Methane losses

The complex TpPtMeH2 was synthesized by reacting TpPtMe(CO) with water (66). While it is stable towards reductive elimination of methane at 55 °C, deuterium incorporation from methanol-c/4 solvent occurs rapidly into the hydride positions and subsequently, more slowly, into the methyl position (Scheme 15). The scrambling into the methyl position has been attributed to reversible formation of a methane complex which does not lose methane under the reaction conditions (75,76). Similar scrambling reactions have been observed for other metal alkyl hydrides at temperatures below those where alkane reductive elimination becomes dominant (77-84). This includes examples of scrambling without methane loss at elevated temperature (78). 

Despite this superficial similarity, however, subtle differences between the behaviour of ionized amines and the analogous ionized alcohols and ethers remain. Thus, metastable ionized 2-butylamine loses 80% ethane in contrast, ionized 2-butanol eliminates both ethane (35%) and methane (40%)85. The latter reaction corresponds to loss of the smaller methyl group and an a-hydrogen atom from the larger ethyl substituent at the branch point. Methane loss does not occur from ionized amines with a methyl substituent on the -carbon, with the solitary exception of ionized isopropylamine which does expel methane (10%). However, ionized 3-hexylamine eliminates both ethane (35%) and propane (20%)85. 

Methane loss was usually small (5-20% ) for reasonable amounts of water formation (AP = 5-20 mm. Hg), so that the initial ratio —d[RH]/ d[H20] could be taken as A[RH]/A[H20] with sufficient accuracy for the analytical treatment discussed below. With computer interpretation, increased accuracy is possible if the initial concentrations of the various reactants are replaced by their mean concentrations during the reaction period. Because it was not considered sufficiently accurate to measure small differences by gas chromatography, the extent of oxidation was determined by oxidizing all the products to carbon dioxide, which was then measured (6). Because of the absence of any significant effect of methane on the rate, concentrations of methane up to 3% were sometimes used instead of the normal 1%. 

Small systems (5-30 MMscfd). Two-stage membrane systems are used to reduce methane loss. In this gas flow range, amine and membrane systems... 

Ion abundances for methane loss from metastable (CH3CD2CHD2)t ions have been found to be in the ratios 87 1 12 (CH4 CH3D CH2D2) after correction for the numbers of equivalent pathways [250]. The reaction is a 1, 2 elimination, so these results indicate both a large primary and a large secondary deuterium isotope effect (see Sect. 7.5.4). On the basis of these intramolecular isotope effects, a mechanism involving a non-classical transition state with a CCH three-centre bond has been proposed for this methane elimination [250]. 

Intermolecular isotope effects observed [534] in metastable ion decompositions of (C3H8)t and (C3D8)t again evidence the fact that intermolecular isotope effects on ion abundances are not a reliable guide to kinetic isotope effects. The isotope effect on methane loss (lcH4/fcD ) is 7 cf. 87 for (ICH4 //ch,d) with CH3CD2CHD2 (above). 

The 16 Da loss corresponds to a methane loss observed, as for the hydrogen loss, when it produces a conjugation or aromaticity gain, especially in cyclic compounds. Thus steroids or bile salts commonly lose methane from an angular methyl group. This 16 Da loss is also observed in N-oxides and sulfoxides, and then results from an oxygen atom loss. 

Oxidation of the surface by methane loss and carbon subduction... 

Methane losses from landfills and enteric fermentation (livestock) are of the same order of magnitude as natural gas venting, but are sources which would be more difficult to capture or control. 

PtMe(H)2(Tp)] is remarkably resistant to methane loss. When it is heated in methanol, methane or hydrogen loss is not observed. 

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