Trivalency

Chiral carbon atoms are common, but they are not the only possible centers of chirality. Other possible chiral tetravalent atoms are Si, Ge, Sn, N, S, and P, while potential trivalent chiral atoms, in which non-bonding electrons occupy the position of the fourth ligand, are N, P, As, Sb, S, Se, and Te. Furthermore, a center of chirality does not even have to be an atom, as shown in the structure represented in Figure 2-70b, where the center of chirality is at the center of the achiral skeleton of adamantane. 

Tracer studies using 253Es show that einsteinium has chemical properties typical of a heavy trivalent, actinide element. 

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

Whereas the differentiation of trivalent carbenium and pentacoor-dinated carbonium ions serves a useful purpose in defining them as limiting cases, it should be clear that in carbocationic systems there always exist varying degrees of delocalization. This can involve participation by neighboring -donor atoms, 7r-donor gronps, or [Pg.148]

Concerning carbocations, previous usage named the trivalent, planar ions of the type carbonium ions. If the name is considered anal-... 

German and French literature, indeed, frequently used the carbenium ion naming for trivalent cations. 

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. 

Whereas the differentiation of limiting trivalent and penta- or higher-coordinate ions serves a useful purpose in establishing the significant differences between these ions, it must be emphasized that these rep-... 

The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, that is, the reaction of trivalent carbocations with molecular hydrogen giving their parent hydrocarbons, involves the same five-coordinate carbonium ions. 

The superacid-catalyzed cracking of hydrocarbons (a significant practical application) involves not only formation of trivalent carbo-cationic sites leading to subsequent /3-cleavage but also direct C-C bond protolysis. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

Trivalent groups derived by the removal of three hydrogen atoms from the same carbon are named by replacing the ending -ane of the parent hydrocarbon with -ylidyne. 

The bivalent radical —NH— linked to two identical radicals can be denoted by the prefix imino-, as well as when it forms a bridge between two carbon ring atoms. A trivalent nitrogen atom linked to three identical radicals is denoted by the prefix nitrilo-. Thus ethylenediaminetetraacetic acid (an allowed exception) should be named ethylenedinitrilotetraacetic acid. 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

Description of the Method. The operational definition of water hardness is the total concentration of cations in a sample capable of forming insoluble complexes with soap. Although most divalent and trivalent metal ions contribute to hardness, the most important are Ca + and Mg +. Hardness is determined by titrating with EDTA at a buffered pH of 10. Eriochrome Black T or calmagite is used as a visual indicator. Hardness is reported in parts per million CaCOs. 

The site preference of several transition-metal ions is discussed in References 4 and 24. The occupation of the sites is usually denoted by placing the cations on B-sites in stmcture formulas between brackets. There are three types of spinels normal spinels where the A-sites have all divalent cations and the B-sites all trivalent cations, eg, Zn-ferrite, [Fe ]04j inverse spinels where all the divalent cations are in B-sites and trivalent ions are distributed over A- and B-sites, eg, Ni-ferrite, Fe Fe ]04 and mixed spinels where both divalent and trivalent cations are distributed over both types of sites,... 

The inorganic flocculating agents are water-soluble salts of divalent or trivalent metals. For all practical purposes these metals are aluminum, iron, and calcium. Sodium siHcate is also used in some appHcations. The principal materials currently in use are described in the foUowing. 

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