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Advanced Organic Chemistry. Part A. Structure and Mechanisms

Francis A. Carey, Richard J. Sundberg

Chapter 1

Chemical Bonding and Molecular Structure - all with Video Answers

Educators

Chapter Questions

02:56

Problem 1

Suggest an explanation for the following observations:
a. Although the hydrocarbon calicene has so far defied synthesis, but it has been estimated that it would have a dipole moment as large as $5.6 \mathrm{D}$.
<smiles>c1ccc(C2CC2)cc1</smiles>
calicenecalicene
b. The measured dipole moment of 4-nitroaniline (6.2 D) is larger than the value calculated using standard group dipoles (5.2 D).
<smiles>Nc1ccc([N+](=O)[O-])cc1</smiles>
4-nitroaniline
c. The dipole moments of furan and pyrrole are in opposite directions.
<smiles>ON(c1ccccc1)c1ccccc1</smiles>
pyrrole

Mercedes Mazza
Mercedes Mazza
Numerade Educator
03:31

Problem 2

Predict the preferred site of protonation for each of the following molecules and explain the basis of your prediction.
a.
<smiles>C(=Nc1ccccc1)c1ccccc1</smiles>
b.
<smiles>CC(N)=O</smiles>
c.
<smiles>c1cc[nH]c1</smiles>
d.
<smiles>Nc1ccccn1</smiles>

Alkendra Singh
Alkendra Singh
Numerade Educator
02:51

Problem 3

What physical properties, such as absorption spectra, bond lengths, dipole moment, etc., could be examined to obtain evidence of resonance interactions in the following molecules. What deviation from "normal" physical properties would you expect?

Dr. Satish Ingale
Dr. Satish Ingale
Numerade Educator
01:09

Problem 4

Consider each of the following physical characteristics of certain amides.
a. Carboxamides have rotational barriers on the order of $20 \mathrm{kcal} / \mathrm{mol}$ for the process:
<smiles>[R]C(=O)N(C)C</smiles>
<smiles>C=CC(=O)N(C)CC(=O)c1ccccc1</smiles>
Develop a structural explanation for the barrier both in resonance and MO terminology.

Hitendra Singh
Hitendra Singh
Numerade Educator
01:55

Problem 5

The AMl semiempirical MO method was used to calculate structure, HOMO-LUMO orbital coefficients, and charge distribution for several substituted cyclopropenones. The results for 2-phenylcyclopropenone, 2phenyl-3-methylcyclopropenone, and 2,3 -diphenylcyclopropenone are given in Table 1.P5a. Based on this information, make predictions about the following reactions:
a. What will be the site of protonation of a cyclopropenone?
b. What will be the site of reaction of a cyclopropenone with a hard nucleophile, such as ${ }^{-} \mathrm{OH}$ ?
c. Is an alkyl or aryl substituent most effective in promoting reaction with a soft nucleophile?

Lottie Adams
Lottie Adams
Numerade Educator
05:36

Problem 6

It is observed that benzo[c] derivatives of furan, pyrrole, and thiophene are less stable and much more reactive than the corresponding benzo[b] derivatives. The differences are apparent for example in $[4+2]$ cycloadditions, which are facile with the benzo[c] but not the benzo[b] derivatives. Some MO properties from AM1 calculations are given in Tables 1.P6a, 1.P6b and 1.P6c. What features of the results are in accord with the experimental observations? Do you find any features of the results than run counter to the observations.
benzo[b]
Table 1.P6. Enthalpy of Reactants and Transition Structures
\begin{tabular}{lccc}
\hline & $\mathrm{O}$ & $\mathrm{NH}$ & $\mathrm{S}$ \\
\hline$\Delta H_{f}(\mathrm{kcal} / \mathrm{mol})$ & & & \\
Benzo[b] & $20.8$ & $55.2$ & $42.4$ \\
Benzo[c] & $27.9$ & $61.7$ & $49.4$ \\
$\Delta H_{f}$ for $4+2 \mathrm{TS}$ & & & \\
Benzo[b] & $75.7$ & $115.3$ & $110.8$ \\
Benzo[c] & $64.5$ & $104.8$ & $102.4$ \\
$\Delta H^{\ddagger}$ for $4+2 \mathrm{TS}$ & & & \\
Benzo[b] & $38.4$ & $43.6$ & $51.9$ \\
Benzo[c] & $20.1$ & $26.6$ & $36.5$ \\
\hline
\end{tabular}

