For meta- and para-substituted

A plot of log (kiku) against pKa (this is called a Br nsted-type plot, as we will see in Section 7.4) for meta- and para-substituted pyridines gave an LFER describing normal behavior. On this same plot the or// o-substituted pyridines all fell below the m and p reference line. The steric constant was defined as the vertical distance between the reference line [whose equation was log (k plkn) = 0.35 pK — 1.73] and the result for the ortho compound, or... 

Good yields of 10-aryl-3,6-dinitroacridones were obtained merely by heating 2,2, 4,4 -tetranitrobenzophenone (419) with an excess of the corresponding aryl amines at 125°C. For example, aniline provided 420 in 80% yield (Eq. 38). The reaction is fairly general for meta- and para-substituted anilines, though it proceeds less readily with orf/io-substituted compounds (79JCS(P1)1364). A method of isolation of the intermediate diarylamine in the synthesis of certain 10-aryl-3,6-dinitroacridones from 2,2, 4,4 -tetranitrobenzophenone has also been described (93JCR(M)2779). 

Happer157 determined 13C substituent chemical shifts for meta- and para-substituted styrenes in seven different solvents. Data for the side-chain carbons, and in the meta series for the ring carbon para to the substituent, were analyzed as a basis for assessing solvent effects on 07, cr, ar (BA) and cr. The 07 values for NO2 varied from 0.60 in DMSO to 0.71 in CDCI3 in EtOH the value was 0.64. The [Pg.505]

Since <7y differs for meta and para substitutions, there are two sets of or values, o meta and crypara. Ortho substitution is excluded since, as we have already seen, proximity effects, which are difficult to separate from electronic factors, may play an important role. Since ArG° = -2.303 RT log Kt, we may write Eq. 8-24 in terms of acidity constants (note that in the following we omit the subscript i to denote the acid function) ... 

Figure 8.7 Hammett plots for meta- and para-substituted phenols, phenylacetic acids, and 3-phenylpropionic acids (data from Serjeant and Dempsey, 1979).

This was very early work, and not enough data for meta- and para-substituted compounds were obtained for an accurate determination of p, but the plot of log k versus pKa of ArCOOH (both at 100°C) for o- and p-substituted methyl benzoates gave a good straight line of slope 0.67. This result, together with that of Eschenmoser et al. 77, emphasizes that steric hindrance at the carbonyl group has little or no effect on the displacement of the ether oxygen of the ester. 

Correlation with Hammett substituent constants alone was not satisfactory. The reaction has been extended to ortho substituted anilines also the results have been treated by the Fujita-Nishioka method. Combining these with the previous data for meta and para substituted anilines, a multiparameter regression equation (2) has been developed (79JCS(P2)219>. 

Reaction Constants for meta- and para-Substituted Benzene Derivatives (RC6H4—)... 

The Hammett equation also fails for open-chain aliphatic derivatives. For example, there is no simple linear relationship between log K for a scries of substituted ethanoic acids (RCH2C02H) and log k for the hydrolysis rates of similarly substituted ethyl ethanoates (RCH2C02C2H5). The freedom of motion available to a flexible open-chain compound permits a much wider range of variations in steric effects than for meta- and para-substituted aromatic compounds. 

The linear correlation between the AEn values for meta- and para-substituted ben-zylic anions and the Hammett a constants has suggested that n conjugation may be used to explain the influence of substituents which have n orbitals.1... 

Structure-Activity Correlations for Meta-and Para-Substituted Trifluoromethane-sulfonanilide Pre-Emergence Herbicides... 

Relative rate data for meta- and para-substituted 2,2-dimethyl 3-phenylbut-3-enoic acids are given in Table 40. Results were shown to fit a a relationship... 

This 2 constitutes the last column in Table 1. From the correlation of pX hb with the Hammett a° constant of the ring substituent (equation 7), many other pTsThb values can be calculated for meta- and para-substituted phenols. 

For meta- and para-substituted phenols, log values spread over 2 log units from 3-dimethylaminophenol to 4-nitro-3-trifluoromethylphenol. Their order is well explained by classical electronic effects. A dual-substituent parameter analysis gives equations 16 and 17, where crp and ctr are the Taft field-inductive and resonance substituent constants, respectively. 

For meta- and para-substituted arylidenebarbiturates (19) the conjugation in the molecule extends from the aromatic moiety over the C-5=C-7 bond to... 

Table XVI gives the coupling constant data reported for meta and para substituted phenyltin compounds. Values of Jsn-C(i) give the best correlation with Hammett a constants values of Usn-c- ethyi also give good correlations for the para compounds (203).

Sn COUPLING CONSTANTS FOR META AND PARA SUBSTITUTED PHENYLTIN COMPOUNDS R-C8H4Sn(CHs)3 ... 

The intermediates react further to give (Alk,Ar)C=CNuCl. With thiolate nucleophiles (EtS and PhS ), Hal = Cl, Br, I, the mechanism is restricted to attack on the halogen (5), but attack on the carbon is also observed in the reaction of EtS and ArC CCl. The second-order rate constants in methanol-water mixtures for meta- and para-substituted l-bromo-2-phenylacetylenes correlate well with Hammett a constants p = IT5. A linear correlation was also observed between ogK and pX of the corresponding thiols. 

FIGURE 14.69 The intermediates for meta and para substitution of toluene. In the intermediate for meta substitution, all three resonance forms are secondary. In the intermediate for para substitution, two resonance forms are secondary but one resonance form is a tertiary carbocation. 

Since electronic effects are also well defined by the Hammett parameters, a, it is not surprising that reasonable correlations between J. and a are observed, in certain families of compounds. This is the case for meta and para substituted anilines for which two linear... 

For meta- and para-substituted aromatic carbonyl compounds, a linear relationship exists between the earbonyl absorption frequency and the Hammett reactivity constant. - - - A relationship between the carbonyl stretching vibration frequency of aromatic carbonyl compound and the pAT " of the corresponding aromatic carboxylic acid has been demonstrated. Correlations with other parameters, such as electronegativities,"" ionization potentials, Taft a values, half-wave potentials, etc., have also been made." For aromatic compounds with ortho- substituents, a combination of factors may be important, such as chelation, steric effects, and field effects (dipole interactions through space). 

Although the Hammett relationship works well for meta- and para-substituted aromatic compound, it frequently fails for either ortho- aromatic (due to steric effects) or aliphatic compounds, and other scales have been proposed, such as that of Taft. 

Fig. 14. X-ray scattering curves for polystyrene (un) and for meta- and para-substituted t-butyl polystyrene (m and p, respectively). Successive curves have been shifted vertically two divisions for clarity. Intensity scales (per division) un - 200 counts/s m -1,000 counts/s p - 400 counts/s.

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