Ionic substitutions

Another desirable property for a ceramic color is a high refractive index. For example, valuable pigments are based on spinels [1302-67-6] ( 2jj = 1.8) and on zircon ( 2j = 1.9), but no valuable pigments are based on apatite ( 2j = 1.6), even though the lattice of apatite is as versatile for making ionic substitutions as that of spinel. 

It is important to note that benzene does not behave like a typical cyclic olefin in that the benzene ring undergoes ionic substitution rather than addition reactions the ring also resists hydrogenation and is chemically more inert. Despite this, it is still a common practice to represent benzene with three double bonds as if it were 2,4,6-cyclohexatriene,... 

Interest has developed recently in cyclopentadienylcobalt carbonyl complexes. Oxidative addition of RI (R = Me or Et) to CpCo(CO)L (L = PPh3, PMe2Ph, or PMePh2) yields initially CpCo(CO)RL I, which then undergoes rapid CO insertion to CpCoL(COR) . The slow step has been studied kinetically 148). Compounds of the type CpCo(CO)(Rp)I afford ionic substitution products when treated with chelating diphosphines 106). 

Aromatic compounds are benzene and its derivatives and compounds that resemble benzene in their behaviour in a chemistry dominated by ionic substitution. Benzene has the formula commonly written as the ring ... 

There have, from time to time, been suggestions that electron transfer may mediate in processes which are formally ionic substitution reactions. Whilst such mechanisms have been established for a small number of systems (Kornblum, 1975 Bunnett, 1978), in other instances the evidence is less substantial. For example, dialkylaminyl spin adducts may be observed when secondary amines are allowed to react with picryl chloride in the presence of MNP (Bil kis and Shein, 1974). This can be interpreted in terms of Scheme 12, but alternatives involving nucleophilic addition to the trap merit consideration. 

This awareness in a short time led to new homolytic aromatic substitutions, characterized by high selectivity and versatility. Further developments along these lines can be expected, especially as regards reactions of nucleophilic radicals with protonated heteroaromatic bases, owing to the intrinsic interest of these reactions and to the fact that classical direct ionic substitution (electrophilic and nucleophilic) has several limitations in this class of compound and does not always offer alternative synthetic solutions. Homolytic substitution in heterocyclic compounds can no longer be considered the Cinderella of substitution reactions. 

Compounds in the two groups differ in a number of ways. The two differ chemically in that the aliphatic undergo free-radical substitution reactions and the aromatic undergo ionic substitution reactions. In this chapter you examine the basics of both ciromatic and heterocyclic ciromatic compounds, concentrating on benzene and related compounds. 

Analogous ionic substitutions work equally well on substituted adamantanes. A wide variety of mono-, di-, tri-, and tetra-bridgehead substituted adamantanes may be readily obtained. 

Under special conditions, an ionic substitution reaction also provides a convenient method for the funtionalization of the methylene positions of adamantane. Treatment of adamantane or 1-hydroxyadamantane with 96 % H2S04 at 77 °C for 5 hours results in a 50—60% yield of adamantanone 67-69). 

The general success of the direct ionic substitutions of the methylene positions of adamantane may be attributed to two main factors ... 

First, as discussed earlier in connection with the aluminum halide catalyzed rearrangements of hydrocarbons (Section II. A. 2), intermolecular hydride transfer reactions appear to be fairly unselective processes. Apparently, charge development in the transition states of these reactions is minimized a penta-coordinate carbon intermediate may be involved. As a result, the strong preference for the bridgehead positions exhibited by most ionic substitution reactions is partially overcome. 

By analogy with adamantane, direct, ionic substitution reactions may be expected to be equally as effective for the synthesis of derivatives of other diamondoid hydrocarbons (e.g. diamantane and triamantane). In these cases, however, a larger number of isomers are possible. Diamantane has one methylene and two nonequivalent bridgehead positions while triamantane has four of each. 