Sana Riaz
Sana Riaz
Numerade Educator
04:05

Problem 7

(Old 1.16) The propellanes are highly reactive in comparison with unstrained hydrocarbons and readily undergo reactions that result in the rupture of the central bond. For example, it has been suggested that the polymerization of propellanes occurs by initial dissociation of the center bond. Perhaps surprisingly, it has been found that [1.1.1]propellane is considerably less reactive than either $[2.2 .1]$ propellane or $[2.1 .1]$ propellane. Use the computational enthalpy data below to estimate the energy required to break the center bond in each of the three propellanes. Assume that the bridgehead $\mathrm{C}-\mathrm{H}$ bonds in each of the bicycloalkanes has a bond enthalpy of $-104 \mathrm{kcal} / \mathrm{mol}$. How might the results explain the relative reactivity of the propellanes.

Rajesh Singh
Rajesh Singh
Numerade Educator
01:15

Problem 8

Examine the heats of hydrogenation shown for unsaturated eight-membered ring hydrocarbons. (a) Discuss the differences among the different compounds in comparison with the standard $\Delta H_{\mathrm{H} 2}$ for an unstrained cis double bond, which is $27.4 \mathrm{kcal} / \mathrm{mol}$. (b) Assigning a strain energy of $9.3 \mathrm{kcal} / \mathrm{mol}$ to cyclooctane, calculate the relative strain of each of the other compounds. (c) What role does conjugation play in relation to the observed $\Delta H_{\mathrm{H} 2} ?$ (d) What conclusion do thesedata permit as to whether cyclooctatetraene is stabilized or destabilized by cyclic conjugation.
\begin{tabular}{lc}
\hline \multicolumn{1}{c}{ Compound } & $\Delta H_{\mathrm{H} 2}(\mathrm{kcal} / \mathrm{mol})$ \\
\hline$Z, Z, Z, Z-1,3,5,7$-Cyclooctatetraene & $97.06$ \\
$Z, Z, Z-1,3,5-$ Cyclooctatriene & $76.39$ \\
$Z, Z, Z-1,3,6-$ Cyclooctatriene & $79.91$ \\
$Z, Z-1,3-$ Cyclooctadiene & $48.96$ \\
$Z, Z-1,4-$ Cyclooctadiene & $52.09$ \\
$Z, Z-1,5-$ Cyclooctadiene & $53.68$ \\
$Z-$ Cyclooctene & $22.98$ \\
$E-$ Cyclooctene & $32.24$ \\
\hline
\end{tabular}

Nikhil Choudhary
Nikhil Choudhary
Numerade Educator
04:48

Problem 9

An isodesmic reaction suitable for calculating the resonance stabilization of benzene relative to cyclohexene is
3
<smiles>[C+]1C=CC=CC=C1</smiles> $\Delta E=-35.3 \mathrm{kcal} / \mathrm{mol}$
A similar calculation can be done with substituent groups in place. The results for several substituents by $\mathrm{G} 3(\mathrm{MP} 2)$ computations are as follows:

Nicholas Sacco
Nicholas Sacco
Numerade Educator
03:21

Problem 10

The explanation of substituent effects on the acidity of substituted carboxylic acids usually focuses on two factors: (a) The ability of the substituent to stabilize the negative charge; and (b) the effect of solvation on the anion. However, there will also be substituent effects on the stability and solvation of the undissociated acid. The studies described on p. XXX resulted in the following values for the gas phase energy (in hartrees) of the acids and anions and the resulting $\Delta G$ for gas phase ionization. Using this information and the solvation energies on p. 53 ,analyze the relative importance of intrinsic anion stabilization and solvation or the observed acidity.
\begin{tabular}{lccc}
\hline \multicolumn{1}{c}{$\mathrm{x}$} & Acid, $E_{\text {gs }}$ & Anion, $E_{\text {gu }}$ & $\Delta G($ kcal/mol $)$ \\
\hline $\mathrm{H}$ & $-189.551612$ & $-189.005818$ & $342.49$ \\
$\mathrm{CH}_{3}$ & $-228.792813$ & $-228.241528$ & $345.94$ \\
$\mathrm{ClCH}_{2}$ & $-687.945725$ & $-687.414262$ & $333.50$ \\
$\mathrm{NCCH}_{2}$ & $-320.909115$ & $-320.386960$ & $327.66$ \\
$\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}$ & $-346.481196$ & $-345.936639$ & $341.71$ \\
\hline
\end{tabular}

