Electrophile electrostatic potential maps

Figure 5.1 Some nucleophiles and electrophiles. Electrostatic potential maps identify the nucleophilic (red negative) and electrophilic (blue positive) atoms.

The electrophile (E ) m this reaction is mtromum ion (0=N=0) The charge distn bution m mtromum ion is evident both m its Lewis structure and m the electrostatic potential map of Figure 12 2 There we see the complementary relationship between the electron poor region near nitrogen of NO, and the electron rich region associated with the TT electrons of benzene... 

The electrophilic site of an acyl cation is its acyl carbon. An electrostatic potential map of the acyl cation from propanoyl chloride (Figure 12.8) illustrates nicely the concentration of positive charge at the acyl carbon, as shown by the blue color. The mechanism of the reaction between this cation and benzene is analogous to that of other electrophilic reagents (Figure 12.9). 

Electron densities, bond densities, and spin densities, as well as particular molecular orbitals may be displayed as graphical surfaces. In addition, the value of the electrostatic potential or the absolute value of a particular molecular orbital may be mapped onto an electron density surface. These maps provide information about the environment around the accessible surface of a molecule. Electrostatic potential maps show overall charge distribution, while orbital maps reveal likely sites for electrophilic and/or nucleophilic attack. Surface displays may be combined with any type of model display. 

Both cis and tran -cyclohexene have been synthesized, but only one of them can be isolated. Electrophilic addition of ROH to one isomer occurs spontaneously, while addition to the other isomer occurs only in the presence of a strong acid, such as sulfuric acid. Calculate the energy of protonation for each isomer cyclohexene protonated cyclohexene, trans-cyclohexene protonated trans-cyclohexene), and identify the more reactive isomer. Also examine electrostatic potential maps. Suggest an explanation to account for both the reactivity difference and the structural changes. (See also Chapter 7, Problem 5.)... 

The first step in the addition of an electrophile such as HBr to an alkyne involves protonation and subsequent formation of an intermediate vinyl cation. Where does propyne protonate Compare energies of 1-methylvinyl and 2-methylvinyl cations. Which is more stable Why Measure CC bond distance in the more stable cation. Does the cation incorporate a full triple bond (as in propyne) or a double bond (as in propene). Examine atomic charges and electrostatic potential maps to locate the positive charge in the two cations. Is the more stable ion the one in which the charge is better delocalized Use the charges together with information about the ions geometry to draw Lewis structures (or a series of Lewis structures) for 1-methylvinyl and 2-methylvinyl cations. 

The molecule below has four stereoisomeric forms exoO exoCH2Br, exoO endoCH2Br, and so on. Examine electrostatic potential maps of the four ions and identify the most nucleophilic (electron-rich) atom in each. Examine the electron-acceptor orbital (the lowest-unoccuped molecular orbital or LUMO) in each and identify electrophilic sites that are in close proximity to the nucleophilic. Which isomers can undergo an intramolecular E2 reaction Draw the expected 8 2 and E2 products. Which isomers should not readily undergo intramolecular reactions Why are these inert ... 

Alcohol attack generates an unstable intermediate that undergoes nucleophilic attack by CL at carbon. Compare electrostatic potential maps of methanol, thionyl chloride intermediate, and phosphorus trichloride intermediate. What features of these maps are consistent with an electrophilic reactive intermediate ... 

Electrostatic interactions can guide alkylation under certain conditions. Examine the electrostatic potential map of the potassium enolate of ethyl acetoacetate. Is carbon or oxygen more electron rich Are electrostatic interactions likely to favor addition of oxygen or carbon Examine atomic charges and electrostatic potential maps for diethylsulfate, ethyl chloride, ethyl bromide and ethyl iodide, pay attention to the backside of the electrophilic carbon. Order the systems from most to least electron poor. Which reaction is most likely to be guided by electrostatics Least likely Can the experimental results be fully explained on this basis ... 

