Benzene and Aromatic Compounds

Question: Draw all possible resonance structures for the antihistamine diphenhydramine, the active ingredient in Benadryl.

Question: What orbitals are used to form the labeled  bonds in the following molecule? Of the labeled bonds, which is the shortest?

Question: Give the IUPAC name for each compound.

a.

b.

c.

d.

Question: Draw the structure corresponding to each name 

1. isobutyl benzene
2. o-dichlorobenzene 
3. cis-1,2-diphenylcyclohexane 
4. m-bromoaniline 
5. 4-chloro-1,2-diethylbenzene
6. 3-tert-butyl-2-ethyltoluene

Question: What is the structure of propofol, which has the IUPAC name 2,6-diisopropylphenol? Propofol is an intravenous medication used to induce and maintain anesthesia. 

Question: What is the structure of a compound of molecular formula ${\mathit{C}}_{\mathbf{10}}{\mathit{H}}_{\mathbf{14}}{\mathit{O}}_{\mathbf{2}}$  that shows a strong IR absorption at 3150–2850 $\mathit{c}{\mathit{m}}^{\mathbf{-}\mathbf{1}}$ and gives the following 1 H NMR absorptions: 1.4 (triplet, 6 H), 4.0 (quartet, 4 H), and 6.8 (singlet, 4 H) ppm

Question: How many ${}^{\mathbf{13}}\mathit{C}$  NMR signals do each compound exhibit?  

a. 

b.

c.

Question: Compounds A and B are both hydrogenated to methylcyclohexane. Which compound has the larger heat of hydrogenation? Which compound is more stable?

Question: Estimate where the protons bonded to the $\mathit{s}{\mathit{p}}^{\mathbf{2}}$ hybridized carbons will absorb in the 1 H NMR spectrum of each compound.

Question: Would [16]-, [20]- or [22]-annulene be aromatic if each ring is planar?

Question: Draw the four resonance structures for anthracene.

Question: Which heterocycles are aromatic?

a. 

b.

c.

d.

Question: (a) How is each N atom in quinine, an effective antimalarial drug that reduces fever, hybridized? (b) In what type of orbital does the lone pair on each N reside?

Question: Januvia, the trade name for sitagliptin, was introduced in 2006 for the treatment of type 2 diabetes. (a) Explain why the five-membered ring in sitagliptin is aromatic. (b) Determine the hybridization of each N atom. (c) In what type of orbital does the lone pair on each N atom reside?

Question: Draw the product formed when cyclohepta-1,3,5-triene (pKa = 39) is treated with a strong base. Why is its pKa so much higher than the pKa of cyclopentadiene?

Question: Rank the following compounds in the order of increasing acidity.

Question: Draw the seven resonance structures for the tropyllium cation.

Question: Assuming the rings are planar, which ions are aromatic?

Question: Compound A exhibits a peak in its ${}^{\mathbf{1}}\mathit{H}$  NMR spectrum at 7.6 ppm, indicating that it is aromatic.

(a) How are the carbon atoms of the triple bonds hybridized?

(b) In what type of orbitals are the $\mathbf{\pi }$ electrons of the triple bonds contained?

(c) How many $\mathbf{\pi }$  electrons are delocalized around the ring in A?

Question: Use the inscribed polygon method to show why the following cation is aromatic.

Question: Use the inscribed polygon method to show why the cyclopentadienyl cation and radical are not aromatic.

Question: How many ${}^{\mathbf{13}}\mathit{C}$  NMR signals does  ${\mathit{C}}_{\mathbf{60}}$ exhibit?

Question: Name each compound and state how many lines are observed in its ${}^{\mathbf{13}}\mathit{C}$ NMR spectrum.

Question: Classify each compound as aromatic, antiaromatic, or not aromatic.

Question: Early structural studies on benzene had to explain the following experimental evidence. When benzene was treated with $\mathit{B}{\mathit{r}}_{\mathbf{2}}$(plus a Lewis acid), a single substitution product of molecular formula  ${\mathit{C}}_{\mathbf{6}}{\mathit{H}}_{\mathbf{5}}\mathit{B}\mathit{r}$ was formed. When this product was treated with another equivalent of $\mathit{B}{\mathit{r}}_{\mathbf{2}}$, three different compounds of molecular formula  ${\mathit{C}}_{\mathbf{6}}{\mathit{H}}_{\mathbf{4}}\mathit{B}{\mathit{r}}_{\mathbf{2}}$  were formed.

1. Explain why a single Kekule structure is consistent with the first result, but does not explain the second result.
2. Then explain why a resonance description of benzene is consistent with the results of both reactions.