Raghvendra Singh
Raghvendra Singh
Numerade Educator
02:39

Problem 11

Cyclic amines such as piperidine and its derivatives show substantial differences in the properties of the axial $\mathrm{C}(2)$ and $\mathrm{C}(6)$ bonds. The axial $\mathrm{C}-\mathrm{H}$ bonds are weaker than the equatorial $\mathrm{C}-\mathrm{H}$ bonds, as indicated by a shifted $\mathrm{C}-\mathrm{H}$ stretching frequency in the IR spectrum. The axial hydrogens also appear at higher field in ${ }^{1} \mathrm{H}-\mathrm{NMR}$ spectra. Axial $\mathrm{C}(2)$ and $\mathrm{C}(6)$ methyl groups lower the ionization potential of the unshared pair of electrons on nitrogen more than equatorial $\mathrm{C}(2)$ and $\mathrm{C}(6)$ methyl

Catherine Lemar
Catherine Lemar
Numerade Educator
06:08

Problem 12

Construct a qualitative $\mathrm{MO}$ diagram for the following systems and discuss how the $\pi$ MOs are modified by addition of the substituent.
a. vinyl fluoride, compared to ethene
b. propenal, compared to ethene
c. acrylonitrile, compared to ethene
d. propene, compared to ethene
e. benzyl cation, compared to benzene
f. fluorobenzene, compared to benzene

Caleb Prus
Caleb Prus
Numerade Educator
01:35

Problem 13

Given below are the $\Delta E$ for some isodesmic reactions. Also given are the AIM and NPA charges at the carbon atoms of the double bond. Provide an explanation for these results in terms of both resonance structure and MO terminology.
a. Draw resonance structures and qualitative MO diagrams that indicate the stabilizing interaction.
b. Explain the order of stabilization $\mathrm{N}>\mathrm{O}>\mathrm{F}$ in both resonance and $\mathrm{MO}$ terminology.
c. Interpret the AIM and NPA charges in relationship to the ideas presented in (a) and (b).
$$
\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{X}+\mathrm{CH}_{2}=\mathrm{CH}_{2} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{3}+\mathrm{CH}_{2}=\mathrm{CHX}
$$
\begin{tabular}{llllll}
\hline $\mathrm{X}$ & $\Delta E$ & $\delta \mathrm{C}(1)^{a}$ & $\delta \mathrm{C}(2)^{a}$ & $\delta \mathrm{C}(1)^{b}$ & $\delta \mathrm{C}(2)^{b}$ \\
\hline $\mathrm{H}$ & 0 & $+0.08$ & $+0.08$ & $-0.25$ & $-0.25$ \\
$\mathrm{CH}_{3}$ & $-3.05$ & $+0.02$ & $+0.08$ & $-0.09$ & $-0.29$ \\
$\mathrm{NH}_{2}$ & $-7.20$ & $+0.51$ & $+0.15$ & $+0.14$ & $-0.36$ \\
$\mathrm{OH}$ & $-6.43$ & $+0.58$ & $+0.18$ & $+0.26$ & $-0.39$ \\
$\mathrm{~F}$ & $-0.99$ & $+0.48$ & $+0.29$ & $+0.30$ & $-0.36$ \\
\hline
\end{tabular}
a. AIM charges
b. NPA charges

