Organic Chemistry

Paula Yurkanis Bruice

Chapter 21

More About Amines • Heterocyclic Compounds - all with Video Answers

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Chapter Questions

Problem 1

Name the following compounds:

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Problem 2

Why is the conjugate acid of morpholine more acidic than the conjugate acid of piperidine?

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Problem 3

a. Draw the structure of 3-quinuclidinone.
b. What is the approximate $\mathrm{p} K_{\mathrm{a}}$ of its conjugate acid?
c. Which has a lower $p K_{a}$, the conjugate acid of 3 -bromoquinuclidine or the conjugate acid
of 3-chloroquinuclidine?

Allison Krajewski

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Problem 4

Explain the difference in the $\mathrm{p} K_{\mathrm{a}}$ values of the piperidinium and aziridinium ions: The piperidinium ion has a $\mathrm{p} K_{\mathrm{a}}$ value expected for an ammonium ion. The $\mathrm{p} K_{\mathrm{a}}$ value of the aziridinium ion is considerably lower. In this ion, the internal bond angle of the threemembered ring is smaller than usual, causing the extemal bond angles to be larger than usual. Because of the larger bond angles, the orbital used by nitrogen to overlap the orbital of hydrogen has more $s$ character than that of a typical $s p^{3}$ nitrogen. This makes nitrogen more electronegative and lowers the $\mathrm{p} K_{\mathrm{a}}$ (Section 1.22 ).

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Problem 5

Give the product of each of the following reactions:

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Problem 6

Draw arrows to show the movement of electrons in going from one resonance contributor to the next in pyrrole.

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Problem 7

When pyrrole is added to a dilute solution of $\mathrm{D}_{2} \mathrm{SO}_{4}$ in $\mathrm{D}_{2} \mathrm{O},$ 2-deuteriopyrrole is formed. Propose a mechanism to account for the formation of this compound.

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Problem 8

Use resonance contributors to explain why pyrrole is protonated on $\mathrm{C}-2$ rather than on nitrogen.

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Problem 9

Explain why cyclopentadiene $\left(\mathrm{p} K_{\mathrm{a}}=15\right)$ is more acidic than pyrrole $\left(\mathrm{p} K_{\mathrm{a}} \sim 17\right),$ even though nitrogen is more electronegative than carbon.

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Problem 10

Will an amide be formed from the reaction of an acyl chloride with an aqueous solution of pyridine? Explain your answer.

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Problem 11

Give the product formed when pyridine reacts with ethyl bromide.

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Problem 12

Compare the mechanisms of the following reactions:

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Problem 13

a. Propose a mechanism for the following reaction: b. What other product is formed?

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Problem 14

Rank the following compounds in order of decreasing ease of removing a proton from a methyl group:

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Problem 15

Why is imidazole a stronger acid $\left(\mathrm{p} K_{\mathrm{a}}=14.4\right)$ than pyrrole $\left(\mathrm{p} K_{\mathrm{a}}=17\right) ?$

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Problem 16

Give the major product of the following reaction:

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Problem 17

List imidazole, pyrrole, and benzene in order of decreasing reactivity toward electrophilic aromatic substitution.

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Problem 18

Imidazole boils at $257^{\circ} \mathrm{C},$ whereas $N$ -methylimidazole boils at $199^{\circ} \mathrm{C}$. Explain this difference in boiling points.

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Problem 19

What percent of imidazole will be protonated at physiological pH (7.3)?

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Problem 20

Why is protonated pyrimidine $\left(\mathrm{p} K_{\mathrm{a}}=1.0\right)$ more acidic than protonated pyridine $\left(\mathrm{p} K_{\mathrm{a}}=5.2\right) ?$

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Problem 21

Is porphyrin aromatic?

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Problem 22

Name each of the following:

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Problem 23

Give the product of each of the following reactions:

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Problem 24

List the following compounds in order of decreasing acidity:

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Problem 25

Which of the following compounds is easier to decarboxylate?

