Amines electrostatic potential

You can examine the structure of methyl amine including its electrostatic potential map in more detail on Learning By Modeling... 

An orbital hybridization description of bonding m methylamme is shown m Figure 22 2 Nitrogen and carbon are both sp hybridized and are joined by a ct bond The unshared electron pair on nitrogen occupies an sp hybridized orbital This lone parr IS involved m reactions m which amines act as bases or nucleophiles The graphic that opened this chapter is an electrostatic potential map that clearly shows the concentration of electron density at nitrogen m methylamme... 

Nucleophiles can also act as acids and bases, and this behavior substantially alters their nucleophilicity. At pH 5, trimethylamine exists mainly as its conjugate acid, trimethylammonium cation. First draw a Lewis structure, and then examine the electrostatic potential for trimethylammonium ion. On the basis of the map, which is the better nucleophile, the cation or the corresponding neutral amine At pH 12, phenol exists mainly as its conjugate base, phenoxide anion. First draw a Lewis structure (or series of Lewis structures), and then examine the electrostatic potential map for phenoxide anion. Which is the better nucleophile, phenoxide or phenol ... 

Calculate activation energies for Sn2 reactions of ammonia and trimethylamine with methyl iodide via transition states ammonia+methyl iodide and trimethyl-amine+methyl iodide, respectively. Is attack by ammonia or trimethylamine more facile Rationalize your observation by comparing electrostatic potential maps for the two transition states. Which transition state requires more charge separation Is this also the higher-energy transition state ... 

Electrostatic potential map for amine-amide shows negatively-charged regions (in red) as likely nucleophilic sites. 

The chemistry of amines ts dominated by the lone pair of electrons on nitrogen, which makes amines both basic and nucleophilic. They react with acids to form acid-base salts, and they react with electrophiles in many of the polar reactions seen in past chapters. Note in the following electrostatic potential map of trimethylamine how the negative (red) region corresponds to the lone-pair of electrons on nitrogen. 

In contrast with amines, amides (RCONH ) are nonbasic. Amides don t undergo substantial protonation by aqueous acids, and they are poor nucleophiles. The main reason for this difference in basicity between amines and amides is that an amide is stabilized by delocalization of the nitrogen lone-pair electrons through orbital overlap with the carbonyl group. In resonance terms, amides are more stable and less reactive than amines because they are hybrids of two resonance forms. This amide resonance stabilization is lost when the nitrogen atom is protonated, so protonation is disfavored. Electrostatic potential maps show clearly the decreased electron density on the amide nitrogen. 

Figure 24.3 Arylamines have a larger positive AG for protonation and are therefore less basic than alkylarnines, primarily because of resonance stabilization of the ground state. Electrostatic potential maps show that lone-pair electron density is delocalized in the amine but the charge is localized in the corresponding ammonium ion.

Amine bases in nucleic acids can react with alkylating agents in typical Sjsj2 reactions. Look at the following electrostatic potential maps, and tell which is the better nucleophile, guanine or adenine. The reactive positions in each are indicated. 

Acetic anhydride, electrostatic potential map of, 791 reaction with alcohols, 807 reaction with amines, 807 reaction with monosaccharides, 988... 

Acid chloride, alcohols from, 804 alcoholysis of, 802-803 amides from, 803-804 amines from, 933-935 amjnolysis of, 803-804 carboxylic acids from, 802 electrostatic potential map of, 791... 

Acyl azide, amines from, 935 Acyl carrier protein, function of, 1 140 Acyl cation, electrostatic potential map of, 558... 

Politzer, P., L. Abrahmsen, and P. Sjoberg. 1984. The Effects of Amine and Nitro Substituents upon the Electrostatic Potential of an Aromatic Ring. J. Am. Chem. Soc. 106, 855. 

Fig. 8.1 Calculated electrostatic potential for aniline (1), in kcal -mol-1, in (a) the plane containing the aromatic ring and amine nitrogen, (b) the plane 1.5 A above the aromatic ring (on the side with the nitrogen lone pair), and (c) the plane perpendicular to the aromatic ring and slicing through the C-N bond and the para carbon. The nuclear positions or their pro-...

Program GRID works in very much the same way, but the objective is to obtain chemically specific information about the molecule. An electrostatic potential does not normally allow one to differentiate between favorable binding sites for a primary or a secondary or a tertiary amine cation, or tetramethyl ammonium or... 

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