Zinc halides characteristics

Catalytic conversions in the monoterpene field have been reviewed recently [13-15]. There is an ongoing transition from conventional homogeneous catalysts (mineral acids, zinc halides) to solid Bronsted and Lewis acid catalysts. Thus, limonene can be alkoxylated with lower alcohols using zeolite H-Beta as the catalyst [16] at room temperature already, with high selectivity and conversion (Scheme 5.3). The alkoxy compounds are applied as fragrances with, dependent on the length of R, characteristic odors. 

A general discussion of the superior thermal stability characteristics of fluorovinylzinc reagents has been presented vide supra). Specifically, the fluorinated zinc reagents exhibit exceptional thermal stability. A sample of a [(Z)-pentafluoroprop-l-enyl]zinc halide in triglyme shows no loss of activity after three days at room temperature and only 10% of the zinc reagent decomposes after 36 days at room temperature. Upon heating at 65 C over three days, only 5 % of the zinc reagent decomposes, and a 25 % loss of activity is observed at the same tern-... 

Analyses of rate measurements for the decomposition of a large number of basic halides of Cd, Cu and Zn did not always identify obedience to a single kinetic expression [623—625], though in many instances a satisfactory fit to the first-order equation was found. Observations for the pyrolysis of lead salts were interpreted as indications of diffusion control. More recent work [625] has been concerned with the double salts jcM(OH)2 yMeCl2 where M is Cd or Cu and Me is Ca, Cd, Co, Cu, Mg, Mn, Ni or Zn. In the M = Cd series, with the single exception of the zinc salt, reaction was dehydroxylation with concomitant metathesis and the first-order equation was obeyed. Copper (=M) salts underwent a similar change but kinetic characteristics were more diverse and examples of obedience to the first order, the phase boundary and the Avrami—Erofe ev equations [eqns. (7) and (6)] were found for salts containing the various cations (=Me). 

Zinc, cadmium and mercury are at the end of the transition series and have electron configurations ndw(n + l)s2 with filled d shells. They do not form any compound in which the d shell is other than full (unlike the metals Cu, Ag and Au of the preceding group) these metals therefore do not show the variable valence which is one of the characteristics of the transition metals. In this respect these metals are regarded as non-transition elements. They show, however, some resemblance to the d-metals for instance in their ability to form complexes (with NH3, amines, cyanide, halide ions, etc.). 

Notably, Zn2+ ions are more easily reduced than naphthalene. This indicates that the electroreduction of the latter is very likely achieved on a recovered electrolytic zinc cathode. What happens under these conditions So far, no data allow us to answer this question. The only information is the experimental observation of a characteristic transient color of the naphthalene anion radical indicating the reduction of this hydrocarbon, simultaneously with the reduction of Zn2+, owing likely to a too high current density set at the cathode11. Moreover, the presence of an alkyl halide under such conditions would lead to its reduction on the zinc deposit, and this reduction would occur more easily than the reduction of naphthalene according to complex processes (equation 25). 

Nickel- or palladium-catalyzed coupling reactions of alkyl Grignard or zinc reagents can be applied to heteroaromatic halides and sulfides. The characteristic features are, therefore, based on those described in the preceding sections and hence details are not repeated in this section. Aspects are summarized by the types of heteroaromatic compounds NiCh(DPPP) is used as catalyst, unless stated otherwise hereafter. 

With these examples the list of the many-sided reactions of the halogen alkyls is not exhausted they are also used for the preparation of the- metallic alkyls, e.g., zinc alkyls for the preparation of the phosphines, and for many other compounds. Finally, attention is called to the characteristic difference between the organic and inorganic halides. While, e.g., potassium chloride, bromide, or iodide in solution act instantly with a silver nitrate solution to form a quantitative precipitate of silver chloride, bromide, or iodide respectively, silver nitrate in a water solution does not act on most organic halides, so that this reagent does not serve in the usual way to show the presence of a halogen. 

The structures of the chlorides, bromides and iodides may be viewed as close-packed arrays of halide ions, but there is a characteristic difference in that zinc ions occupy tetrahedral interstices whereas cadmium ions occupy octahedral ones. Actually, there are at least three polymorphs of ZnCl2 known, two of which are similar and in which the presence of zinc ions in tetrahedral holes has been demonstrated. Raman spectra show that, depending on the concentration, the species present in aqueous solutions of ZnCl2 are [Zn(H20)6]2 +, ZnCl+(aq), ZnCl2(aq) and [ZnCl4(H20)2]2-, but no indication was found of [ZnCl3]- or [ZnClJ2- (see below). 

This concept was further extended to totally eliminate the use of iodonium salts as the component of the photoinitiating system [KAH 09]. The cationic polymerization of vinyl ethers was initiated upon irradiation at A = 350 nm with vinyl halides in the presence of zinc iodide. A mechanism involving the formation of an adduct between the monomer and the products yielded from the photoinduced homolysis of the vinyl halide followed by electron transfer is proposed. In the subsequent step, the terminal carbon-halide bond in this adduct is activated by the coordinating effect of zinc iodide. This polymerization exhibited some characteristics of pseudo-living cationic polymerization. 

F.t.i.r. spectra of both anomers of a variety of peracetylated 2,4-dinitrophenyl D-gluco- and galacto-pyranosides and peracetylated aryl 1-thioglycosides have been recorded. The anomers give characteristic absorptions. F.t.i.r. spectra of adducts formed between D-glucose and zinc(II), cadmium(II) and mercury(II) halides in the solid (KBr pellets) and in solution (film cast) have been discussed in detail. ... 

Reduction. Most sulphonyl halides respond to the following test. Boil 0 2 g of the original compound with 5 ml of dilute hydrochloric acid and a pinch of zinc dust. The characteristic odour of a thiophenol indicates the presence of a sulphonyl halide. 

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