Base pair electrostatic potential maps

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... 

Active Figure 2.5 The reaction of boron trifluoride, a Lewis acid, with dimethyl ether, a Lewis base. The Lewis acid accepts a pair of electrons, and the Lewis base donates a pair of nonbonding electrons. Note how the movement of electrons from the Lewis base to the Lewis acid is indicated by a curved arrow. Note also how, in electrostatic potential maps, the boron becomes more negative (red) after reaction because it has gained electrons and the oxygen atom becomes more positive (blue) because it has donated electrons. Sign in atwww. thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

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. 

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An alternative view is provided by a pair of electrostatic potential maps. Electron-rich heteroatoms line up with the electron-poor (acidic) hydrogens. Attraction between the two bases may be thought of as due to favorable Coulombic interactions. 

Examples of neutral Lewis acids are halides of group 3A elements, such as BF3. Boron trifluoride, a colorless gas, is an excellent Lewis acid because the boron atom in the trigonal planar BF3 molecule is surrounded by only six valence electrons (Figure 15.12). The boron atom uses three sp2 hybrid orbitals to bond to the three F atoms and has a vacant 2p valence orbital that can accept a share in a pair of electrons from a Lewis base, such as NH3. The Lewis acid and base sites are evident in electrostatic potential maps, which show the electron poor B atom (blue) and the electron rich N atom (red). In the product, called an acid-base adduct, the boron atom has acquired a stable octet of electrons. 

The method for calculating the electrostatic potential map is outlined in Reference 54, and an example of the electrostatic potential around a nucleic-acid base pair is shown in Figure 17.14. The electron density at the distance r from an atomic nucleus may be represented, as in Chapter 9, Equation 9.4, by... 

The nitrogen atom of an amine possesses a lone pair that represents a region of high electron density. This can be seen in an electrostatic potential map of trimethylamine (Figure 23.1). The presence of this lone pair is responsible for most of the reactions exhibited by amines. Specifically, the lone pair can function as a base or as a nucleophile ... 

Amides also represent an extreme case of lone-pair delocalization. The lone pair on the nitrogen atom of an amide is highly delocalized by resonance. In fact, the nitrogen atom exhibits very little electron density, as can be seen in an electrostatic potential map (Figure 23.4). For this reason, amides do not function as bases and are very poor nucleophiles. 

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