Benzene electrostatic potential map for

Display the electrostatic potential map for benzene. Which areas are most electron rich Which are most electron poor Would you expect an electrophile to attack from above and below the plane of the molecule or in the plane of the molecule ... 

The electrostatic potential map for benzene clearly shows that the K face is electron rich (red color) and the periphery is electron poor (blue color), consistent with the observation that electrophilic attack on benzene occurs at the nface. 

Fig. 2 Electrostatic potential maps for benzene, phenol, and indole, models for the side cnains of phenylalanine, tyrosine, and tryptophan. View this art in color at www.dekker.com.)...

The electrostatic potential map of benzene (Figure 11 3c) shows regions of high electron density above and below the plane of the ring which is where we expect the most loosely held electrons (the rr electrons) to be In Chapter 12 we will see how this region of high electron density is responsible for the characteristic chemical reactivity of benzene and its relatives... 

Compare atomic charges and electrostatic potential maps for the three cations. For each, is the charge localized or delocalized Is it associated with an empty a-type or Tt-type orbital Examine the lowest-unoccupied molecular orbital (LUMO) of each cation. Draw all of the resonance contributors needed for a complete description of each cation. Assign the hybridization of the C" atom, and describe how each orbital on this atom is utilized (o bond, n bond, empty). How do you explain the benzene ring effects that you observe ... 

Draw and compare Lewis structures for benzene and pyridine. How many 7C electrons does each molecule have Where are the most accessible electrons in each Display the electrostatic potential map for pyridine and compare it to the corresponding map for benzene. Would you expect electrophilic attack on pyridine to occur analogously to that in benzene If so, should pyridine be more or less susceptible to aromatic substitution than benzene If not, where would you expect electrophilic attack to occur Explain. 

Such a representation is referred to as a local ionization potential map. Local ionization potential maps provide an alternative to electrostatic potential maps for revealing sites which may be particularly susceptible to electrophilic attack. For example, local ionization potential maps show both the positional selectivity in electrophilic aromatic substitution (NH2 directs ortho para, and NO2 directs meta), and the fact that TC-donor groups (NH2) activate benzene while electron-withdrawing groups (NO2) deactivate benzene. 

The electrostatic potential map for the parallel dimer has been constructed by artificially holding the two benzenes together. 

Does formation of bromobenzenium ion lead to disruption of the aromaticity of benzene Is the ion highly delocalized Examine the geometry of bromobenzenium ion, and measure CC bond distances. Are they all the same (as in benzene) or do you see alternation between short and long distances How do they compare to bond distances in benzene, and to typical single and double bond distances (1.54A and 1.32A, respectively). Draw a Lewis structure (or series of Lewis structures) to convey what you observe. Examine atomic charges as well as the electrostatic potential map for bromobenzenium ion. Where is the positive charge Is it localized on a single center or delocalized over several centers ... 

Take a minute to compare the electrostatic potential maps for anisole, benzene, and nitrobenzene. Notice that an electron-donating substituent (OCH3) makes the ring more red (more negative), whereas an electron-withdrawing substituent (NO2) makes the ring less red (less negative). 

Electrostatic potential maps of benzene and fluorobenzene. The high electronegativity of fluorine causes the TT electrons of fluorobenzene to be more strongly held than those of benzene. This difference is reflected in the more pronounced red color associated with the TT electrons of benzene. The color scale is the same for both models. 

Formation of this bond interrupts the cyclic system of it electrons, because in the formation of the arenium ion the carbon that forms a bond to the electrophile becomes sp hybridized and, therefore, no longer has an available p orbital. Now only five carbon atoms of the ring are s hybridized and still have p orbitals. The four it electrons of the arenium ion are delocalized through these fivep orbitals. A calculated electrostatic potential map for the arenium ion formed by electrophilic addition of bromine to benzene indicates that positive charge is distributed in the arenium ion ring (Fig. 15.2), just as was shown in the contributing resonance structures. 

Electrostatic potential map of benzene shows the electron density (red color) above and below the plane of the molecule. For simpUdty, only the framework of the molecule is shown. 

Figure 15.2 A calculated structure for the arenium ion intermediate formed by electrophilic addition of bromine to benzene (Section 15.3). The electrostatic potential map for the principal location of bonding electrons (indicated by the solid surface) shows that positive charge (blue) resides primarily at the ortho and para carbons relative to the carbon where the electrophile has bonded. This distribution of charge is consistent with the resonance model for an arenium ion. (The van der Waals surface is indicated by the wire mesh.)...

Figure 15-2 Orbital picture of the bonding in benzene. (A) The a framework is depicted as straight lines except for the bonding to one carbon, in which the p orbital and the sp hybrids are shown explicitly. (B) The six overlapping p orbitals in benzene form a ir-electron cloud located above and below the molecular plane. (C) The electrostatic potential map of benzene shows the relative electron richness of the ring and the even distribution of electron density over the six carbon atoms.

The connection between a molecule s electron density surface, an electrostatic potential surface, and the molecule s electrostatic potential map can be illustrated for benzene. The electron density surface defines molecular shape and size. It performs the same function as a conventional space-filling model by indicating how close two benzenes can get in a liquid or crystalline state. 

Resonance is an extremely useful concept that we ll return to on numerous occasions throughout the rest of this book. We ll see in Chapter 15, for instance, that the six carbon-carbon bonds in so-called aromatic compounds, such as benzene, are equivalent and that benzene is best represented as a hybrid of two resonance forms. Although an individual resonance form seems to imply that benzene has alternating single and double bonds, neither form is correct by itself. The true benzene structure is a hybrid of the two individual forms, and all six carbon-carbon bonds are equivalent. This symmetrical distribution of electrons around the molecule is evident in an electrostatic potential map. 

Further evidence for the unusual nature of benzene is that all its carbon-carbon bonds have the same length—139 pm—intermediate between typical single (154 pm) and double (134 pm) bonds. In addition, an electrostatic potential map shows that the electron density in all six carbon-carbon bonds is identical. Thus, benzene is a planar molecule with the shape of a regular hexagon. All C-C—C bond angles are 120°, all six carbon atoms are sp2-hybridized. and each carbon has a p orbital perpendicular to the plane of the six-membered ring. 

To see how an electrostatic potential map (and by implication any property map) is constructed, first consider both a size surface and a particular (negative) potential surface for benzene. 

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