Preliminary results confirm the expected utility of direct ionic substitution reations for the preparation of derivatives of diamantane. In refluxing bromine, diamantane gives 1-bromodiamantane (74) 26S> as well as a variety of disubstituted bridgehead bromides (Eq. (64)) 266 If, on the other hand, the reaction is carried out in the presence of AlBr3, significant amounts of 4,9-dibromodiamantane, 77, are also observed (Eq. (65) 265l Treatment of this dibromide with one equivalent of trialkyltin hydride yields the 4-mono-substituted diamantane, 78. 

Finally, 2,6-disubstituted adamantanes may be obtained either by direct synthesis or as minor components of specific ionic substitution reactions. Treatment of 2,6-bicyclo[3.3.1]nonanedione with pyrolidine and methylene iodide followed by acid hydrolysis provides pure 2,6-adamantanedione (42) in an overall yield of approximately 20 % (Eq. (76)) 288>. 

Thus, the historical context reveals that the term C-H bond activation was introduced with a clear purpose to distinguish metal-mediated C-H cleavage from traditional radical and ionic substitution, and as such was essentially a mechanistic term [8], As a result we may formulate the organometallic definition the term C-H bond activation refers to the formation of a complex wherein the C-H bond... 

Since the primary minerals are electrically neutral a compensation must occur when differently charged elements replace each other. In plagioclase feldspars Si4+ can be replaced by Al3+ but, simultaneously Na+ is replaced by Ca2+. Large, well formed crystals are prized as gems and their colours depend on these ionic substitutions. Corundum (A1203) is colourless, yet just a few Cr3+ions turn it into a ruby (Burns, 1983). For a fuller discussion of igneous rocks the reader is referred to Dercourt and Paquet (1985). 

The reason why mixes with AR > 1.7 do not yield any CjjA, on independent crystallization is that the solid phases are not pure CjA, QAF and CjS. For AR = 2.71, the quaternary liquid in equilibrium with C,S, C S and CjA at 1400X contains 55.7% CaO, 27.1% AljOj, 10.0% FcjOj and 7.2% SiOj (S8). This composition can be closely matched by a mixture of aluminate (63%), ferrite (30%) and belite (7%) with the normal compositions given in Table 1.2, the bulk composition of this mixture being 54.4% CaO, 26.4% AI2O3, 9.7% Fe Oj, 5.6% SiO and 1.8% MgO, with <1% each of TiOj, Mn20j, NajO and KjO. Independent crystallization can thus yield a mixture of the three phases. The liquid composition cannot be matched by a mixture of pure CjA, C AF and CjS, which is relatively too high in CaO, so that if no ionic substitutions occurred, some C,2A7 would also be formed. A strict comparison would be with the actual composition of the clinker liquid, which is modified by minor components, but lack of adequate data precludes this. 

Effects of ionic substitutions, defects and variation in polymorph... 

The ionic substitutions are again governed by definite criteria known as Hume-Rothery rales. Size of the atoms is the most important factor in these rales. Substitution of one atom by another in a crystal structure is most likely when their ionic radii are within 15% it is less likely when sizes differ by 15-30%, and unlikely beyond that range. Note that these substitutions must also maintain overall charge balance, because the crystal structure must be neutral. 

The most common apatite is Ca5(P04)30H and is called hydroxyapatite. Other forms include chloroapatite (Ca5(P04)3Cl), fluoroapatite (Ca5(P04)3F) and carbonate apatite or dahllite (Ca5(P04)3C03). These minerals are in pure forms, but it is also possible to generate them by partial replacement of one anion by another or one cation by another. For example, Ca may be replaced by Pb by ionic substitution, yielding pyromorphites [Pb5(P04)3(0H,Cl,F)]. As we shall see in Chapter 16, this mineral is very important in stabilizing the hazardous metal Pb. Also as discussed in Chapter 2 and shall be seen in later chapters, Mg-based CBPCs have many applications, and hence minerals such as Mg5(P04)3(0H,Cl,F) are also very common. 

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