Methide ion

The alkylate contains a mixture of isoparaffins, ranging from pentanes to decanes and higher, regardless of the olefins used. The dominant paraffin in the product is 2,2,4-trimethylpentane, also called isooctane. The reaction involves methide-ion transfer and carbenium-ion chain reaction, which is cataly2ed by strong acid. 

This reaction shows that the methide ion is a very strong Bmnsted base. The species C22 is the acetylide ion, and the carbides that contain it are called acetylides. The acetylide ion is also a strong Bronsted base, and acctylides react with water to produce ethyne (acetylene) and the corresponding hydroxide. Calcium carbide, CaC2, is the most common saline carbide. 

Thus, the process of hydride ion abstraction from a primary position is approximately thermoneutral, and hence we must conclude that it is an energetically allowed process, although possibly with a relatively small reaction rate. A process competing with primary H abstraction (Reaction 13) is methide ion abstraction (Reaction 11, loss of CH4 from the... 

Values of AGacid for the gas phase protonation55 of substituted methide ions, XCII2, were correlated with the LDRA equation ... 

Since the proton affinity P(Rn=) of the branched olefin Rn= is certainly greater than P(C2H4), the reaction would be endothermic a similar argument applies to the transfer of carbonium ions. This may account for the low yields observed even in the heterogeneous cationic oligomerisation of ethylene. The most likely transfer reactions are hydride ion and methide ion transfers between oligomer ions and molecules (see below), leading to conjunct polymerisation [12]. 

Methide ion is eliminated more easily than the ethide ion because the methide ion has greater stability. This stability results from the electron-donating ability (inductive effect) of the methyl group. This agrees with the... 

Organoboron rings containing three or more boron centers may be viewed as inverse crown ethers. DFT calculations suggest that triboramacrocycle (180), in which the boron centers are coimected via ethylene bridges, effectively coordinates halide, hydride and methide ions (equation Good selectivity for fluoride over other... 

Reactions 148 and 149 are thought to proceed analogously however, the methide ion must find different reaction pathways to escape as methane from the intermediate complex. It does so by proton abstraction from a methyl group (reaction 151 358->359) or by inducing an SN2 displacement reaction on the methoxyl group (358->330)156b 164b. 

In some cases, the propagation reaction is accompanied by intramolecular rearrangements due to hydride ion (H ) or methide ion (CH3 ) shifts. Such polymerizations are referred to as isomerization polymerizations. Consider, for example, the polymerization of 3-methyl-1,2-butene in which the carbocation (XV), formed initially, isomeiizes by a 1,2-hydride shift. The resulting ion (XVI), being a tertiary carbocation, is more stable than (XV) which is a secondary carbocation. 

The commonly accepted mechanism for catalyst activation involves abstraction of a methide ion from a metal methyl complex see eq 10. Marks and co-... 

Iodide ion-initiated decomposition of the mercurials (13) proceeds faster with X = CF3 than with X = F. The fluorobis(trifluoromethyl)methide ion intermediate is less stable than tris(trifluoromethyl)methide (68). The efficiency of dichlorocarbene interception by various anions has been compared (69) and these fall into the anticipated sequence I Br > Cl F, NOf, ClOf. However, F is more reactive than Cl toward difluorocarbene (70). 

Dinitromethide ions add to methyl acrylate in the Michael fashion. When the remaining hydrogen of the methide ions is replaced by fluorine, the Michael addition proceeds with considerable rate enhancement (61). The phenomenon has been attributed to anion destabilization through C-F bond weakening by the more electronegative trigonal carbon atom and the repulsion of the delocalized p electrons by the fluorine lone-pair electrons. The effect can simply be viewed as a consequence of the inharmonious union of the hard fluorine with the soft, delocalized carbanion center. 

The treatment of orthothiocarbonates with -BuLi generates tri(alkylthio)-methide ions (89). 

The product results from transfer of a methyl group and its electron pair (a methide ion, CH ) from the quaternary carbon atom to the primary carbon atom. This methide shift converts the primary car-bocation to a more stable tertiary carbocation. 

CH2F NOJ, and its seemingly high pJlTa value (cn. 9 55 in HjO at 25 "C cf. MeNOs, 10-22), like that of difluoronitromethane (12-4), supports the generalization that ot-fluorine does not stabilize a carbanion to the extent expected from a consideration of inductive withdrawal of electron density from carbon, and in fact can destabilize a conjugated anion. Fluorodinitro-methide ion in 50% dioxan or water adds to methyl acrylate about 2000... 

The methide ion is the strongest base because the single negative charge is concentrated in a much smaller molecular orbital than any of the others. This orbital size effect even overcomes the trend in electronegativities (H, 2.1 C, 2.5 N, 3.0 O, 3.5 and F, 4.0). 

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