Question: Draw all aromatic hydrocarbons that have molecular formula ${\mathit{C}}_{\mathbf{8}}{\mathit{H}}_{\mathbf{10}}$. For each compound, determine how many isomers of molecular formula ${\mathit{C}}_{\mathbf{8}}{\mathit{H}}_{\mathbf{9}}\mathit{B}\mathit{r}$ would be formed if one H atom on the benzene ring were replaced by a Br atom.

Question: Give the IUPAC name for each compound.

Question: Draw a structure corresponding to each name.

1. p-dichlorobenzene 
2. p-iodoaniline
3. o-bromonitrobenzene
4. 2,6-dimethoxytoluene
5. 2-phenylprop-2-en-1-ol
6. trans-1-benzyl-3-phenylcyclopentane

Question: a. Draw the 14 constitutional isomers of molecular formula ${\mathit{C}}_{\mathbf{8}}{\mathit{H}}_{\mathbf{9}}\mathit{C}\mathit{l}$  that contain a benzene ring.

b. Name all compounds that contain a trisubstituted benzene ring.

c. For which compound(s) are stereoisomers possible? Draw all possible stereoisomers.

Question: How many  electrons are contained in each molecule?

Question: Which compounds are aromatic? For any compound that is not aromatic, state why this is so.

Question: Which of the following heterocycles are aromatic?

Question: Label each compound as aromatic, antiaromatic, or not aromatic. Assume all completely conjugated rings are planar.

Question: Hydrocarbon A possesses a significant dipole, even though it is composed of only C-C and C-H bonds. Explain why the dipole arises and use resonance structures to illustrate the direction of the dipole. Which ring is more electron rich?

Question: Pentalene, azulene, and heptalene are conjugated hydrocarbons that do not contain a benzene ring. Which hydrocarbons are especially stable or unstable based on the number of  electrons they contain? Explain your choices.

Question: The purine heterocycle occurs commonly in the structure of DNA.

a. How is each N atom hybridized?

b. In what type of orbital does each lone pair on a N atom reside?

c. How many $\mathbf{\pi }$ electrons does purine contain?

d. Why is purine aromatic?

Question: Methotrexate, a drug that inhibits the metabolism of folic acid, is used in the treatment of a variety of cancers and autoimmune disorders such as rheumatoid arthritis. (a) Give the hybridization of each N atom in methotrexate. (b) In what type of orbital does the lone pair of each N reside? (c) Explain why the bicyclic ring system that contains four N atoms is aromatic.

Question:

a. How many $\mathbf{\pi }$ electrons does C contain?

b. How many $\mathbf{\pi }$ electrons are delocalized in the ring?

c. Explain why C is aromatic.

Question: AZT was the first drug approved to treat HIV, the virus that causes AIDS. Explain why the six-membered ring of AZT is aromatic.

Question: Explain the observed rate of reactivity of the following $\mathbf{2}\mathbf{°}$alkyl halides in an ${\mathit{S}}_{\mathbf{N}}\mathbf{1}$ reaction.

Question: Draw a stepwise mechanism for the following reaction.

Question: Explain why  $\mathit{\alpha }$-pyrone reacts with $\mathit{B}{\mathit{r}}_{\mathbf{2}}$ to yield a substitution product (like benzene does),rather than an addition product to one of its $\mathit{C}\mathbf{=}\mathit{C}$ bonds.

Question: Draw additional resonance structures for each species.

Question: The carbon–carbon bond lengths in naphthalene are not equal. Use a resonance argument to explain why bond (a) is shorter than bond (b).

Question:

a. Draw all reasonable resonance structures for pyrrole and explain why pyrrole is less resonance stabilized than benzene.

b. Draw all reasonable resonance structures for furan and explain why furan is less resonance stabilized than pyrrole.

Question: Which compound in each pair is the stronger acid?

Question: Treatment of indene with $\mathit{N}\mathit{a}\mathit{N}{\mathit{H}}_{\mathbf{2}}$ forms its conjugate base in a Brønsted–Lowry acid-base reaction. Draw all reasonable resonance structures for indene’s conjugate base, and explain why the $\mathit{p}{\mathit{K}}_{\mathbf{a}}$of indene is lower than the $\mathit{p}{\mathit{K}}_{\mathbf{a}}$ of most hydrocarbons.

Question: Considering both 5-methylcyclopenta-1,3-diene (A) and 7-methylcyclohepta-1,3,5-triene (B), which labeled H atom is most acidic? Which labeled H atom is least acidic? Explain your choices.

Question: Draw the conjugate bases of pyrrole and cyclopentadiene. Explain why the $\mathit{s}{\mathit{p}}^{\mathbf{3}}$ hybridized C-H bond of cyclopentadiene is more acidic than the N-H bond of pyrrole.

Question: a. Explain why protonation of pyrrole occurs at C2 to form A, rather than on the N atom toform B.

b. Explain why A is more acidic than C, the conjugate acid of pyridine.