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Advanced Organic Chemistry. Part A. Structure and Mechanisms

Francis A. Carey, Richard J. Sundberg

Chapter 8

Aromaticity - all with Video Answers

Educators

Chapter Questions

01:45

Problem 1

The reaction of 1,2 -diphenylcyclobutadiene (generated in situ by oxidation of its iron tricarbonyl complex) with $p$-benzoquinone yields adduct 1-A as the exclusive product. A completely analogous structure is obtained using maleimide as the dienophile. However, with the more reactive dienophiles tetracyanoethylene and dicyanomalemide, two isomeric adducts of type 1-B and 1-C are found in a 1:7 ratio in each case. Discuss these results and explain how they relate to the issue of a square versus a rectangular structure for the cyclobutadiene ring.

Raghvendra Singh
Raghvendra Singh
Numerade Educator
01:37

Problem 2

A single resonance structures is shown below for each of several molecules. Consider other resonance structures and identify those that would be expected to make a major stabilizing contribution to the molecule in question.

Freddie Montague
Freddie Montague
Numerade Educator
01:01

Problem 3

a. A synthesis of tropone (cycloheptatrienone) entails treating 1 -methoxy- $1,3,5$ cyclopheptatriene with bromine. A salt is produced that yields tropone on treatment with aqueous $\mathrm{NaHCO}_{3}$. What is a likely structure for the salt? Write a mechanism for its formation and for the formation of tropone on hydrolysis.
b. The optically active dichlorophenylcyclobutenone $\mathbf{3}-\mathbf{A}$ undergoes racemization in acetic acid at $100^{\circ} \mathrm{C}$. Suggest an experiment to determine if the enol (a hydroxycyclobutadiene) is an intermediate.

Narayan Hari
Narayan Hari
Numerade Educator
01:21

Problem 4

Predict whether or not the following structures would show strong delocalization and stabilization (aromatic), weak stabilization by conjugation (nonaromatic), or strong destabilization (antiaromatic) relative to acyclic model structures. Explain the basis for your prediction.

Hitendra Singh
Hitendra Singh
Numerade Educator
03:39

Problem 5

Bicyclo[6.2.0]deca- $2,4,6,8,10$-pentaene has been synthesized, and a number of MO and MM calculations have been performed to assess its aromaticity or antiaromaticity. Consider the structure and discuss the points below.
a. What aspects of the structure suggest that antiaromaticity might be observed?
b. What aspects of the structure suggest that aromaticity might be observed?
c. What are some of the experimental and computational criteria that could be applied to assess aromaticity or antiaromaticity? Cite at least three such probes and indicate the nature of the observation and interpretation.

Temi Ajayi
Temi Ajayi
Numerade Educator
01:57

Problem 6

Using the empirically chosen energy equivalents for bond types given on p. 748 and a standard compilation of HMO calculations, determine the resonance energies of the following molecules by the Hess-Schaad procedure (p. 747). Do you find any discrepancies between the predicted and observed properties, as described in Section 8.5?

David Collins
David Collins
Numerade Educator
01:06

Problem 7

The completely conjugated cyclic polyenones have attracted considerable interest. Consider the following aspects of their properties:
a. The relative basicity of carbonyl oxygens can be measured by studying the strength of hydrogen bonding with a hydrogen donor such as phenol. The $K_{\mathrm{eq}}$ for $1: 1$ complexation of the following substituted cyclenones was determined. What conclusions do you draw from these data?

Hailey Tomashek
Hailey Tomashek
Numerade Educator
07:49

Problem 8

The isodesmic reaction series shown below has been used to compare the stabilities of the cyclopolyene ketenes. The total energies (HF/6-31G*) are given in hartrees. Calculate the stabilization found for each cyclopolyene ketene for $\mathrm{n}=1-3$. Account for the differences in stabilization and compare the results of these exocyclic ketenes to the corresponding cyclopolyenones (Problem 8.7).

Ian Kaigh
Ian Kaigh
Numerade Educator
04:23

Problem 9

One criterion of aromaticity is the diamagnetic ring current, which is indicated by a substantial chemical shift difference between hydrogens in the plane of a conjugated system and those above or below the plane. The chemical shifts of two isomeric hydrocarbons are given below. In qualitative terms, which compound appears to be more aromatic? (Because the chemical shift owing to ring current depends on the detailed geometry, a quantitative calculation would be necessary to confirm the correctness of the qualitative assessment.) Does HMO theory predict a difference in the aromaticity of these two compounds?