Electrostatic potential map for nitronium cation shows most positively-charged regions (in blue) as likely electrophilic sites. 

Display the electrostatic potential map for 2-methyl anisole. For which ring site, para to methyl or para to methoxy, is the electrostatic potential more negative Where do you expect electrophilic attack to occur Is your result consistent with the relative stabihties of intermediate benzenium ions formed upon addition of the electrophile Compare energies for 3-methyl-4-methoxybenzenium ion and 4-methyl-3-methoxybenzenium ion. 

Electrostatic potential map for 2-methylanisole shows negatively-charged regions (in red) as the likely sites of electrophilic attack. 

Friedel-Crafts acylation involves electrophilic attack by acyl cation (CHsCO ) on the ring, and the ring s electronic character should indicate its susceptibility to attack. Compare electrostatic potential maps of ferrocene and acetylferrocene. Which molecule contains the most electron-rich ring Which acylation reaction should be faster Does an acetyl substituent enhance or diminish ring reactivity What should be the major product when ferrocene is combined with one equivalent of acetic anhydride ... 

Examine atomic charges and electrostatic potential maps of these ions. Which ion has the most electron-poor electrophilic carbon Which has the least electrophilic carbon Is the variation in charge consistent with the observed reactivity patterns Explain. 

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 ... 

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. 

Examine the highest-occupied molecular orbital (HOMO) of singlet methylene. Where is the pair of electrons, inplane or perpendicular to the plane Next, examine the electrostatic potential map. Where is the molecule most electron rich, in the o or the 7t system Where is the most electron poor Next, display the corresponding map for triplet methylene. Which molecule would you expect to be the better nucleophile The better electrophile Explain. Experimentally, one state of methylene shows both electrophilic and nucleophilic chemistry, while the other state exhibits chemistry typical of radicals. Which state does which Elaborate. 

An electrostatic potential map of boron trifluoride is shown. Is BF3 likely to be a nucleophile or an electrophile Draw a Lewis structure for BF3, and explain your answer. 

Electrostatic potential maps of (a) formaldehyde (CH20) and (b) methanethiol (CH3SH) are shown. Is the formaldehyde carbon atom likely to be electrophilic or nucleophilic What about the methanethiol sulfur atom Explain. 

Figure 16.4 The mechanism of electrophilic nitration of an aromatic ring. An electrostatic potential map of the reactive electrophile N02+ shows that the nitrogen atom is most positive (blue).

Figure 16.9 Mechanism of the Friedel-Crafts acylation reaction. The electrophile is a resonance-stabilized acyl cation, whose electrostatic potential map indicates that carbon is the most positive atom (blue).

Judging from the following electrostatic potential maps, which kind of carbonyl compound has the more electrophilic carbonyl carbon atom, a ketone or an acid chloride Which has the more nucleophilic carbonyl oxygen atom Explain. 

One further comparison aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes because the electron-donating resonance effect of the aromatic ring makes the carbonyl group less electrophilic. Comparing electrostatic potential maps of formaldehyde and benzaldehyde, for example, shows that the carbonyl carbon atom is less positive (less blue) in the aromatic aldehyde. 

What kind of chemistry do enols have Because their double bonds are electron-rich, enols behave as nucleophiles and react with electrophiles in much the same way that aikenes do. But because of resonance electron dona lion of a lone-pair of electrons on the neighboring oxygen, enols are more electron-rich and correspondingly more reactive than aikenes. Notice in the following electrostatic potential map of ethenol (BbC CHOH) how there is a substantial amount of electron density (yellow-red) on the a carbon. 

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. 

Acetanilide, electrophilic aromatic substitution of, 939-940 Acetate ion, bond lengths in, 43 electrostatic potential map of, 43, 53, 56, 757 resonance in, 43 Acetk acid, bond angles in, 755 bond lengths in, 755 dimer of, 755 dipole moment of, 39 electrostatic potential map of, 53, 55... 

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