Resonance contributors

This free radical (1) exists as three resonance contributors. It reacts with oxygen to yield a conjugated peroxy free radical such as (2). In a molecule, like trilinolein with its multiple functionaHty, there are many possible hydroperoxy products that are formed (21) radical (2) iUustrates the principal type of stmcture formed. 

Measure the carbon-carbon bond length in benzene. Would you describe it as a single bond, a double bond, or somewhere in between Draw whatever resonance contributors are needed to justify your conclusion. 

The situation is more eomplieated when the set of reasonable eontributing struetures are not all equivalent. Examine the geometry and atomie eharges forphenoxide anion. Do these data fit any one of the possible resonanee struetures (draw all reasonable possibilities), or is a eombination of two or more resonance contributors necessary ... 

Draw all reasonable resonance contributors for both planar and perpendicular conformers of benzyl cation. Identify the site(s) of the positive charge in each. Which cation would you expect to be more stable Which is the more stable Compare energies of planar and perpendicular conformers of benzyl cation. 

Draw the two resonance contributors that are needed to describe each transition state. Identify all partial carbon-carbon double bonds () and measure their distances. Are these values like that found in benzene, or do transition states have their own characteristic partial double bond... 

Does your transition state drawing look more like a sing Lewis structure or a resonance hybrid If the latter, whi resonance contributors must you combine to generate a of the features of this hybrid ... 

Display and examine electrostatic potential maps for ethyl cation, 2-propyl cation and 2-methyl-2-propyl cation. Which cation shows the greatest localization of positive charge If you find that the methyl groups delocalize the positive charge, where does the charge go Write resonance contributors for the three cations to rationalize your conclusion. (Note You may need to draw resonance contributors that contain a CC double bond and are missing a CH bond see also Chapter 7, Problem 8.)... 

Compare atomic charges and electrostatic potential maps of the two carboeations. In which is the positive charge more delocalized For each carbocation, draw whatever resonance contributors are needed to account for all of your observations. Which carbocation is better stabilized by resonance ... 

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 ... 

Compare energies for the two alternative conjugate acids of methyl acetate (protonated methyl acetate and methoxy protonated methyl acetate) and dimethylacetamide (N-protonated dimethylacetamide and 0-protonated dimethylacetamide). Which acid in each pair is more stable Draw resonance contributors for the more stable conjugate acid for each system. 

Examine the structures and charge distributions of the lowest-energy anions, and draw all of the resonance contributors necessary to describe these ions. What features account for the fact that these anions are more stable than their alternatives ... 

Examine the geometry and electrostatic potential map for acetone enolate. Are the CC and CO bonds in the enolate more similar to those in acetone or propen-2-ol precursors Is the negative charge primarily located on oxygen or on carbon Assuming this enolate is a hybrid of the two resonance contributors as shown above, which, if either, appears to be the major contributor ... 

Compare electrostatic potential maps of enolates derived from 2-butanone, 4,4-dimethyl-2-pentanone, 4,4,4-trifluoro-2-butanone and l-phenyl-2-propanone with those of acetone. Which substituents cause significant changes in the electronic structure of these enolates and what are the nature of these changes Justify your answers by making drawings of any important resonance contributors. 

Which of the two enolates enolate A or enolate B) is lower in energy Rationalize your observation by comparing their structures, charge distributions and electrostatic potential maps. Draw all of the resonance contributors needed to describe each enolate. Which enolate is generated by reaction with NaH ... 

Next, examine the dianion resulting from double deprotonation of ethyl acetoacetate. Draw all the resonance contributors needed to describe this ion. Next, examine the geometry, atomic charges and electrostatic potential map for dianion. Are you able to decide which resonance contributors are important and which are not ... 

Since the final proton transfer is essential for a successful condensation, it is important to understand what factors drive the proton transfer. Examine the electrostatic potential map of the carbanion, and draw all of the resonance contributors that are needed to describe this ion. How does this ion differ from the others Which product, if either, would be expected from the following condensation Explain. 

