15.12: Additional Problems

15 • Additional Problems 15 • Additional Problems

Visualizing Chemistry

Give IUPAC names for the following substances (red = O, blue = N): (a)

(b)

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

(b)

(c)

(d)

(e)

(f)

Problem 15-19 Draw structures corresponding to the following names: (a)

3-Methyl-1,2-benzenediamine

(b) 1,3,5-Benzenetriol (c) 3-Methyl-2-phenylhexane (d) o-Aminobenzoic acid (e) m-Bromophenol (f) 2,4,6-Trinitrophenol (picric acid)

Dinitrobenzene

(b) Bromodimethylbenzene (c) Trinitrophenol

Draw and name all possible aromatic compounds with the formula C₇H₇Cl.

Draw and name all possible aromatic compounds with the formula C₈H₉Br. (There are 14.)

Structure of Aromatic Compounds

C₉H₁₂; gives only one C₉H₁₁Br product on substitution of a hydrogen on the aromatic ring with bromine

(b) C₁₀H₁₄; gives only one C₁₀H₁₃Cl product on substitution of a hydrogen on the aromatic ring with chlorine (c) C₈H₁₀; gives three C₈H₉Br products on substitution of a hydrogen on the aromatic ring with bromine (d) C₁₀H₁₄; gives two C₁₀H₁₃Cl products on substitution of a hydrogen on the aromatic ring with chlorine

Look at the three resonance structures of naphthalene shown in Section 15.6, and account for the fact that not all carbon–carbon bonds have the same length. The C1–C2 bond is 136 pm long, whereas the C2–C3 bond is 139 pm long.

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.

Look at the five resonance structures for phenanthrene (Problem 26), and predict which of its carbon–carbon bonds is shortest.

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?

Draw an energy diagram for the three molecular orbitals of the cyclopropenyl system (C₃H₃). How are these three molecular orbitals occupied in the cyclopropenyl anion, cation, and radical? Which of the three substances is aromatic according 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.

Which would you expect to be most stable, cyclononatetraenyl radical, cation, or anion? (Cyclononatetraene has a ring of nine carbons and four double bonds.)

How might you convert 1,3,5,7-cyclononatetraene to an aromatic substance?

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.

How many π electrons does indole have?

(b)

What is the electronic relationship of indole to naphthalene?

Ribavirin, an antiviral agent used against hepatitis C and viral pneumonia, contains a 1,2,4-triazole ring. Why is the ring aromatic?

Spectroscopy

Compound A, C₈H₁₀, yields three substitution products, C₈H₉Br, on reaction with Br₂. Propose two possible structures for A. The ¹H NMR spectrum of A shows a complex four-proton multiplet at 7.0 δ and a six-proton singlet at 2.30 δ. What is the structure of A?

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)

C₁₀H₁₄

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

(b)

C₁₀H₁₄

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

General Problems

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.

Show the relative energy levels of the seven π molecular orbitals of the cycloheptatrienyl system. Tell which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation.

1-Phenyl-2-butene has an ultraviolet absorption at λ_max = 208 nm (ε = 8000). On treatment with a small amount of strong acid, isomerization occurs and a new substance with λ_max = 250 nm (ε = 15,800) is formed. Propose a structure for this isomer, and suggest a mechanism for its formation.

1H NMR spectra: (a)

C₈H₉Br

IR absorption at 820 cm^–1

(b)

C₉H₁₂

IR absorption at 750 cm^–1

(c)

C₁₁H₁₆

IR absorption at 820 cm^–1

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.

Normal CarbonDEPT-135DEPT-90  29 ppm Positive No peak  50 Negative No peak  55 Positive No peak 114 Positive Positive 126 No peak No peak 130 Positive Positive 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.

Normal CarbonDEPT-135DEPT-90 118.0 ppm Positive Positive 119.5 No peak No peak 128.0 Positive Positive 140.0 No peak No peak

Aromatic substitution reactions occur by addition of an electrophile such as Br⁺ to an aromatic ring to yield an allylic carbocation intermediate, followed by loss of H⁺. Show the structure of the intermediate formed by reaction of benzene with Br⁺.

The substitution reaction of toluene with Br₂ can, in principle, lead to the formation of three isomeric bromotoluene products. In practice, however, only o- and p-bromotoluene are formed in substantial amounts. The meta isomer is not formed. Draw the structures of the three possible carbocation intermediates (Problem 51), and explain why ortho and para products predominate over meta products.

(b)

Problem 15-54

After the following reaction, the chemical shift of H_a 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.

Problem 15-56 Compounds called azo dyes are the major source of artificial color in textiles and food. Part of the reason for their intense coloring is the conjugation from an electron-donating group through the diazo bridge (−N=N−) to an electron-withdrawing group on the other side. For the following azo dyes, draw a resonance form that shows how the electron-donating group is related to the electron-withdrawing group on the other side of the diazo bridge. Used curved arrows to show how the electrons are reorganized. (a)

(b)