Cameron Oden
Cameron Oden
Numerade Educator
03:13

Problem 10

Offer an explanation for the following observations:
a. Hydrocarbon $\mathbf{1 0 - A}(\mathrm{p} K \approx 14)$ is considerably more acidic than $\mathbf{1 0}-\mathbf{B}(\mathrm{p} K \approx 22)$.
b. Hydrocarbon $\mathbf{1 0 - C}$ has an unusually small separation of its oxidation and reduction potentials, as established by electrochemical measurements. It is both easily reduced and easily oxidized. Both mono- and dications and monoand dianions can be readily formed.
c. The barrier for rotation about the marked bond in 10-D is only about 14 $\mathrm{kcal} / \mathrm{mol}$.
d. The hydrocarbon $\mathbf{1 0}-\mathbf{E}$ is easily reduced to a dianion. The ${ }^{1} \mathrm{HNMR}$ spectrum of the dianion shows an average downfield shift relative to the hydrocarbon. The central carbon shows a large upfield shift in the ${ }^{13} \mathrm{C}-\mathrm{NMR}$ spectrum.

Shazia Naz
Shazia Naz
Numerade Educator
01:43

Problem 11

The HMOs for acenaphthene are shown below. The atomic coefficients for the orbital that is the LUMO in the neutral compound and the HOMO in the dianion are given at the right.
$$
\begin{aligned}
&-2.36 \\
&-1.92 \\
&-1.43 \\
&-1.31 \\
&-1.00 \\
&-0.28 \\
&+0.63 \\
&+0.83 \\
&+1.00 \\
&+1.69 \\
&+2.47
\end{aligned}
$$
Comment on the aromaticity, antiaromaticity, or nonaromaticity of acenaphthene and its dianion on the basis of the following physical measurements:
a. The bond lengths of acenaphthene are given below. Compare them with the bond lengths for naphthalene given on p. 18. What conclusions can you draw about the aromaticity of acenaphthene?
b. Both X-ray crystallography and NMR data indicate that the $\mathrm{C}(1)-\mathrm{C}(2)$ bond lengthens significantly in the dianion, as indicated below (X-ray data). There is also a different pattern of bond length alternation. What conclusions can you draw about the aromaticity of the acenaphthene dianion?
c. The ${ }^{1} \mathrm{H}-$ and ${ }^{13} \mathrm{C}-\mathrm{NMR}$ shifts for acenaphthene and its dianion $\left(\mathrm{Na}^{+}\right.$ counterion) are given below. What conclusions about charge density and aromaticity can be drawn from these data?
$$
\begin{array}{llccccccc}
\hline & & 1,2 & 3,8 & 4,7 & 5,6 & 2 \mathrm{a}, 8 \mathrm{a} & 5 \mathrm{a} & 8 \mathrm{~b} \\
\hline{ }^{1} \mathrm{H} & \text { Neutral } & 7.04 & 7.65 & 7.50 & 7.78 & & & \\
& \text { Dianion } & 4.49 & 4.46 & 5.04 & 3.34 & & & \\
{ }^{13} \mathrm{C} & \text { Neutral } & 129.9 & 124.7 & 128.3 & 127.8 & 140.7 & 129.1 & 129.3 \\
& \text { Dianion } & 86.1 & 97.0 & 126.8 & 82.6 & 123.4 & 149.3 & 137.7 \\
\hline
\end{array}
$$

Hitendra Singh
Hitendra Singh
Numerade Educator
04:04

Problem 12

The ${ }^{1} \mathrm{H}-\mathrm{NMR}$ and ${ }^{13} \mathrm{C}-\mathrm{NMR}$ spectra of both the anion and cation derived from phenalene have been observed. The HMO pattern for phenalene is given below.
The chemical shifts observed for the cation and anion are given below.
$$
\begin{array}{lllllll}
\hline & {{ }^{1} \mathrm{H}-\mathrm{NMR}} & &{{ }^{13} \mathrm{C}-\mathrm{NMR}} \\
\hline & \mathrm{C}(1) & \mathrm{C}(2) & \mathrm{C}(1) & \mathrm{C}(2) & \mathrm{C}(9 \mathrm{~b}) & \mathrm{C}(3 \mathrm{a}) \\
{[\text { Cation }]} & 9.38 & 8.55 & 155.5 & 133.7 & 123.7 & 133.7 \\
{[\text { Anion }]} & 5.36 & 6.10 & 103.4 & 128.0 & 139.6 & 145.0 \\
\hline
\end{array}
$$
What conclusions can be drawn about electron distribution in the cation and anion from the NMR data and how does it relate to the HMO pattern?

