Meta position, definition

The discrepancies in the correlations for the electrophilic side-chain reactions are best illustrated by the apparent <7+-parameters. An analytic approach to the problem is achieved by a definition of p for each side-chain reaction on the basis of the meta reactivities and Hammett crm-values. Again, this choice is attractive because of the unimportance of resonance interactions at the meta position (Brown... 

In these equations, k or k2 refer to the rate or equilibrium constant for the unsubstituted molecule, p characterizes the sensitivity of the reaction to electronic substituent effects, and a is the relative electronic effect of the substituent. A substituent at the meta position has a different a value than that of the same substituent at the para position. By definition, p = 1.0 for the pi a of substituted benzoic acids and a is the substituent effect on this reaction. As an example, Figure 3.1 shows the relationship between Hammett a and the log of the acid dissociation constant (log Kf of metu-substituted phenols. The correlation coefficient of this relationship is 0.97. The data is presented in Table 3.1. 

Because the rate of substitution varies with position, in a benzene derivative it is more informative and frequently more useful to talk about partial rate factors than about relative rates. A partial rate factor is defined as the rate at one particular position in the benzene derivative relative to the rate of substitution at one position in benzene. Let us, for example, calculate the para and meta partial rate factors (pf and mf, respectively) for bromination of toluene with bromine in aqueous acetic acid. Toluene brominates 605 times faster than benzene under these conditions. The product is 66.8 percent p-, 0.3 percent m-, and 32.9 percent o-bromotoluene. Attack at the para position of toluene occurs 0.668 x 605 times as fast as attack at all six positions of benzene but (0.668 x 605 x 6 = 2420) times as fast as at one position of benzene. Therefore pfCH for bromination of toluene under these conditions is 2420. There are only three times as many total carbons in benzene as meta carbons in toluene. Therefore mfca3 = 0.003 x 605 x 3 = 5.5. The definitions of the partial rate factors for monosubstituted benzenes (—R) are given in Equations 7.78-7.80. 

Ounnar and co-workers [31,32] widely apply in their QSRR studies the approach called correspondence factor analysis (CFA). CFA is mathematically related to PCA, differing in the preprocessing and scaling of the data. Those authors often succeeded in assigning definite physical sense to abstract factors, e.g., they identified the Hammett constants of substituents in meta and para positions of 72 substituted /V-benzylideneanilines (NBA) in determining the first factorial axis resulting from the CFA analysis of retention data of NBA in diverse normal-phase HPLC systems. 

Di-substitution Products.—Usually however the di-substitution products are designated by numbers as first indicated. The names ortho, meta and para are also sometimes used exactly as in the benzene products together with other similar names applying to definite pairs of positions. By examining the formula we shall find that ten isomeric di-substitution products of naphthalene are possible in case the two substituents are the same. These ten with their numerical designations and names are as follows ... 

In general, the Perkin reaction is limited to aromatic aldehydes. The activity of substituted benzaldehydes in the Perkin reaction is similar to the trends observed in other reactions involving the carbonyl group. A halogen (28) or nitro (34) group in any position increases the rate of reaction and the yield a methyl group (26) in any position decreases the rate and yield, and this effect falls off in the order ortho > meta > para A methoxy group in the ortho position (30) has a small favorable influence, but in the para position (32) it has a definite unfavorable effect on the rate and yield. 

Note that n > 0 by definition (exceptions are meta-materials [3]) even if in metals it is often ei < 0. The sign of k is also positive ... 

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