briefly go over some natural examples where both thermodynamic and kinetic factors are important. This brief chapter cannot provide the depth of treatment found in a textbook fully devoted to these physical chemical subjects. Those who wish a more detailed discussion of these concepts might turn to one of the following texts Atkins (1994), Levine (1995), Alberty and Silbey (1997). 

Inorganic bases have been also employed in this system. When Butcher first used alkali carbonates [50], it was reported that, in DMF and at ambient temperature, the carbamation of primary and secondary aliphatic amines (or also arylamines) with alkyl halides under a C02 atmosphere (0.1 MPa) was effectively promoted by Cs2C03 [50, 51]. The Cs+ cations in the solvent used (DMF) did not form ion pairs with counterions, and favored the formation of naked carbamate anions that were more reactive at the O-ends with alkyl halides. Jung further found that the addition of tetrabutylammonium iodide (TBAI) to the system RR NH/ C02/RX/Cs2C03/DMF promoted the carbamation process with a higher yield and selectivity with respect to N-alkylation [51]. The process has been successfully extended to the synthesis of carbamate functionalities on solid phases. In this case, resin-bound carbamates are readily released from the resin by treatment with LiAlH4 in THF, yielding the respective N-methyl secondary amines [51]. 

Finally it should also be mentioned that the quasi-spin treatment can be extended to the two-particle Coulomb interaction. This will not be considered here since in the case of the (t2g )3 multiplets the quasi-spin characteristics of the Coulomb operator do not give rise to additional selection rules. 

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