Heteroelements

Asphaltenes have high concentrations of heteroelements sulfur, nitrogen, nickel and vanadium. Their content varies widely in petroleum oils (Table 1.5). They cause a number of problems throughout the petroleum industry. 

The refining industry generally seeks either to eliminate asphaltenes or to convert them to lighter materials because the presence of heteroelements cause pollution problems, e.g., sulfur and nitrogen, catalyst poisoning, and corrosion (formation of metal vanadates during combustion). 

When a boron atom of a borane is replaced by a heteroelement, the compounds are called carbaboranes, phosphaboranes, thiaboranes, a2aboranes, etc, by an adaptation of organic replacement nomenclature. The numbering of the skeleton in heteroboranes is such that the heteroelement is given the lowest possible number consistent with the conventions of the parent borane. Thus C2B2H is dicarba- /(9j (9-pentaborane(5) and could occur as the 1,2-, 2,3-, or 1,5-isomeric forms (l,2-dicarba- /(9j (9-pentaborane(5) [23777-70-0] 2,3-dicarba- /(9j (9-pentaborane(5) [30396-61-3] and... 

Polyhedral Expansion. The term polyhedral expansion is used to describe a host of reactions in which the size of the polyhedron is increased by the addition of new vertex atoms whether boron, heteroelements, or metals. In the case of the boranes, the pyrolysis of B2H has been used to obtain B H and industrially. Although a subject of much study, the mechanism of such pyrolytic expansions is not well understood. 

Heteroboranes contain heteroelements classified as nonmetals. The heteroatoms known to form part of a borane polyhedron include C, N, Si, P, As, S, Se, Sb, and Te either alone or in combination. In principle, most heteroboranes could have a wide range of skeletal sizes. However, with the primary exception of the carbaboranes, extensive chemistry has emerged only for the thiaboranes and azaboranes, which have the greatest availabiLity and demonstrated scope of chemistry. 

SPh determination of silicon and phosphorus in form of Si-Mo and P-Mo heteropolyacids are used successfully for series determination of these heteroelements in OEC and polymers (polysiloxanes, polyphosphazenes, etc.). 

Despite many publications on carbynes, their existence has not been universally accepted and the literature has been characterised by conflicting claims and counter claims [e.g., 27-29]. This is particularly tme of meteoritic carbynes. An interesting account of die nature of elemental carbon in interstellar dust (including diamond, graphite and carbynes) was given by Pillinger [30]. Reitmeijer [31] has re-interpreted carbyne diffraction data and has concluded that carbynes could be stratified or mixed layer carbons with variable heteroelement content (H,0,N) rather than a pure carbon allotrope. 

Figure 15.44 Some heterocyclic S-N compounds incorporating a third heteroelement.

The Ireland-Claisen reaction of ( )-vinylsilanes has been applied to the stereoselective synthesis of syn- and c/nti-2-substituted 3-silyl alkcnoic acids. a R-2-Alkyl-3-silyl acids are prepared by rearrangement of ( )-silyl ketene acetals which are generated in situ from the kinetically formed (Z)-enolate of the corresponding propionate ester40. Chelation directs the stereochemistry of enolization of heteroelement-substituted acetates in such a way that the syn-diastereomers are invariably formed on rearrangement403. 

Definitive evidence on the role of heteroelements in the regjochemical control of the hydrozirconation may be exemplified with the reaction of phospholene-1 (7) with 1. The gem-phosphazirconocene compound 8 is selectively formed with P-coor-dination on the metal fragment. The ]3-zirconated product 10 was prepared with 1... 

This book is divided in eight chapters and each of them is devoted to the state-of-the art of the homogeneously catalysed addition of E-H or E-E heteroelement bonds to unsaturated substrates with C=C, C=C and C=X functions (X = O, S). Em-... 

By this term we understand bonds between heteroelements in the sense of classical organic chemistry, i.e. bonds which do not include the element carbon. 

