Acetonitrile, electrostatic potential

Examine electrostatic potential maps for methyl acetate (X-OMc), dimethylacetamide (X=NMe2), and acetonitrile. What is the most electron-rich site in each molecule Does this site correspond to a bonding pair of electrons or a nonbonding pair of electrons Assuming that protonation occurs onto the most electron-rich site, where would you expect each molecule to protonate ... 

The multilayered character of acetonitrile adsorption creates a pseudo-stationary phase of significant volume on the surface, which acts as a suitable phase for the ion accumulation. In the low organic concentration region (from 0 to 20 v/v% of acetonitrile), studied ions show significant deviation from the ideal retention behavior (decrease in ion retention with increase in acetonitrile composition) due to the formation of the acetonitrile layer, and significant adsorption of the chaotropic anions was observed. This creates an electrostatic potential on the surface in which there is an adsorbed acetonitrile layer, which provides an additional retentive force for the enhancement of the retention of protonated basic analytes. When the dielectric constant is lower than 42 [167], this favors the probability of ion pair formation in this organic enriched layer on top of the bonded phase. 

Use SpartanView to examine electrostatic potential maps of deprotonated acetonitrile, IV-methylimidazole, and 1-methylcyclohexcne. Assuming that the most negative atom is also the most nucleophilic, identily the two most nucleophilic atoms in each molecule. 

Suppose a chemist wants to use KF as a nucleophilic reagent in an 8 2 reaction. Use Spartan View to compare electrostatic potential maps of water, acetonitrile, and DMSO, and tell which solvent(s) you expect to bind strongly to F" and to K. Which solvent(s) should promote a rapid Sn2 reaction ... 

Figure 20.1 shows the structures and electrostatic potentials of the various derivatives of acetic acid-acetyl chloride, acetic anhydride, ethyl acetate, acetamide, and acetonitrile. Like the other carbonyl-containing compounds that we ve studied, acyl chlorides, anhydrides, esters, and amides all have a planar arrangement of bonds to the carbonyl group. 

Electrostatic potential map acetamide. 111 acetate ion, 741, 742 acetic acid, 739, 742 acetic anhydride. 111 acetone enol, 701 acetonitrile. 111 acetyl chloride, 114, 111 acetylene, 339, 342 amino acids, 1053 aniline, 862 benzene, 398 benzyne, 930... 

An electrostatic potential map for acetonitrile (CH3CN), which is polar, is shown at left. From this map, determine the geometry for how two acetonitrile molecules would interact with each other. Draw structural formulas, using the three-dimensional bond notation introduced in Section 10.4, to illustrate the geometry of the interaction. 

Figure 20-3 (A) Orbital picture of the nitrile group, showing the sp hybridization of both atoms in the C=N function. (B) Molecular structure of ethanenitrile (acetonitrile), which is similar to that of the corresponding alkyne. (C) Electrostatic potential map of ethanenitrile, depicting the positively polarized cyano carbon (blue) and the relatively negatively polarized nitrogen (green) with its lone electron pair (red).

The electrostatic terms can be reasonably well handled in solvents of high dielectric constant, but problems are raised by some solvents of widespread use in spin trapping, for example dichloromethane ( ) = 8.9), chloroform (D = 4.8) and benzene (D = 2.3), in which the electrostatic terms calculated as above for acetonitrile become -24.8, -46 and —96 kcal mol-1, respectively. Already in dichloromethane the effective standard potential of Fe(CN)6 /Fe(CN)6- is increased by 1.08 V and in benzene by an absurdly high 4.2 V ... 

Anion receptors incorporating cobaltocenium have been studied extensively due to the combination of an accessible redox couple and potential favourable electrostatic interactions of the cationic organometallic metallocene complex with anions. The first anion receptor based on this species was reported by Beer and co-workers in 1989 [6]. The macrocyclic bis-cobaltocenium receptor 1 was shown to bind (via electrostatic interaction) and to electro chemically sense bromide in acetonitrile solvent media. 

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