Zubair Abdulla
Zubair Abdulla
Numerade Educator
02:24

Problem 13

The ${ }^{13} \mathrm{C}-\mathrm{NMR}$ spectrum of octalene $(\mathbf{1 3}-\mathbf{A}, \mathbf{1 3}-\mathbf{B}$, or $\mathbf{1 3 - C})$ is temperature dependent. At $-150^{\circ} \mathrm{C}$, there are signals for 14 different carbons. At $-100^{\circ} \mathrm{C}$, these collapse to seven different signals. Above $80^{\circ} \mathrm{C}$, all but one of the remaining signals become broad. Although not attained experimentally, because of decomposition, it would be expected that only four different signals would be obtained at still higher temperature. (1) Show that these data rule out structures 13-A and 13-B for the room temperature structure of octalene, and favor structure 13-C.
(2) What is the nature of the dynamic process that converts the 14-line spectrum to a 7-line spectrum? (3) What would be the nature of the process that converts the 7 -line spectrum to a 4 -line spectrum?

Lottie Adams
Lottie Adams
Numerade Educator
02:37

Problem 14

When alcohol 14-A is dissolved in $\mathrm{FSO}_{3} \mathrm{H}$ at $-136^{\circ} \mathrm{C}$ and then brought to $-110^{\circ} \mathrm{C}$, it gives rise to ${ }^{13} \mathrm{C}-\mathrm{NMR}$ spectrum having five lines in the intensity ratio $2: 1: 2: 2: 2$.
a. Suggest several possible structures for this cation and discuss stabilizing features that might favor a particular structure.
b. Figure 8.P14(a, b) gives the computed minimum-energy structure at the MP4(SDQ)/6-31G $(d)$ level. Diagram (c) is the $-\nabla^{2} \rho(\mathbf{r})$ in the $\mathrm{C}(1)-\mathrm{C}(2)-\mathrm{C}(9)$ plane. Does this structure correspond to any of those you have suggested in Part (a)? What structural representation would be most consistent with the calculated minimum-energy structure?

Shazia Naz
Shazia Naz
Numerade Educator
03:36

Problem 15

a. The heats of combustion $\left(\Delta H_{\mathrm{c}}\right)$ and heats of hydrogenation $\left(\Delta H_{\mathrm{H}_{2}}\right)$ for addition of $1 \mathrm{~mol}$ of $\mathrm{H}_{2}$ and the estimated stabilization energy (SE) for benzene and cyclooctatetraene (in $\mathrm{kcal} / \mathrm{mol}$ ) are given below. The $\Delta H_{\mathrm{c}}$ and $\Delta H_{\mathrm{H}_{2}}$ are also given for [16]annulene. Compare the stabilization energy of [16] annulene with benzene and cyclooctatetraene on a per $\mathrm{CH}$ basis.
$$
\begin{array}{llcc}
\hline & \text { Benzene } & \text { Cyclooctatetraene } & \text { [16]Annulene } \\
\hline \Delta H_{\mathrm{c}} & 781 & 1086 & 2182 \\
\Delta H_{\mathrm{H}_{2}} & -5.16 & 25.6 & 28.0 \\
\mathrm{SE} & 36 & 4 & ? \\
\hline
\end{array}
$$
b. The enthalpies of the reaction of the cyclooctatetraene and [16]annulene dianions with water have been measured.
$$
\begin{aligned}
&2 \mathrm{Na}^{+}\left(\mathrm{C}_{n} \mathrm{H}_{n}\right)^{2-}+2 \mathrm{H}_{2} \mathrm{O}_{(1)} \rightarrow \mathrm{C}_{n} \mathrm{H}_{\mathrm{n}+2}+2 \mathrm{NaOH} \\
&\Delta H=-33.33 \mathrm{kcal} / \mathrm{mol} \text { for cyclooctatetraene } \\
&\Delta H=-10.9 \mathrm{kcal} / \mathrm{mol} \text { for }[16] \text { annulene. }
\end{aligned}
$$
Using these data and the enthalpy of the reaction of sodium with water:
$$
2 \mathrm{Na}_{\text {(s) }}+2 \mathrm{H}_{2} \mathrm{O}_{\text {(1) }} \rightarrow 2 \mathrm{NaOH}_{\text {(aq) }}+\mathrm{H}_{2} \Delta H=-88.2 \mathrm{kcal} / \mathrm{mol}
$$
calculate $\Delta H$ for the reaction:
$$
2 \mathrm{Na}_{(\mathrm{s})}+\mathrm{C}_{n} \mathrm{H}_{n} \rightarrow 2 \mathrm{Na}^{+}+\left(\mathrm{C}_{n} \mathrm{H}_{n}\right)^{2-}
$$
Why might the reaction of $\left[\mathrm{C}_{16} \mathrm{H}_{16}\right]^{2-}$ with water be less exothermic than for $\left[\mathrm{C}_{8} \mathrm{H}_{8}\right]^{2-}$ ? How do you interpret the difference in the heat of reaction of the two hydrocarbons to form the respective dianions?

