Para, meaning

Note that this is the para para (abbreviated as p,p ) isomer. The term para means that the chlorine atoms are positioned on the benzene ring across from the linkage to the rest of the molecule. DDT won Paul Muller the Nobel Prize in medicine in 1948 for malaria control because DDT killed the mosquitoes that transmitted this disease. At least 2 billion kilograms of DDT has been used worldwide since about 1940. Because of problems with calcium metabolism in birds (egg shell thinning), DDT has been banned in most industrialized countries since about 1970-1975. By the loss of HC1, p,//-DDT degrades relatively quickly to DDE (see below), which is almost permanently stable in the environment. DDT, however, is still produced and used to control malaria in developing countries because it is cheap to manufacture and simple to use. 

If the methyl radicals are on the top and bottom of the ring, it is referred to as the para position para means opposite. The structure and molecular formula for paraxylene are shown in Figure 5.42. 

Phenol may be nitrated with dilute nitric acid to 3deld a mixture of o- and nitrophenols the 3deld of p-nitrophenol is increased if a mixture of sodium nitiute and dilute sulphuric acid is employed. Upon steam distilling the mixture, the ortho isomer passes over in a substantially pure form the para isomer remains in the distillation flask, and can be readily isolated by extraction with hot 2 per cent, hydrochloric acid. The preparation of m-nitrophenol from wt-nitroaniline by means of the diazo reaction is described in Section IV,70. 

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

The Gassman indole synthesis has one serious limitation. Attempts to use anilines with an ortho/para- methoxy moiety failed to indolize. One means to overcome this was synthesis of the corresponding oxindoles followed by reduction to the indoles. ... 

Treatment of that intermediate with aluminum chloride leads to selective demethylation of that ether para to the carbonyl group (23). Cyclization by means of tosic acid gives the dihydronaphthalene nucleus (24). Alkylation of the phenol with N-(2-chloroethyl)pyrrolidine affords nafoxidine (25). ... 

Nucleophilic aromatic substitution of the anion from ary lace ton itrile 113 on the dichloroni-trobenzene 114 results in replacement of the para halogen and formation of 115. Reduction of the nitro group gives the corresponding aniline (116). Acylation of the amine with 3,5-diiodoacetylsa-licylic acid 117 by means of the mixed anhydride formed by use of ethyl chloroformate, gives, after alkaline hydroly.sis, the anthelmintic agent closantel (118) [28]. 

The meto-oxybenzaldehyde is then nitrated and methylated, by which means para-nitrowcfa-methoxy-benzaldehyde... 

Equation 35 is very similar to Eq. 33, although the correlation is not as sharp. The most striking difference between Eqs. 35 and 36 is that the latter does not contain a term in E. This means that para substituents have no effect on hydrolysis through... 

In these equations, Dmax is the larger of the summed values of STERIMOL parameters, Bj, for the opposite pair 68). It expresses the maximum total width of substituents. The coefficients of the ct° terms in Eqs. 37 to 39 were virtually equal to that in Eq. 40. This means that the a° terms essentially represent the hydrolytic reactivity of an ester itself and are virtually independent of cyclodextrin catalysis. The catalytic effect of cyclodextrin is only involved in the Dmax term. Interestingly, the coefficient of Draax was negative in Eq. 37 and positive in Eq. 38. This fact indicates that bulky substituents at the meta position are favorable, while those at the para position unfavorable, for the rate acceleration in the (S-cyclodextrin catalysis. Similar results have been obtained for a-cyclodextrin catalysis, but not for (S-cyclodextrin catalysis, by Silipo and Hansch described above. Equation 39 suggests the existence of an optimum diameter for the proper fit of m-substituents in the cavity of a-cyclodextrin. The optimum Dmax value was estimated from Eq. 39 as 4.4 A, which is approximately equivalent to the diameter of the a-cyclodextrin cavity. The situation is shown in Fig. 8. A similar parabolic relationship would be obtained for (5-cyclodextrin catalysis, too, if the correlation analysis involved phenyl acetates with such bulky substituents that they cannot be included within the (5-cyclodextrin cavity. 

The special substituent constants for + R para-substituents are denoted by a, and those for — R para-substituents are denoted by a+ 54. They are based respectively on the reaction series discussed above. Selected values are given in Table 1. Characteristic a or a+ values are sometimes distinguished for meta-substituents also, but only for a minority of substituents which show very marked + R or — R effects do these differ significantly from ordinary a values. The range of applicability of the Hammett equation is greatly extended by means of a and cr+, notably to nucleophilic (by a ) and to electrophilic (by cr+) aromatic substitution. 

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