Resistance catalytic

The most notable chemistry of the biscylopen-tadienyls results from the aromaticity of the cyclopentadienyl rings. This is now far too extensively documented to be described in full but an outline of some of its manifestations is in Fig. 25.14. Ferrocene resists catalytic hydrogenation and does not undergo the typical reactions of conjugated dienes, such as the Diels-Alder reaction. Nor are direct nitration and halogenation possible because of oxidation to the ferricinium ion. However, Friedel-Crafts acylation as well as alkylation and metallation reactions, are readily effected. Indeed, electrophilic substitution of ferrocene occurs with such facility compared to, say, benzene (3 x 10 faster) that some explanation is called for. It has been suggested that. 

The sulfonyl group in sulfones resists catalytic hydrogenation. Double bonds in a, -unsaturated sulfones are reduced by hydrogenation over palladium on charcoal (yield 94%) [686, 687] or over Raney nickel (yield 62%) without the sulfonyl group being affected [686]. In p-thiopyrone-1,1-dioxide both double bonds were reduced with zinc in acetic acid but the keto group and the sulfonyl group survived [655]. Raney nickel may desulfurize sulfones to hydrocarbons [673]. 

Chemical evidence for the ketonic nature of pyroaconitine (CXLI), formed by p3Tolysis of aconitine, was demonstrated by subjecting it to Wolff-Kishner reduction (85). The product of this reaction contained no carbonyl absorption in its IR spectrum, showing that not only had the ketone been reduced but also that the benzoyloxy group had been saponified. Further, one of the methoxyl groups had been eliminated. The Wolff-Kishner product was thus assumed to possess structure CXLIII since it resisted catalytic hydrogenation. A later correlation involving... 

In brief, the key issue in high-temperature catalytic combustion is how to develop heat-resistant catalytic materials in particular, retention of large surface area is one of the main indicators used to rank catalyst quality. 

Dai Y, Lim B, Yang Y, Cobley CM, Li W, Cho EC, Grayson B, Fanson FT, Campbell CT, Sun Y, Xia Y (2010) A Sinter-resistant catalytic system based on platinum nanoparticles supported on Xi02 nanofibers and covered by porous silica. Angew Chem Int Ed 49 8165-8168... 

The sandwich bis-7r-cyclopentadienyl complexes are thermally rather stable, and many melt without decomposition at about 173X. They are stable to hydrolysis, and the C5H5 rings resist catalytic hydrogenation. Their stability to oxidation, however, varies greatly with the nature of the metal. At room temperature the 18-electron complex ferrocene is inert to molecular oxygen, whereas chromocene (which possesses a 16-electron configuration) is pyrophoric in air. 

Amongst examples of reactions of 2-thiothiazolines with active methylene groups, their interactions with cyanoacetic ester (see also these Reports, Vol. 1, p. 408) and rhodanines have been studied. 2-Alkylthio-A -thiazolines (129) or their quaternary salts are converted by the former reagents into 2-(ethoxycarbonylcyanomethylene)thia-zolidine (130). Their tetrasubstituted double bond resists catalytic... 

Dehydration or Chemical Theory. In the dehydration or chemical theory, catalytic dehydration of ceUulose occurs. The decomposition path of ceUulose is altered so that flammable tars and gases are reduced and the amount of char is increased ie, upon combustion, ceUulose produces mainly carbon and water, rather than carbon dioxide and water. Because of catalytic dehydration, most fire-resistant cottons decompose at lower temperatures than do untreated cottons, eg, flame-resistant cottons decompose at 275—325°C compared with about 375°C for untreated cotton. Phosphoric acid and sulfuric acid [8014-95-7] are good examples of dehydrating agents that can act as efficient flame retardants (15—17). 

Eriedel-Crafts reaction of naphthalene or tetrahydronaphthalene derivatives with those of styrene or alkylbenzenes has been used in the preparation of high viscous fluids for traction drive (195). Similarly, Eriedel-Crafts reaction of tetraline and a-methylstyrene followed by catalytic hydrogenation provided l-(l-decalyl)-2-cyclohexyl propane, which is used as a highly heat resistant fluid (196). 

A polyester-type fluorescent resin matrix (22) is made by heating trimellitic anhydride, propylene glycol, and phthaUc anhydride with catalytic amounts of sulfuric acid. Addition of Rhodamine BDC gives a bright bluish red fluorescent pigment soluble in DME and methanol. It has a softening point of 118°C. Exceptional heat resistance and color brilliance are claimed for products of this type, which are useful for coloring plastics. 

Reduction. Just as aromatic amine oxides are resistant to the foregoing decomposition reactions, they are more resistant than ahphatic amine oxides to reduction. Ahphatic amine oxides are readily reduced to tertiary amines by sulfurous acid at room temperature in contrast, few aromatic amine oxides can be reduced under these conditions. The ahphatic amine oxides can also be reduced by catalytic hydrogenation (27), with 2inc in acid, or with staimous chloride (28). For the aromatic amine oxides, catalytic hydrogenation with Raney nickel is a fairly general means of deoxygenation (29). Iron in acetic acid (30), phosphoms trichloride (31), and titanium trichloride (32) are also widely used systems for deoxygenation of aromatic amine oxides. 

The metal parts of the injection molder, ie, the liner, torpedo, and nozzle, that contact the hot molten resin must be of the noncatalytic type to prevent accelerated decomposition of the polymer. In addition, they must be resistant to corrosion by HCl. Iron, copper, and zinc are catalytic to the decomposition and caimot be used, even as components of alloys. Magnesium is noncatalytic but is subject to corrosive attack, as is chromium when used as plating. Nickel alloys such as Duranickel, HasteUoy B, and HasteUoy C are recommended as constmction materials for injection-molding metal parts. These and pure nickel are noncatalytic and corrosion-resistant however, pure nickel is rather soft and is not recommended. 

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