ES
Eugene Schneider
University of Minnesota - Twin Cities
01:34

Problem 16

Consider the two structures shown for kekulene, one suggesting inner and outer annulenes and the other a series of phenanthrene-like units. ${ }^{1} \mathrm{H}$-NMR and bond length data are given. The NICS values are calculated as $-4.3$ for the angularly fused rings and $-10.8$ for the linearly fused rings. Indicate properties that you would expect to be associated with each structure. Do you consider the properties to be more consistent with the double-annulene or the phenanthrenelike structures?

HC
Hashim Choudhry
Numerade Educator
06:11

Problem 17

Acepentalene is a rather unstable molecule, but its dianion can be formed quite readily. The structure and properties of acepentalene and its dianion and dication have been calculated (B3LYP/6-31G*) and are given below. The computed lowest-energy structure is slightly pyramidal, with an inversion barrier of $7.1$ $\mathrm{kcal} / \mathrm{mol}$. The structure of the lithium salt of the dianion is given in the Figure 8.P17. The calculated inversion barrier for the dianion is $5.4 \mathrm{kcal} / \mathrm{mol}$. The chemical shift of the ${ }^{1} \mathrm{H}$ signal in the dianion is at $-8.2$.
a. What evidence in terms of aromaticity/antiaromaticity can you offer for the apparently greater stability of the $12 \pi$-electron dianion as compared with the $10 \pi$-electron neutral? What accounts for the pyramidal as opposed to planar structures for the neutral and dianion?
b. The HMO orbitals of acepentalene are given below. How do the predictions of HMO theory accord with the experimental and B3LYP results?

VS
Vivek Singh
Numerade Educator
09:12

Problem 18

Arene oxides are important intermediates in the metabolism of aromatic compounds. Although they are highly reactive, both valence tautomerism to oxepins and acid-catalyzed ring opening to phenols can be observed and studied.
Surprisingly, the rate of acid-catalyzed ring opening is less than that for dehydration of "2,4-cyclohexadienol" even though they lead to similar cations. Normally, epoxide ring opening is much faster than alcohol dehydration. For example, the epoxide of cyclohexadiene is about $10^{7}$ more reactive than cyclohexenol.

Matthew Lueckheide
Matthew Lueckheide
Numerade Educator
04:50

Problem 19

Using isodesmic reactions and the G2(MP2) energies given below, determine if $1,2,3$-trichlorocyclopropenium ion is more or less stable than the tert-butyl carbocation. Are the chlorine substituents stabilizing or destabilizing with respect to cyclopropenium ion?
$$
\begin{array}{lc}
\hline {\text { Substance }} & \begin{array}{c}
\text { G2(MP2) energy } \\
\text { (hartrees) }
\end{array} \\
\hline \text { Trichlorocyclopropenium } & -1492.916633 \\
\text { Cyclopropenium ion } & -115.492839 \\
\text { tert } \text {-Butyl carbocation } & -157.169332 \\
1,2,3,3 \text {-tetrachlorocyclopropene } & -1952.950661 \\
3 \text {-Chlorocyclopropene } & -575.525577 \\
\text { tert } \text {-Butyl chloride } & -617.226718
\end{array}
$$

Julia G.
Julia G.
Numerade Educator
00:50

Problem 20

Triquinacene is a hydrocarbon that might be stabilized by homoaromaticity.
The calculated (B3LYP/6-3111+G*) $\Delta H_{\mathrm{H}_{2}}$ for the successive double bond are $-27.6,-27.3$, and $-26.8 \mathrm{kcal} / \mathrm{mol}$. The $\Delta H_{\mathrm{H}_{2}}$ for cyclopentene is $-26.9 \mathrm{kcal} / \mathrm{mol}$. The existence of homoaromatic stabilization of triquinacene might be assessed by the following homodesmotic reaction, where $\Delta E=-0.8 \mathrm{kcal} / \mathrm{mol}$ :
Do these data indicate homoaromatic stabilization of triquinacene? Why or why not?

Mishal Gul
Mishal Gul
Numerade Educator