15 • Additional Problems
15 • Additional Problems
All-cis cyclodecapentaene is a stable molecule that shows a single absorption in its 1H NMR spectrum at 5.67 δ. Tell whether it is aromatic, and explain its NMR spectrum.
1,6-Methanonaphthalene has an interesting 1H NMR spectrum in which the eight hydrogens around the perimeter absorb at 6.9 to 7.3 δ, while the two CH2 protons absorb at –0.5 δ. Tell whether it is aromatic, and explain its NMR spectrum.
The following molecular model is that of a carbocation. Draw two resonance structures for the carbocation, indicating the positions of the double bonds.
Azulene, an isomer of naphthalene, has a remarkably large dipole moment for a hydrocarbon (μ = 1.0 D). Explain, using resonance structures.
Naming Aromatic Compounds
Structure of Aromatic Compounds
Anthracene has four resonance structures, one of which is shown. Draw the other three.
Phenanthrene has five resonance structures, one of which is shown. Draw the other four.
In 1932, A. A. Levine and A. G. Cole studied the ozonolysis of o-xylene and isolated three products: glyoxal, 2,3-butanedione, and pyruvaldehyde:
In what ratio would you expect the three products to be formed if o-xylene is a resonance hybrid of two structures? The actual ratio found was 3 parts glyoxal, 1 part 2,3-butanedione, and 2 parts pyruvaldehyde. What conclusions can you draw about the structure of o-xylene?
Aromaticity and Hückel’s Rule
3-Chlorocyclopropene, on treatment with AgBF4, gives a precipitate of AgCl and a stable solution of a product that shows a single 1H NMR absorption at 11.04 δ. What is a likely structure for the product, and what is its relation to Hückel’s rule?
Cyclopropanone is highly reactive because of its large amount of angle strain. Methylcyclopropenone, although even more strained than cyclopropanone, is nevertheless quite stable and can even be distilled. Explain, taking the polarity of the carbonyl group into account.
Cycloheptatrienone is stable, but cyclopentadienone is so reactive that it can’t be isolated. Explain, taking the polarity of the carbonyl group into account.
Calicene, like azulene (Problem 17), has an unusually large dipole moment for a hydrocarbon. Explain, using resonance structures.
Pentalene is a most elusive molecule that has been isolated only at liquid-nitrogen temperature. The pentalene dianion, however, is well known and quite stable. Explain.
Ribavirin, an antiviral agent used against hepatitis C and viral pneumonia, contains a 1,2,4-triazole ring. Why is the ring aromatic?
What is the structure of a hydrocarbon that has M+ = 120 in its mass spectrum and has the following 1H NMR spectrum?
7.25 δ (5 H, broad singlet); 2.90 δ (1 H, septet, J = 7 Hz); 1.22 δ (6 H, doublet, J = 7 Hz)
1H NMR: 7.18 δ (4 H, broad singlet); 2.70 δ (4 H, quartet, J = 7 Hz); 1.20 δ (6 H, triplet, J = 7 Hz)
IR absorption at 745 cm–1
1H NMR: 7.0 δ (4 H, broad singlet); 2.85 δ (1 H, septet, J = 8 Hz); 2.28 δ (3 H, singlet); 1.20 δ (6 H, doublet, J = 8 Hz)
IR absorption at 825 cm–1
On reaction with acid, 4-pyrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hückel 4n + 2 rule, explain why the protonated product is so stable.
Bextra, a COX-2 inhibitor once used in the treatment of arthritis, contains an isoxazole ring. Why is the ring aromatic?
N-Phenylsydnone, so-named because it was first studied at the University of Sydney, Australia, behaves like a typical aromatic molecule. Explain, using the Hückel 4n + 2 rule.
Propose a structure for a molecule C14H12 that has the following 1H NMR spectrum and has IR absorptions at 700, 740, and 890 cm–1:
The proton NMR spectrum for a compound with formula C10H12O2 is shown. The infrared spectrum has a strong band at 1711 cm–1. The normal carbon-13 NMR spectral results are tabulated along with the DEPT-135 and DEPT-90 information. Draw the structure of this compound.
|29 ppm||Positive||No peak|
|126||No peak||No peak|
|159||No peak||No peak|
|207||No peak||No peak|
The proton NMR spectrum of a compound with formula C6H5NCl2 is shown. The normal carbon-13 and DEPT experimental results are tabulated. The infrared spectrum shows peaks at 3432 and 3313 cm–1 and a series of medium-sized peaks between 1618 and 1466 cm–1. Draw the structure of this compound.
|119.5||No peak||No peak|
|140.0||No peak||No peak|
After the following reaction, the chemical shift of Ha moves downfield from 6.98 ppm to 7.30 ppm. Explain.
The following compound is the product initially formed in a Claisen rearrangement, which we’ll see in Section 18.4. This product is not isolated, but tautomerizes to its enol form. Give the structure of the enol and provide an explanation as to why the enol tautomer is favored.