Consider 1,3-pentadiene and 1,4-pentadiene. Which, if either, would benefit from the type of resonance described above Draw appropriate resonance contributors for this isomer. Indicate the likely importance of different zwitterionic structures which you might draw. Compare the energies of 1,3-pentadiene and 1,4-pentadiene. Which one is more stable ... 

Buckminsterfullerene (Cm or Buckyball ) is structurally related to corannulene. In which molecule would you expect 7U-orbital overlap be more effective Explain. How many chemically unique carbons are there in C6o Measure CC bond distances. How many unique distances are there Is each benzene fully delocalized or is one resonance contributor more important than the other ... 

Delocalized cations, represented by two or more resonance contributors, are usually more stable than localized cations. However, the fact that several resonance contributors can be drawn for a molecule does not guarantee that the molecule will actually be resonance stabilized (see also Chapter 12, Problem 10). 

The pKa of 1,3-cyclopentadiene is 15, making it more acidic than water, as well as more acidic than almost any other hydrocarbon. This unusual acidity is presumably due to resonance stabilization of the conjugate base, which can be drawn as a hybrid of five resonance contributors. 

Obtain the energy of each cation that might be generated by electrophilic addition of Br to biphenyl (biphenyl+Br+). Which one is most stable Are there others of comparable stability Examine the structure of the most stable cation(s), and draw all of the resonance contributors needed to describe this ion(s). Predict the product(s) of biphenyl bromination. Will the reaction be highly selective, moderately selective or unselective ... 

Compare the geometry of para-nitroaniline to those of both aniline and nitrobenzene. Is there any evidence for push-pull resonance contributors Is there shortening of bonds to the amino and nitro groups Are the bonds in the ring localized Is the dipole moment for para-nitroaniline smaller, larger or about the same as the sum of the dipole moments for aniline and nitrobenzene What does your result say about the importance of push-pull resonance contributors ... 

Examine the structure, atomic charges and electrostatic potential map of phenyl diazonium ion. Which atom(s) appears to carry most of the positive charge Is the electron distribution around this atom(s) uniform, or are some regions more electron rich and others more electron poor Draw appropriate resonance contributors. 

Examine the structures and atomic charges for the various conjugate bases. How do they differ What distinctive features, if any, characterize the most stable conjugate base Draw all of the resonance contributors needed to account for the electron distribution and geometry of the most stable conjugate base. 

Examine the geometry of the most stable radical. Is the bonding in the aromatic ring fuUy delocalized (compare to model alpha-tocopherol), or is it localized Also, examine the spin density surface of the most stable radical. Is the unpaired electron localized on the carbon (oxygen) where bond cleavage occurred, or is it delocalized Draw all of the resonance contributors necessary for a full description of the radical s geometry and electronic structure. 

Examine the electrostatic potential map and spin density surface of Q radical anion (Q ). Draw all of the resonance contributors needed to account for these data. Examine the CO bond distances and spin density surface of QH radical (QH ). Draw all of the resonance contributors needed to account for these data. 

Onto which atoms (carbon, nitrogen or both) is the unpaired electron in tricyanomethyl radical delocalized Rationalize your result by drawing resonance contributors. 

First, try to draw resonance contributors for both ground state and triplet anthrone. Then display a spin density surface for the triplet state of anthrone. (Note that the spin density surface shows the location of both unpaired electrons, one of which may be in a 7t orbital and one of which may be in a o orbital.) Where are the two unpaired electrons Are they localized or delocalized Given that spin delocalization generally leads to stabilization, would you expect the triplet state of anthrone to be stable ... 

Draw the most important resonance contributors for nitrobenzene (include all of the contributors needed to explain for the variation in electrostatic potential, charge and chemical shift relative to benzene). Do these resonance contributors account for the different behavior of 6meta and 6paia Explain. 

Resonance structures or resonance contributors are connected by double-headed arrows (<- ) => the real molecule, radical, or ion is a hybrid of all of them. 

At this stage, the valence shells for the two oxygen atoms are closed, but the sulfur atom is two electrons short of a complete octet. If we complete the octet for sulfur by converting a lone pair of electrons on the right hand side oxygen atom into a sulfur-to-oxygen n-bond, we end up generating the resonance contributor (A) shown below ... 

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