Metal acids weaker than alcohols

The occurrence of intermediates in the reaction of metal ions with porphyrins has been noted on several occasions. For example, Fleischer and Wang [39] have reported the formation of an intermediate in the reaction of protoporphyrin dimethyl ester with Fe(II), Fe(III), Cr(III) and other metal ions in acetone and chloroform solution. Russian workers [40] have also detected intermediates in the reaction betwen chlorophyllic acid and the salts ZnClj, CoClj, FeClj and Co(OAc)2 in glacial acetic acid. The intermediates appear to be unstable in media containing even small amounts of polar strongly coordinating solvents such as water, alcohols and pyridine, suggesting that the thermodynamic stability of the SAT complex is much weaker than that of the solvent complex MSg. Polar solvent molecules immediately displace the porphyrin from the intermediate. 

Pyrrole is soluble in alcohol, benzene, and diethyl ether, but is only sparingly soluble in water and in aqueous alkaUes. It dissolves with decomposition in dilute acids. Pyrroles with substituents in the -position are usually less soluble in polar solvents than the corresponding a-substituted pyrroles. Pyrroles that have no substituent on nitrogen readily lose a proton to form the resonance-stabilized pyrrolyl anion, and alkaU metals react with it in hquid ammonia to form salts. However, pyrrole pK = ca 17.5) is a weaker acid than methanol (11). The acidity of the pyrrole hydrogen is gready increased by electron-withdrawing groups, eg, the pK of 2,5-dinitropyrrole [32602-96-3] is 3.6 (12,13). 

Alcohols are only slightly weaker acids than water, with a K. value of approximately 1 x 10 . The reaction of ethanol with sodium metal (a base) produces sodium ethoxide and hydrogen gas. 

Since an alcohol is a weaker acid than water, an alkoxide is not prepared from the reaction of the alcohol with sodium hydroxide, but is prepared instead by reaction of the alcohol with the active metal itself. 

The pfCa of acetic acid is 4.7, while the pfCa of ethanol is approximately 16. This means that acetic acid is almost a hundred thousand million (or 1011) times more acidic than ethanol. Alcohols are much weaker acids than water and in biological systems are considered to be neutral. To bring about the ionisation of an alcohol requires the use of a very strong base such as metallic sodium. 

Table 1.1 illustrates the relatively high acidity of compounds in which a C—H bond is activated by two or more carbonyl (or cyano) groups. It is therefore possible to use a comparatively weak base, such as a solution of sodium ethoxide in ethanol, in order to form the required enolate anion. An equilibrium is set up, as illustrated in Scheme 1.4, in which the conjugate acid of the base (BH) must be a weaker acid than the active methylene compound. Another procedure for preparing the enolate of an active methylene compound is to use sodium hydride (or finely divided sodium or potassium metal) in tetrahydrofuran (THF), diethyl ether (Et20) or benzene. The metal salt of the enolate is formed irreversibly with evolution of hydrogen gas. p-Diketones can often be converted into their enolates with alkali-metal hydroxides or carbonates in aqueous alcohol or acetone. 

Sodium and potassium alkoxides can be prepared by treating alcohols with sodium or potassium metal or with the metal hydride (Section 6.15B). Because most alcohols are weaker acids than water, most alkoxide ions are stronger bases than the hydroxide ion. 

Oxidative addition of the P-H and S-H bonds in phosphines and thiols are less well studied but are more favorable than oxidative aditions of N-H and 0-H bonds for thermodynamic and kinetic reasons. S-H and P-H bonds in thiols and phosphines are weaker and more acidic " than the 0-H and N-H bonds in alcohols and amines. Moreover, the resulting products from additions to late transition metal complexes possess proportionately stronger metal-sulfur and metal-phosphorus bonds. " ... 

One of synthetic approaches for the iron nanoparticles is based on the widely used decomposition of iron pentacarbonyl [19, 361, 362], The novelty of the approach is the surfactant system used. Studies with a number of strongly bound surfactants have resulted in decreased magnetic response, due to surface oxidation, disturbing the electronic structure of the surface atoms, or some other mechanism. With this in mind, ones chose to work with a weak surfactant, a p-diketone. P-diketones do have a history as adhesion promoters in bonds between metals and polymers [363], The limited reactivity of p-diketones is as an advantage the P-diketone is much weaker oxidizer than carboxylic acids or alcohols and will not oxidize iron, it is not as nucleophilic as phosphines, yet it is known to be capable of chelating iron. 

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