Ferrocene compounds containing heteroelements are versatile building blocks for polynuclear complexes. They possess electrochemically active iron centers as well as unique cylindrical shapes. An example of ferrocene-substituted thiolato ligands is l,l -ferrocenedithiolate (S2fc). Its complex with Pd(II) lacks a Pd—Fe bond. 

Me3SiCl reacts with phosphinomethanides I (R=Me) with at least one hydrogen as carbanion substituent (X = Y=H X=H, Y=SiMe3, PMej) via Si-C bond formation to give heteroelement substituted phosphinomethanides [4]. With fully C-heteroatom-substituted I, the reaction depends on the nature of X and Y, as shown by Eqs. (l)-(3) ... 

It is well known that char formation during polymer burning is an important mechanism by which polymers resist burning. For example Van Krevelen (1-2) has shown that a correlation exists between the oxygen index (OI) of a polymer containing no heteroelements and the amount of char it forms when pyrolyzed in the absence of air. As shown in Table I, a similar correlation is seen to hold for a number of common engineering thermoplastics. 

Defects in MWCNTs are always present. We can briefly differentiate between topological defects which lead to rehybridization (C5 and C7 rings instead of C6 lead to rehybridization between sp2 and sp3) and incomplete bonding defects (vacancies, dislocation) (Fig. 16.2). Functionalization or doping with heteroelements may add further modifications with respect to the ideal ordered structure, but are also the sites which allow for anchoring supported metals or metal oxides, or to functionalize the CNTs with organic groups. 

The synthesis, chemistry, and complexing behavior of phospholide (6, with 6a, 6b, and 6c resonance structures) and polyphospholide anions have been reviewed recently.The [C H Ps J series with n = 0—4 is a complete set of structures with a successive replacement of CH units by the same heteroelement, P. The counterions are alkaline metals (e.g., Ps K+, which, together with K2HP7, has been obtained from red P in refluxing DMF in a yield of... 

In some cases it is necessary to add a reactive gas to either assist the loss of ligands and/or reach better decomposition conditions, or to introduce a heteroelement into the deposit such as for nitride formation for instance. 

D. Carbon and Other Heteroelement Atom Considerations (Rule 3]... 

The structures, relative stabilities, and relative Lowry-Bronsted acidities of carboranes and boranes as well as related anions, Lewis base adducts, and heteroelement analogs are rationalized primarily on the basis of rudimentary coordination numbers. The principal factors, in decreasing order of importance, are (a) the various deltahedra and deltahedral fragments, (b) the placement of bridge and endohydrogens, (c) the placement of carbon and other heteroelements, and d) the resulting coordination number of boron. 

The 1971 hypothesis 164) (throughout this paper referred to as rule 1) may now be expanded by the addition of supplementary rules that deal with the placement of BE hydrogens (rule 2), the placement of carbon and other heteroelements (rule 3), and the consequent coordination situation of boron (rule 4). 

The following sections expand on the deltahedron-deltahedral fragment hypothesis (structural rule 1), and in the order of decreasing importance add three additional rules, one involving the placement of BE hydrogens (rule 2), next the placement of the various heteroelements (with emphasis on carbon) (rule 3) 172), and finally the structural accommodations of boron, including the influences of endohydrogens (rule 4). All are shown to have their roots firmly and simply embedded in CNPB, considerations. 

Upon closer scrutiny it becomes apparent that these considerations revealed by carbon s predilections are general and apply to the other heteroelement carboranes as well. 

The modified element name sila indicates replacement in the carbon skeleton, and similar treatment can be applied to other element names. The parent hydride names of Table 5.2 may all be adapted in this way and used in the same fashion as in the oxa-aza nomenclature of organic chemistry. In inorganic chemistry, a major use is in names of cyclic derivatives that have heteroelement atoms replacing carbon atoms in structures. It may be possible to name such species by Hantzsch-Widman procedures (see p. 77), and these should always be used when applicable. 

FIVE-MEMBERED RINGS WITH THREE OR FOUR HETEROATOMS INCLUDING AT LEAST ONE GROUP IV HETEROELEMENT OTHER THAN SILICON... 

---