Sodium iodide, dissociation

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

In another important experiment (1989) Zewail and his group studied the dissociation of sodium iodide (Nal) Na+ r Na + I. The pump pulse excites the ion pair Na+r which has an equilibrium distance of 2.8 A between... 

Figure 20.3. The time evolution of photoexcited sodium iodide. The signal intensity proportional to covalent ([Na—I] ) stmctures oscillates following the femtosecond pump pulse the [Na—I] vibrates between covalent and ionic structures before crossing to the lower potential energy surface and dissociating to give Na and I atoms. [Reproduced with pemtission from A. H. Zewail, J. Phys. Chem. A. 2000, 104, 5660.]...

Lil in boiling pyridine or other weak nucleophilic bases can cleave alkyl esters to alkyl iodides and lithium carboxylates (Scheme 28). The reaction is mainly used for mild, aprotic cleavage of esters to car-boxylates. The high degree of dissociation for Lil and the nucleophilic strength of the iodide ion explain the reaction with esters, which is not useful with the other halides. Trimethylchlorosilane and sodium iodide also give alkyl iodides from esters. ... 

As pointed out earlier, when ionic compounds dissolve in water, they break apart completely into their component cations and anions. To be more realistic, the equations should show the dissociation of dissolved ionic compounds into ions. Therefore, returning to the reaction between sodium iodide and lead nitrate, we would write... 

To study the complexation selectivity of sodium iodide for the linear pentamine isomer, n-HMTP (Compound 7), and not the branched isomer (as in the analogous tetramine separation), both n-HMTP Nal and iso-HMTP Nal were prepared and studied. No substantial difference in solubilities of the pentamine-sodium iodide complexes in benzene could be observed. However, the stability of n-HMTP Nal in benzene seemed to be appreciably greater than that of iso-HMTP Nal in the same solvent. The latter complex dissociated in benzene to give free iso-HMTP and insoluble sodium iodide at 40 °C. The corresponding dissociation temperature for n-HMTP Nal in benzene was 72°C. The complexation selectivity found by combining N-permethylated TEPA with sodium iodide may be rationalized if the above results indeed were to reflect a difference in the relative stabilities of the two complexes. 

As we have already seen, the ground-state potential of sodium iodide is ionic in nature and correlates with a pair of ions, i.e. Na and (see Figure 15.6). However, what is not generally appreciated is that all molecules have ion-pair states (these are often electronically excited states) and such states can photo-dissociate to yield pairs of ions, once the ion-pair dissociation limit is reached, provided that formation of a stable negative ion fragment is possible, i.e. 

Since most of the dissociation reaction of an isolated molecule occurs in a very short timescale ( 100 fsec), the direct observation of its transition from reagents to products has to wait until the discovery of femtosecond laser technology. With femtosecond wave packet spectroscopy, the trajectories of particles can be monitored during their motions on a potential energy surface. Zewail s group in 1989, for the first time, directly followed the evolution in space and time of such trajectories during the breakage of a chemical bond in the dissociation of sodium iodide. 

Reduction. BrezeHus attempted the first reduction of zirconium in 1824 by the reaction of sodium with potassium fiuorozirconate. However, the first pure ductile metal was made in 1925 by the iodide thermal-dissociation method. The successfiil commercial production of pure ductile zirconium via the magnesium reduction of zirconium tetrachloride vapor in an inert gas atmosphere was the result of the intense research efforts of KroU and... 

Although the reaction with thiosulfate and with iodide ions may be a mere reduction of the halide, the reaction with sodium benzoate would appear to be a radical dissociation induced by the attack of a negative ion. The fate of the benzoate ion is unknown. Tris-( -nitrophenyl)-methyl benzoate is a stable substance which does not dissociate into radicals.23... 

Figure 7.4 Thermodynamic data needed in evaluation of the enthalpy of formation of MX(s). (a) Lattice enthalpy of sodium halides (b) lattice enthalpy of alkali iodides (c) electron gain and dissociation enthalpies of halides (d) ionization and atomization enthalpies of alkali metals.

Until fairly recently, little was known of the stmcmres and properties of the organozinc compounds occurring as intermediates in varions reactions. Interestingly, the complexforming ability of organozinc componnds had already been recognized very early. In 1858, Wanklyn reported the formation of the ionic sodium triethylzinc complex. One year later, Frankland observed that the formation of dimethylzinc from methyl iodide and zinc was accelerated by the addition of dimethyl ether or diethyl ether. It appeared that separation of the dimethylzinc from the ether was impossible, bnt it lasted nntil 1962 when it was established that dimethylzinc and dimethyl ether form a 1 1 complex in solntion, which is appreciably dissociated in the vaponr phase. ... 

When treated with water, bromine and iodine are set free hydrochloric acid liberates iodine sodium or ammonium hydroxides give a black precipitate of nitrogen iodide, and the mother liquid liberates bromine when treated with hydrochloric acid. Sulphurous acid gives hydrobromic and hydriodic acids. F. Ephraim. found the dissociation press, of NI IB to be 65 mm. at 107° 204 mm. at 134° 465 mm. at 154° and 640 mm. at 161°. 

The reduction of l-methyl-2-carbomethoxypyridinium iodide (2, I ) with sodium amalgam leads to the pyridinyl radical dimer (2-2) (see sect. 4.1). The dimer is in equilibrium with the monomer (2 ) (K jssoc ca- 1.3-2.0 x 10 M by titration or EPR measurements) Dissociation of the dimer yields 2 , isolated via low temperature distillation (ca. 40-50 °C) with condensation at 77 K (Eq. 5). Dimerization occurs on warming the condensate above ca. 120 K. The spectroscopic properties of the radical 2" are determined with a new thin film spectroKopy apparatus, a device which dramatically simplifies experiments on stable radicals (S rt. 3.1.1). 

It has been found that sodium chloride, which is frequently used in biochemical experiments, is completely dissociated at any temperature and has a solubility of 0.1 M in mixed solvents of 50% volumic ratio. Other neutral salts, such as sodium and potassium iodides, are even more soluble in these conditions. Thus, it is easy to preserve the ionic strength of many samples in cooled aqueous-organic mixtures. 

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