Acidity of methane

Another class of acids of interest in organic chemistry is the group of carbon acids. Here we may discern three kinds of effects on acidity. The first of these is illustrated by the acidity of methane (pKa a 48) compared with that of cyclohexane (pKa a 52) (Table 3.1). It would appear that the trend is in the direction of decreasing acid strength with substitution of hydrogen by alkyl. Note that the tendency here is in the direction opposite to the effect in alcohols if we take Brauman s gas-phase results to be the more accurate indication of intrinsic acid strength. The hydrocarbon data are from solution measurements subject to considerable uncertainty, and the differences are small. It seems risky to interpret the results in terms of intrinsic molecular properties. 

Ingold, C. K. Powell, W. J. Experiments on the synthesis of the polyacetic acids of methane. Part II. Some abnormal condensations of malonic and cyanoacetic esters with haloge-nated methanes./. Chem. Soc. 1921, 119, 1222-1229. 

Revealed for example, the increasing acidity of methane, ethene and ethyne, see e.g. Smith MB, March J (2001) March s advanced organic chemistry. Wiley, New York, pp 331, 348... 

The overall order of alkane acidities deduced from the decomposition of silicon anions is the same as that obtained from the decomposition of alkoxides " " . The ethyl anion has the highest proton affinity, i.e., ethane is the weakest acid. The acidities of propane (secondary hydrogen) and of cyclobutane are both lower than that of methane, but all other alkanes are more acidic. The acidity of methane is particularly enhanced by a cyclopropyl substituent and by a t-butyl group. 

It is interesting to examine the effect of a- and j5-methyl substitution on the acidity of methane. One a-methyl group decreases the acidity of methane, two a-methyl groups also decrease the acidity but by a smaller amount and the three a-methyl groups of the isobutane actually increase the acidity. (This last result is at odds with the theoretical results.)... 

Rodriquez, C. R, Sirois, S., and Hopkinson, A. C. "Effect of Multiple Halide Substituents on the Acidity of Methanes and Methyl Radicals. Electron Affinities of Chloro and Fluoromethyl Radicals."... 

CH2CI2. A colourless liquid with a chloroform-like odour b.p. 4I°C. Prepared by heating chloroform with zinc, alcohol and hydrochloric acid manufactured by the direct chlorination of methane. Decomposed by water at 200°C to give methanoic and hydrochloric acids. Largely used as a solvent for polar and non-polar substances, particularly for paint removal (30%), dissolving cellulose acetate and degreasing (10%). It is more stable than carbon tetrachloride or chloroform especially towards moisture or alkali. It is somewhat toxic. U.S. production 1981 280000 tonnes. 

The high acidity of superacids makes them extremely effective pro-tonating agents and catalysts. They also can activate a wide variety of extremely weakly basic compounds (nucleophiles) that previously could not be considered reactive in any practical way. Superacids such as fluoroantimonic or magic acid are capable of protonating not only TT-donor systems (aromatics, olefins, and acetylenes) but also what are called (T-donors, such as saturated hydrocarbons, including methane (CH4), the simplest parent saturated hydrocarbon. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Table 1 7 shows that hydrocarbons are extremely weak acids Among the classes of hydrocarbons acetylene is a stronger acid than methane ethane ethylene or benzene but even its K is 10 ° smaller than that of water... 

A representative technical grade of methyl chloride contains not more than the following indicated quantities in ppm of impurities water, 100 acid, such as HCl, 10 methyl ether, 20 methanol, 50 acetone, 50 residue, 100. No free chlorine should be detectable. Traces of higher chlorides are generally present in methyl chloride produced by chlorination of methane. The boiling point should be between —24 and —23° C, and 5—95% should distill within a range of about 0.2°C. It should be clear, colorless, and free from visible impurities. 

Table 7.5. EquUibriimi Acidities of Substituted Methanes in Dimethyl Sulfoxide ...

NMe is now commercially available and is prepd by the vapor phase nitration of methane at a ratio of 9 moles of methane to I mole of nitric acid at 475° and a residence time of 0.18sec (Ref 12) or by the similar nitration of aliphatic hydrocarbons (Ref 8). Other prepns are from Me sulfate and Na nitrite (Ref 26) by the oxidn of Me amine with dinitrogen trioxide in the gas phase or in methylene chloride, yield 27%... 

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