Alkali metals reducing abilities

The slow development of heavy alkali organometallic chemistry is due to high reactivity, as rationalized by the increase of polar character of the metal-ligand bond due to the reduced polarizing ability of the metals. The increase in ionic character on descending the group of alkali metals is clearly demonstrated by the increase in ionic radii with Li+(0.69A), Na+(0.97A), K+(1.33A), Rb+(1.47A), and Cs+(1.67A), resulting in a radius of Cs+ that is more than double of that of Li+. 

The ion pairing ability of cations can also be reduced by addition of complexing agents. For example, the use of alkali metal salts in the presence of cryptands 114, 114a) such as 4,7,13,16,21,24-hexaoxa-l,10-diazabi-cyclopentatriacontane (222-crypt) ... 

For reactions of this type, the relative reducing powers of the alkali metals can be predicted from the first ionization energies listed in Table 12.9. Since it is much easier to remove an electron from a cesium atom than from a lithium atom, cesium should be the better reducing agent. The expected trend in reducing ability is... 

This order is observed experimentally for direct reactions between the solid alkali metals and nonmetals. However, this order of reducing ability is not observed when the alkali metals react in aqueous solution. For example, the reduction of water by an alkali metal is very vigorous and exothermic ... 

From the characterization of the hydrolytically solubilized coal, the following observations can be made (1) the polycyclic aromatic rings are being reduced, and a possible mechanism is hydride transfer from the solvent (2) oxygen heteroaromatics are destroyed (3) aryl ethers are being cleaved by this process (4) the ability of the glycols to chelate the positive alkali metal ions could contribute to the enhanced yields of soluble coal compared to other protic solvents. 

The following sequence of overall acidity (density of Bronsted and Lewis acid sites) measured via t.p.d. of ammonia was found for dealuminated Y zeolites exchanged with alkali-metal ions H-D-Y Li-D-Y > Na-D-Y > Cs-D-Y > K-D-Y the strength of the acid sites was particularly low in the case of K-D-Y. The lower the overall acidity of the support, the lower was the observed average oxidation number of Mo. The zeolites with reduced overall acidity exhibited a higher ability to decompose ammonia. 

In order to obtain more fundamental catalytic activity data of the catalytic materials of interest a number of model catalysts consisting of alkali metal and precious metal were prepared and tested for their ability to promote the reactions of water and carbon dioxide with solid carbon. These tests provide basic information about the ability of the catalysts to catalyse soot combustion with CO2, H2O and O2. Results are summarized in Table 2. Both alkali metal and precious metal (PM) doped supports were used. Two supports were used which can be categorised as an inert and a reducible oxide support. Clearly the presence of the alkali metal has a significant effect on catalysing the soot combustion as anticipated. The effect of the reducible oxide support is not significant. In addition to the experiments summarised in Table 2 two further samples of alkali metal supported on an alumina foam and cordierite wall flow filter were prepared and coated with soot in a similar manner to that described above. Measurement of the soot combustion characteristics of these samples in O2, CO2 and H2O were very similar to the powder samples. 

The ability of sodium-cooled reactors to produce steam at current commercial conditions of superheat and pressure has resulted in considerable design effort to prevent this direct leakage. At present, two methods are available to reduce the probability of direct leakage to a safe level. The first of these employs an intermediate alkali-metal heat-transfer system between the sodium reactor coolant and the steam generating system. This method is generally applicable only when space limitations are not present. 

One of the few catalysts to give reasonably selective metathesis to form ethene and hex-3-ene, reaction (6), is M0O3/AI2O3 doped with 2% alkali metal ions (Bradshaw 1967 Alkema 1968) or thallium ions (Kobylinski 1972). The suppression of the isomerization is more effective the larger the cation Cs+ > T > Rb" > K > Na" " > Li it also correlates with a diminished ability to adsorb ammonia. The polarizability of the cation thus appears to be an important factor in reducing the surface acidity which is the cause of isomerization. 

Another interesting feature in HMPA is its ability to dissolve alkali metals i the blue solutions are strong reducing agents. 

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