Para-critical

It has been known since the early days that behavior of the aromatic polyamides (aramids or PARA) critically depends on the type of isomeric substitutions — para resulted in crystalline, while meta in amorphous resins [Kwolek et al, 1962]. 

It is quite common to refer to the somewhat ill-defined region where such changes are noticable as the near-critical region. The synonym para-critical may be occasionally found in older literature. Technically, the near-critical region extends all around the critical point, but the expression is commonly used to refer to the nonsupercritical section only. The very similar expression compressible region refers to the area around the critical point in which the... 

PNOTE - MOOIEIEO PARA tETEPS INCLUDING CRITICAL TEMPERATURE PRESSURE VOLUME OtPOLP MOMENT,... 

Properties of Light and Heavy Hydrogen. Vapor pressures from the triple point to the critical point for hydrogen, deuterium, tritium, and the various diatomic combinations are Hsted in Table 1 (15). Data are presented for the equiUbrium and normal states. The equiUbrium state for these substances is the low temperature ortho—para composition existing at 20.39 K, the normal boiling point of normal hydrogen. The normal state is the high (above 200 K) temperature ortho—para composition, which remains essentially constant. 

Tubular reactors are used for some polycondensations. Para-blocked phenols can be reacted with formalin to form linear oligomers. When the same reactor is used with ordinary phenol, plugging will occur if the tube diameter is above a critical size, even though the reaction stoichiometry is outside the region that causes gelation in a batch reactor. Polymer chains at the wall continue to receive formaldehyde by diffusion from the center of the tube and can crosslink. Local stoichiometry is not preserved when the reactants have different diffusion coefficients. See Section 2.8. 

In Table III are shown values of p, p, X, n, SD, / and Hammett p values obtained for additional para BA type reaction series which contain data for sufficient numbers ( ) and kinds of substituents to provide a reasonably critical analysis. We do not believe, however, that the analysis is as critical for sets which do not meet the minimal requirements for a basis set. The reaction series are segregated according to type of measurement equilibrium, rate, and fluorine nmr (F-nmr) shift. Any data set in Table III and in all tables hereafter (other than designated basis sets) which meets our minimal basis set criterion is indicated by an asterisk. 

Thus, the critical value determining para-selectivity r2k/D can be replaced by two readily measured quantities, a and t0.3 ... 

Brazed joints are limited to tubular lap or butt-lap joints and shall meet the more stringent requirements of this Code or the engineering design. See Nonmandatory Appendix A, para. A-3.3 for AWS C3.3, Recommended Practices for the Design, Manufacture, and Examination of Critical Brazed Components. 

The internal line shall be leak tested on the basis of the internal or external design pressure, whichever is more critical. This test must be performed before the jacket is completed if it is necessary to provide visual access to joints of the internal line as required by para. IP-10.6.3(a). 

The triad annihilations in Schemes 12 and 13 again provide the mechanistic basis for analysing these apparently disparate results. Thus the difference between Me2PyN02 and MeOPyNOj in (i) and (ii), respectively, mirrors that observed for toluene (vide supra). Accordingly, the statistical ortho/ para pattern obtained from Me2PyN02 in (i) can be attributed to a similar dominance of the homolytic annihilation of AN+- to produce the critical Wheland intermediate in aromatic nitration (Scheme 17). Indeed, the... 

Moderate to good enantioselectivities were obtained for nearly all examples, but the products from 83a-c could be recrystallized to higher enantiomeric purity. Addition of iodine was critical for catalysis as was the use of a ligand with electron-poor para-fluorophenyl groups on the phosphorous atom. Substitution at the 3 position of the pyridine ring was described as being difficult for both the quinolines and pyridine systems. The resulting hydrazine derivatives could be easily converted to piperdines by reduction with Raney nickel or under Birch conditions. 

Sulfanilamides are antibiotics that serve as structural analogs of para-aminobenzoic acid (PABA), a substrate In the formation of folic acid by many bacteria. Substitution of the sulfanilamide compound In place of PABA In the reaction prevents formation of the critical coenzyme folic acid. 

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