Methyl anion, shape

Problem 2.3 Besides free radicals, we shall encounter two other kinds of reactive particles, carbonium ions (positive charge on carbon) and carbanions (negative charge on carbon). Suggest an electronic configuration, and from this predict the shape, of the methyl cation, CHj of the methyl anion, CH3 . 

However, because there is only one electron in that orbital, it does not have the same repulsive effect as a pair of electrons. As a consequence the carbon/hydrogen bonds are not forced together as much as they are in, for example, the methyl anion, CH3, which adopts an almost regular tetrahedron shape. Suggest the direction and degree of polarisation that is present in the methyl radical. 

FIGURE 2.16 The structure of the methyl anion, CH3.The hybridization of the carbon in this carbanion is approximately 1. The molecular shape is pyramidal. 

Methane can be substituted in many ways through replacement of one or more hydrogens with another atom or groups of atoms. In principle, removal of a hydrogen from methane can lead to the methyl anion ( CH3), the methyl radical ( CH3), or the methyl cation ( CH3) depending on the nature of the hydrogen removed ( "H, H or H). In this chapter, we have discussed only the shapes of these intermediates—reactions are coming later. It will be important to remember that carbocations are flat and s/> hybridized and that simple carbanions are pyramidal and approximately s/> hybridized. 

Another way to assess nucleophilic reactivity is to examii the shape of the nucleophile s electron-donor orbital (th is the highest-occupied molecular orbital or HOMC Examine the shape of each anion s HOMO. At which ato would an electrophile, like methyl bromide, find the be orbital overlap (Note This would involve overlap of tl the HOMO of the nucleophile and the lowest-unoccupif molecular orbital or LUMO of CH3Br.) Draw all of tl products that might result from an Sn2 reaction wi CHaBr at these atoms. 

During the last 5 years, there have been several reports of multiblock copolymer brushes by the grafting-from method. The most common substrates are gold and silicon oxide layers but there have been reports of diblock brush formation on clay surfaces [37] and silicon-hydride surfaces [38]. Most of the newer reports have utilized ATRP [34,38-43] but there have been a couple of reports that utilized anionic polymerization [44, 45]. Zhao and co-workers [21,22] have used a combination of ATRP and nitroxide-mediated polymerization to prepare mixed poly(methyl methacrylate) (PMMA)Zpolystyrene (PS) brushes from a difunctional initiator. These Y-shaped brushes could be considered block copolymers that are surface immobilized at the block junction. 

Fig. 8.6. Liquid-liquid equilibria of alcohol-ionic liquid mixtures [105], The left side shows the LLE curves of l-butyl-3-methyl-imidazolium-PF6 mixtures with alcohols (ethanol, blue 1-propanol, red and 1-butanol, green symbols). The experimental curves (solid symbols) show a shape different from the calculated LLE curves, but the upper critical-solution temperatures (UCST) are surprisingly well met. On the right side, the trends of the UCST with a modification of the 1-alkyl-group of the anion (butyl = 4, octyl = 8) is shown. Again, the COSMO-RS predictions (open symbols, same color code as on the left) are in surprisingly good agreement with the experimental data.

If NaCl is replaced by the butyl-3-methyl imidazolium (BMIM) chloride IL, a 30% decrease in retention factor associated with a remarkable peak shape improvement is observed. In this case, the IL cation adsorbs on the C18 stationary phase more than Cl", thereby preventing detrimental attractive silanophilic interaction of the cationic additive. Charge-charge repulsion occurs, the retention factor is lower, and the peak shape is better. The analyte cation is largely retained by hydrophobic fast interactions. When BMIM BF4 IL replaces NaCl, both the cation and anion of the IL adsorb on the C18 surface and all the interactions cited above take place simultaneously and contradict each other. Global retention depends on the extent to which one interaction is stronger than the other [124],... 

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