Benzene and Aromaticity

Give IUPAC names for the following compounds

e) 

Tell whether the following compounds are ortho-, meta-, or para-disubstituted:

Give IUPAC names for the following compounds:

Draw structures corresponding to the following IUPAC names: 

(a) p-Bromochlorobenzene 

(b) p-Bromotoluene 

(c) m-Chloroaniline 

(d) 1-Chloro-3,5-dimethylbenzene

Pyridine is a flat, hexagonal molecule with bond angles of 120°. It undergoes substitution rather than addition and generally behaves like benzene. Draw a picture of the orbitals of pyridine to explain its properties. Check your answer by looking ahead to Sections 15-5.

To be aromatic, a molecule must have 4n+2 p electrons and must have a planar, monocyclic system of conjugation. Cyclodecapentaene fulfills one of these criteria but not the other and has resisted all attempts at synthesis. Explain.

Draw the five resonance structures of the cyclopentadienyl anion. are all carbon-carbon bonds equivalent? How many absorption lines would you expect to see in the ${}^{\mathbf{1}}\mathbf{H}$NMR and ${}^{\mathbf{13}}\mathbf{C}$NMR spectra of the anion?

 Cyclooctatetraene readily reacts with potassium metal to form the stable cyclooctatetraene dianion, ${\mathbf{\text{C}}}_{\mathbf{8}}{{\mathbf{\text{H}}}_{\mathbf{8}}}^{\mathbf{2}\mathbf{-}}$. Why do you suppose this reaction occurs so easily? What geometry do you expect for the cyclooctatetraene dianion?

The relative energy levels of the five molecular orbitals of the cyclopentadienyl system are similar to those in benzene. That is, there is a single lowest energy MO, above which the orbitals come in degenerate pairs. Draw a diagram like that in Figure 15-5, and tell which of the five orbitals are occupied in the cation, radical, and anion.

Draw an orbital picture of furan to show how the molecule is aromatic.

Thiamin, or vitamin B1, contains a positively charged five-membered nitrogen–sulfur heterocycle called a thiazolium ring. Explain why the thiazolium ring is aromatic.

Azulene, a beautiful blue hydrocarbon, is an isomer of naphthalene. Is azulene aromatic? Draw a second resonance form of azulene in addition to that shown.

How many electrons does each of the four nitrogen atoms in purine contribute to the aromatic system?

Which would you expect to be most stable, Cyclononatetraenyl radical, cation, or anion?

Cycloheptatrienone is stable, but cyclopentadienone is so reactive that it can’t be isolated. Explain, taking the polarity of the carbonyl group into account.

Cycloheptatrienone                                                Cyclopentadienone

Cyclopropane 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.

Cyclopropanone                                           Methylcyclopropenone

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?

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)

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)

3-Chlorocyclopropene, on treatment with AgBF₄, gives a precipitate of AgCl and a stable solution of a product that shows a single  ¹HNMR absorption at 11.04 d. What is a likely structure for the product, and what is its relation to Hückel’s rule?

       3-Chlorocyclopropene

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?

Look at the five resonance structures for phenanthrene (Problem 15-26), and predict which of its carbon-carbon bonds is the shortest.

Phenanthrene has five resonance structures, one of which is shown. Draw the other four.

Anthracene has four resonance structures, one of which is shown. Draw the other three

                                               Anthracene

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

Propose structures for aromatic hydrocarbons that meet the following descriptions:

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

(b)C₁₀ H₁₄ ; gives only one C₁₀H₁₃Cl product on substitution of hydrogen on the aromatic ring with chlorine 

(c) C₈H₁₀; gives three C₈H₉Br  products on substitution of hydrogen on the aromatic ring with bromine 

(d)C₁₀ H₁₄  ; gives two C₁₀H₁₃Cl products on substitution of hydrogen on the aromatic ring with chlorine

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

Calicene, like azulene (Problem 15-17), has an unusually large dipole moment for a hydrocarbon. Explain, using resonance structures.

Calicene

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.

Pentalene

Pentalene dianion

Indole is an aromatic heterocycle that has a benzene ring fused to a pyrrole ring. Draw an orbital picture of indole.

 (a) How many p electrons does indole have?

 (b) What is the electronic relationship of indole to naphthalene?

Indole

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

Ribavirin

Compound A, ${\mathbf{C}}_8{\mathbf{H}}_{10}$ , yields three substitution products, ${\mathbf{C}}_8{\mathbf{H}}_9\mathbf{Br}$, on reaction with ${\mathbf{Br}}_2$. Propose two possible structures for A. The ${}^1\mathbf{H}$NMR spectrum of A shows a complex four-proton multiplet at 7.0 d and a six-proton singlet at 2.30 d. What is the structure of A?

What is the structure of a hydrocarbon that has ${\mathbf{M}}^{+}\mathbf{=}\mathbf{120}$ in its mass spectrum and has the following ${}^1\mathbf{H}$NMR spectrum? 7.25 d (5 H, broad singlet); 2.90 d (1 H, septet, J 5 7 Hz); 1.22 d (6 H, doublet, J 5 7 Hz).

Give IUPAC names for the following substances (red 5 O, blue 5 N):

All-cis cyclodecapentaene is a stable molecule that shows a single absorption in its NMR spectrum at $\mathbf{5}\mathbf{.}\mathbf{67}\mathit{\delta }$. 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 d, while the two protons absorb at 20.5$\mathit{\delta }$. Tell whether it is aromatic, and explain its NMR spectrum.

The substitution reaction of toluene with Br2 can, in principle, lead to the formation of three isomeric Bromo toluene products. In practice, however, only o- and p-Bromo toluene are formed in substantial amounts. The meta isomer is not formed. Draw the structures of the three possible carbocation intermediates (Problem 15-51), and explain why ortho and para products predominate over meta products.

Draw all of the resonance forms for each. What patterns emerge?

After the reaction below, the chemical shift of${\mathbf{H}}_{\mathbf{a}}$moves downfield from 6.98 ppm to 7.30 ppm. Explain.

The compound below is the product initially formed in a Claisen rearrangement (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.

Give IUPAC names for the following compounds

a) 

Give IUPAC names for the following compounds

b)

Give IUPAC names for the following compounds

c) 

Give IUPAC names for the following compounds

d) 

Give IUPAC names for the following compounds

f) 

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)

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)

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 ($\mathbf{\mu }\mathbf{=}$5 1.0 D). Explain, using resonance structures.

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)