Reduction processes, polynuclear compounds

Reduction Processes. Because of the presence of many polypyridine ligands, each capable of undergoing several reduction processes, the electrochemical reduction of the polynuclear compounds produces very complex patterns. 

Rubbers. Plasticizers have been used in mbber processing and formulations for many years (8), although phthaHc and adipic esters have found Htde use since cheaper alternatives, eg, heavy petroleum oils, coal tars, and other predominandy hydrocarbon products, are available for many types of mbber. Esters, eg, DOA, DOP, and DOS, can be used with latex mbber to produce large reductions in T. It has been noted (9) that the more polar elastomers such as nitrile mbber and chloroprene are insufficiendy compatible with hydrocarbons and requite a more specialized type of plasticizer, eg, a phthalate or adipate ester. Approximately 50% of nitrile mbber used in Western Europe is plasticized at 10—15 phr (a total of 5000—6000 t/yr), and 25% of chloroprene at ca 10 phr (ca 2000 t/yr) is plasticized. Usage in other elastomers is very low although may increase due to toxicological concerns over polynuclear aromatic compounds (9). 

These results allow a complete description of metal-ammonia processes, as shown in Scheme IV. The upper pathway represents the classical Birch reduction (i.e., benzenes), whereas polynuclear aromatic compounds react by one of the lower routes. As indicated in Scheme IV, the protonation of dianions rather than radical anions by ammonia is favored. We previously suggested dianion protonation (8) because radical anions are not very basic (10). Recently, Mullen et al. (12) pointed out that, in some cases, dianions and radical anions of the same neutral precursor are protonated at different sites in the cases they investigated, protonation occurred with the dianions. 

Researchers have often suggested (3, 4) that methyl-ammonia reduction must be carried out with carefully purified polynuclear aromatic compounds and scrupulously dried ammonia and ethyl ether cosolvents. Hence Burkholder and I (8) were surprised to learn that commercial anthracene could be reduced almost quantitatively without prior purification of ammonia or THF (or ethyl ether) and with a rather wide variation in the amount of metal used. More recently, my co-workers and I (14) found that 0.5 g of anthracene in ammonia-THF containing 3 mL of H20 can be reduced in excellent yield by the addition of 2.5 mol of sodium (Scheme V). These results indicate that electron addition to polynuclear aromatic compounds is a very fast process, and destruction of the metal by water is simply not competitive under these conditions. 

In this discussion of the metal-ammonia processes involving polynuclear aromatic compounds, some potentially important topics were omitted. One such topic is the stereochemistry of reduction, which is quite important... 

It is known that, when heavy feedstocks such as heavy vacuum gas oil are processed simultaneously with all the hydrocracking reactions, heavy polynuclear aromatics are formed. These compounds are concentrated in the heaviest fraction of the effluent they have a detrimental effect on the activity of the catalyst and on its life. For an operation with recycling for complete conversion, these heavy polynuclear aromatics accumulate in the liquid recycle stream causing fouling and rapid deactivation of the catalyst. A solution often practised is to accept a slight reduction in conversion by purging part of ftie liquid recycle stream catalyst activity and stability are thereby improved. 

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