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Problem 26

Rank the following compounds in order of decreasing reactivity in an electrophilic aromatic substitution reaction:

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Problem 27

One of the following compounds undergoes electrophilic aromatic substitution predominantly at $\mathrm{C}-3,$ and one undergoes electrophilic aromatic substitution predominantly at C-4. Which is which?

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Problem 28

Benzene undergoes electrophilic aromatic substitution reactions with aziridines in the presence of a Lewis acid such as $\mathrm{AlCl}_{3}$
a. What are the major and minor products of the following reaction? b. Would you expect epoxides to undergo similar reactions?

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Problem 29

The dipole moments of furan and tetrahydrofuran are in the same direction. One compound has a dipole moment of $0.70 \mathrm{D},$ and the other has a dipole moment of 1.73 D. Which is which?

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Problem 30

A tertiary amine reacts with hydrogen peroxide to form a tertiary amine oxide.
Tertiary amine oxides undergo a reaction similar to the Hofmann elimination reaction (Section 10.9 ), called a Cope elimination reaction. In this reaction, a tertiary amine oxide, rather than a quaternary ammonium ion, undergoes elimination. A strong base is not needed for a Cope elimination because the amine oxide acts as its own base. Does the Cope elimination have an alkene-like transition state or a carbanion-like transition state?

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Problem 31

Give the products that would be obtained by treating the following tertiary amines with hydrogen peroxide followed by heat:

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Problem 32

The chemical shifts of the $C$ - 2 hydrogen in the spectra of pyrrole, pyridine, and pyrrolidine are 2.82 ppm, 6.42 ppm, and 8.50 ppm. Match each heterocycle with its chemical shift.

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Problem 33

Explain why protonation of aniline has a dramatic effect on the compound's UV spectrum, whereas protonation of pyridine has only a small effect on that compound's UV spectrum.

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Problem 34

Explain why pyrrole $\left(\mathrm{p} K_{\mathrm{a}} 17\right)$ is a much stronger acid than ammonia $\left(\mathrm{p} K_{\mathrm{a}} 36\right)$

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Problem 35

Propose a mechanism for the following reaction:

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Problem 36

Quinolines, heterocyclic compounds that contain a pyridine ring fused to a benzene ring, are commonly synthesized by a method known as the Skraup synthesis, in which aniline reacts with glycerol under acidic conditions. Nitrobenzene is added to the reaction mixture to serve as an oxidizing agent. The first step in the synthesis is the dehydration of glycerol to propenal. a. What product would be obtained if parn-ethylaniline were used instead of aniline?
b. What product would be obtained if 3-hexen-2-one were used instead of glycerol?
c. What starting materials are needed for the synthesis of 2,7 -diethyl- 3 -methylquinoline?

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Problem 37

Propose a mechanism for each of the following reactions:

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Problem 38

Give the major product of each of the following reactions:

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Problem 39

a. Draw resonance contributors to show why pyridine-N-oxide is more reactive than pyridine toward electrophilic aromatic substitution
b. At what position does pyridine-N-oxide undergo electrophilic aromatic substitution?

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Problem 40

Propose a mechanism for the following reaction:

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Problem 41

Pyrrole reacts with excess para-(N,N-dimethylamino)benzaldehyde to form a highly colored compound. Draw the structure of the colored compound.

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Problem 42

2-Phenylindole is prepared from the reaction of acetophenone and phenylhydrazine, a method known as the Fischer indole synthe Propose a mechanism for this reaction.

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Problem 43

What starting materials are required to synthesize the following compounds, using the Fischer indole synthesis?

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Problem 44

Organic chemists work with tetraphenylporphyrins rather than with porphyrins because tetraphenylporphyrins are much more resistant to air oxidation. Tetraphenylporphyrin can be prepared by the reaction of benzaldehyde with pyrrole. Propose a mechanism for the formation of the ring system shown here:

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