Methyl cations ammonia

Draw Lewis structures for methyl anion, ammonia and hydronium cation. How many electrons are left over in each after all bonds have been made Display and compare electron density surfaces for methyl anion, ammonia and hydronium cation. Which is the smallest molecule Which is the largest Rationalize your observation. (Hint Compare the number of electrons in each molecule, and the nuclear charge on the central atom in each molecule.)... 

Electron density surfaces can also be used to uncover trends and build qualitative descriptions. For example, size surfaces for the isoelectronic molecules, methyl anion, ammonia and hydronium cation show a marked decrease in overall size. 

With TAA salts of small alkyl groups (e.g., ethyl, methyl), cation reduction is usually the limiting cathodic reaction. The anodic limiting reaction for ammonium ions is their oxidation to nitrogen and protons. It should be emphasized that atmospheric contaminants are supposed to influence the above cathodic and anodic limits of liquid ammonia, as they do for the other nonaqueous systems discussed in the previous sections. 

The much more highly charged silicon atom can interact far more readily with nucleophiles. Silyl cations may even be complexed simultaneously and symmetrically by two electron pair donors (hypercoordination), in contrast to carbocations. With ammonia, the methyl cation gives the very stable protonated methyl amine, H3C-NH3 a second ammonia molecule is only weakly bound to this complex. If both NH3 groups are forced to be equidistant from carbon, a Sn2 transition state results, 20 kcal mol" higher in energy than the minimum. 

Barley, M.H., M.R. Rhodes, and T.J. Meyer (1987). Electrocatalytic reduction of nitrite to nitrous oxide and ammonia based on the N-methylated, cationic iron por-ph5nin complex [Fe (H20)(TMPyR)] +. Inorg. Chem. 26, 1746-1750. 

Third, there is a possible fundamental difference in stereochemistry between the and S l reactions due to the fact that the reaction involves an intermediate carbanion instead of a carbonium ion. Since the methyl cation (CH3 ) is planar, the same should be true of its derivatives. If, then, an S l reaction takes place at an asymmetric center in RX, the asymmetry will be lost in the intermediate cation R, so the reaction should lead to loss of configuration. Reactions of this type invariably lead to partial or complete racemization. The methyl anion (CH3 ) is, however, pyramidal, being isoelectronic with ammonia, and the same should be true of its derivatives unless these are stabilized by E or -f groups adjacent to the anionic carbon. When such a pyramidal anion R is formed by heterolysis of RX, the stereochemistry at the reaction center is retained. If the subsequent reaction of R with the electrophile X is much faster than inversion of the pyramidal anion, the product will then be formed with complete retention of configuration. 

An interesting variant of cationic-resin capture has recently been reported wherein a strongly acidic cation exchange resin mediated sequential amine deprotection and resin capture (Scheme 6).78 Protected aminoalco-hols were reacted with an excess of isocyanates to form /V-BOC-amine carbamates in solution phase. Methanol was subsequently added to quench excess isocyanates as the neutral methyl carbamate byproducts. Sulfonic acid resin 51 was then used to affect amine-BOC group deprotection and resin capture of the deprotected amines. Washing of the resin bed and release (ammonia/methanol) afforded purified amine carbamate products. 

The reaction mixture is heated at 80°C for 5 min to cease the reaction, followed by centrifugation. The supernatant is run onto a column of 100 parts by volume of cation-exchange resin [Amberlite IRC-50, NH4+-form]. The column is washed with water, and then eluted with 1 N-aqueous ammonia to give fractions which contain kanamycin B-3 -phosphate. The fractions are collected and concentrated under reduced pressure, and then the concentrate is run onto a column of 100 parts by volume of cation-exchange resin [carboxy-methyl Sephadex C-25, NH4+-form]. The column is washed with water, and eluted with 0.2 N-aqueous ammonia to give fractions which contain kanamycin B-3 -phosphate. The fractions are collected, concentrated and lyophilized, whereby 4.5 parts of kanamycin B-3 -phosphate. 

Substitution by Alkyl Groups As an example, consider the relative basicities of ammonia and methylamine. Alkyl groups are electron-donating toward cations, and methylamine has a methyl group to help stabilize the positive charge on nitrogen. 

Substituted 4- or 5-halopyrimidines with sodium amide in liquid ammonia give 4-methyl-1,3,5-triazines (Scheme 317) the reaction is general and yields are good cf., the related conversion of 6-substituted 2-bromopyridines into pyrimidines (Section 4.3.3.3.4). 4-Amino-5-nitrosopyrimidines are converted into 1,3,5-triazines by acetic anhydride or phosphorus oxychloride (Scheme 318). 1,3,5-Triazines can be obtained from 1,3,5-oxadiazinium cations (Section 3.2.1.6.1.3). 

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