Hitendra Singh
Hitendra Singh
Numerade Educator
14:05

Problem 14

In the Hückel MO treatment, orbitals on nonadjacent atoms are assumed to have no interaction. The concept of homoconjugation suggests that such orbitals may interact, especially in rigid structures in which the orbitals are directed toward one another. Consider, e.g, norbornadiene, (bicyclo $[2.2 .1]$ hepta-2,5-diene).
a. Construct an MO diagram according to HMO theory and assign orbital energies.
b. Construct the qualitative MO diagram that would result from significant overlap between the $C(3)$ and $C(5)$ and $C(2)$ and $C(6)$ orbitals.
c. The ionization potentials (IP) of some 2 -substituted norbornadienes are given below. The two IP values pertain to the $\pi$ system. Use PMO theory to analyze the effect these substituents have on the IP. Use a qualitative MO diagram to show how the substituents interact with the two double bonds and how this affects the IP. Discuss the effect the substituents have on the IP.
\begin{tabular}{lll}
\hline \multicolumn{1}{c}{$\mathrm{X}$} & $\mathrm{IP}_{1}$ & $\mathrm{IP}_{2}$ \\
\hline $\mathrm{H}$ & $8.69$ & $9.55$ \\
$\mathrm{CH}_{3} \mathrm{O}$ & $8.05$ & $9.27$ \\
$\mathrm{CN}$ & $9.26$ & $10.12$ \\
\hline
\end{tabular}

Susan Hallstrom
Susan Hallstrom
Numerade Educator
02:13

Problem 15

a. Sketch the nodal properties of the Hückel HOMO orbital of the pentadienyl cation.
b. The orbital coefficients of two of the $\pi$ MOs of pentadienyl are given below. Specify which is of lower energy; classify each orbital as bonding, nonbonding, or antibonding; and specify whether each orbital is $S$ or $A$ with respect to a plane bisecting the molecule perpendicular to the plane of the structure.
$$
\begin{aligned}
&\psi_{x}=0.50 \varphi_{1}+0.50 \varphi_{2}-0.50 \varphi_{4}-0.50 \phi_{5} \\
&\psi_{y}=0.58 \varphi_{1}-0.58 \varphi_{3}+0.58 \varphi_{5}
\end{aligned}
$$

Catherine Lemar
Catherine Lemar
Numerade Educator
00:40

Problem 16

There has been discussion as to whether unsaturated EWGs such as formyl or cyano stabilize or destabilize carbocations.
<smiles>O=CCc1ccccc1</smiles>
<smiles>CCC(C)C</smiles>
<smiles>[O-]C=Cc1ccccc1</smiles>
<smiles>N#CC=Cc1ccccc1</smiles>
<smiles>N#CC=Cc1ccccc1</smiles>The diagrams below give $\mathrm{STO}-3 \mathrm{G}$ bond lengths and Mulliken charge dens for the benzyl cation and for its $\alpha$-formyl and $\alpha$-cyano derivatives. Analyze effect of the substituents on the carbocation.
<smiles>CC(C)(C)C1CCCCC1</smiles>
<smiles>CC(C=O)C1CCCCC1</smiles>
<smiles>CC1CCCCC1(C)C=CC#N</smiles>
$1.411$
$1.435$
$1.414$
$1.411$
<smiles> O = [ N + ] ( [ O - ] ) C C 1 C ( C</smiles>
<smiles> O = C ( O ) C = C C 1 C</smiles>

Mishal Gul
Mishal Gul
Numerade Educator
05:10

Problem 17

a. Calculate the HMO energy levels and atomic orbital coefficients for $1,3-$ butadiene.
b. Estimate the delocalization energy, in units of $\beta$, of the cyclobutadienyl dication $\mathrm{C}_{4} \mathrm{H}_{4}{ }^{2+}$ from HMO theory.
c. Estimate, in units of $\beta$, the energy associated with the longest-wavelength UVVIS absorption of $1,3,5,7$-octatetraene. Does it appear at a longer or shorter wavelength than the corresponding absorption in $1,3,5$-hexatriene?

Guilherme Barros
Guilherme Barros
Numerade Educator
11:06

Problem 18

Spiropentane has unusual strain and hybridization. Consider the following facets of its structure.
a. The strain energy of spiropentane (62.5 $\mathrm{kcal} / \mathrm{mol}$ ) is considerably more than twice that of cyclopropane ( $27.5 \mathrm{kcal} / \mathrm{mol}$ ). Suggest an explanation.
b. The structure of spiropentane has been determined by X-ray crystallography. The endocyclic angles at the spiro carbon are about $62^{\circ}$, and the bond angles between $\mathrm{C}-\mathrm{C}$ bonds in the adjacent rings are about $137^{\circ}$. How would you relate the strain to the hybridization of each carbon in spirocyclopentane based on these bond angles?
c. The fractional $s$ character in a $\mathrm{C}-\mathrm{C}$ bond can be estimated from ${ }^{13} \mathrm{C}-{ }^{13} \mathrm{C}$ coupling constants using the equation
$$
J_{C i-C j}=K\left(s_{i}\right)\left(s_{j}\right)
$$
where $K$ is a constant $=550 \mathrm{~Hz}$ and $s$ is the fractional $s$ character of each atom. In spiropentane, the $J$ for coupling between $\mathrm{C}(1)$ and $\mathrm{C}(3)$ is $20.2 \mathrm{~Hz}$. The $J$ between $\mathrm{C}(2)$ and $\mathrm{C}(3)$ is about $7.5 \mathrm{~Hz}$. Calculate the $s$ character of the $\mathrm{C}(1)-\mathrm{C}(3)$ and $\mathrm{C}(2)-\mathrm{C}(3)$ bonds.
spiropentane

Anish Wadhwa
Anish Wadhwa
Numerade Educator
00:43

Problem 19

Predict whether the following gas phase reactions will be thermodynamically favorable or unfavorable. Explain your answer.
a. $\mathrm{CH}_{4}+-<\mathrm{CH}_{3}{ }^{-}+\cdots<$
b. $\mathrm{CH}_{4}+\mathrm{N} \equiv \mathrm{CCH}_{2}^{+} \rightleftharpoons \mathrm{CH}_{3}^{+}+\mathrm{N} \equiv \mathrm{CCH}_{3}$
c. $\mathrm{CHF}_{3}+\mathrm{CH}_{3}{ }^{-} \rightleftharpoons \mathrm{CF}_{3}{ }^{-}+\mathrm{CH}_{4}$

Mishal Gul
Mishal Gul
Numerade Educator
03:19

Problem 20

O. For each reaction, predict which compound would react faster $(k)$ or give the more complete $(K)$ reaction. Explain the basis for your prediction.
a.
b.
<smiles>NC(=O)c1ccccc1</smiles>
Ph
c.

Raghvendra Singh
Raghvendra Singh
Numerade Educator
02:57

Problem 21

Computational comparison of the structures of the benzyl cation (A) and singlet phenyl carbene (B) indicate a much greater degree of double-bond character for the exocyclic bond in $\mathbf{A}$ than in $\mathbf{B}$. Provide a rationale for this difference, both in $\mathrm{VB}$ and $\mathrm{MO}$ terminology.

Ahmed Ali
Ahmed Ali
Numerade Educator
06:40

Problem 22

Interesting stability and structural trends have been found with $\mathrm{MP} 2 / 6-31 \mathrm{G}^{*}$ calculations on substituted imines. The data below give the $\Delta E$ for the isodesmic reaction:
$$
\mathrm{YX}-\mathrm{N}=\mathrm{CH}_{2}+\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2} \longrightarrow \mathrm{CH}_{3} \mathrm{~N}=\mathrm{CH}_{2}+\mathrm{YX}-\mathrm{CH}=\mathrm{CH}_{2}
$$
The data suggest that $\Delta E$ increases with $\chi_{\mathrm{XY}}$, the group electronegativity of the substituent. The $\mathrm{X}-\mathrm{N}=\mathrm{CH}_{2}$ bond angle also decreases with $\chi_{\mathrm{AB}}$. The NPA

Chase Crook
Chase Crook
Numerade Educator
10:53

Problem 23

Table 1.P23a shows NPA charges for planar and twisted (C-N rotation) for formamide, 3 -aminoacrolein and squaramide. Table $1 . \mathrm{P} 23 \mathrm{~b}$ gives computed ${ }^{17} \mathrm{O}$ chemical shifts for the planar and twisted forms. Figures 1.P23A-F are maps showing the potential for interaction with a particle of charge $+0.5 e$. The barriers

Katie Mcalpine
Katie Mcalpine
Numerade Educator