Chapter 9, Aromatic Substitution Video Solutions, Advanced Organic Chemistry. Part A. Structure and Mechanisms

05:54

Problem 1

Predict qualitatively the isomer ratio for nitration of each of the following compounds:

Tom Rutherford

Numerade Educator

03:48

Although $N, N$-dimethylaniline is extremely reactive toward electrophilic aromatic substitution and is readily substituted by weak electrophiles, such as diazonium and nitrosonium ions, this reactivity is greatly diminished by introduction of an alkyl substituent in an ortho position. Explain.

Zubair Abdulla

Numerade Educator

01:01

Problem 3

Toluene is 28 times more reactive than benzene, whereas isopropylbenzene is 14 times more reactive than benzene toward nitration in the organic solvent sulfolane. The $o: m: p$ ratio for toluene is 62:3:35. For isopropylbenzene, the ratio is 43:5:52. Calculate the partial rate factors for each position in toluene and isopropylbenzene. Discuss the significance of the partial rate factors. Compare the reactivity at each position of the molecules, and explain any significant differences.

Narayan Hari

Numerade Educator

06:12

Problem 4

Some bromination rate constants are summarized below. Compare the correlation of the data with both $\sigma$ and $\sigma^{+}$substituent constants. What is the value of $\rho$ ? What information do the results provide about the mechanism of bromination?

Matthew Lueckheide

Numerade Educator

02:38

Problem 5

Compare the product distribution results given below for the alkylation of $p$-xylene at two different temperatures after $2 \mathrm{~h}$. The ratio of aromatic reagent:halide: $\mathrm{AlCl}_{3}$ was 1.0:0.5:0.1.

Ramesh Singh

Numerade Educator

01:54

Problem 6

The table below gives first order-rate constants for the reaction of substituted benzenes with $m$-nitrobenzenesulfonyl peroxide. From these data, calculate the relative reactivity and partial rate factors. Does this reaction fit the pattern of an electrophilic aromatic substitution? If so, does the active electrophile exhibit low, intermediate, or high reactant and position selectivity?

Banhishikha Sinha

Numerade Educator

01:06

Problem 7

Propose a structure for the products of the following reactions:

Lottie Adams

Numerade Educator

03:40

Problem 8

In $100 \% \mathrm{H}_{2} \mathrm{SO}_{4}$ the cyclization shown below occurs. If one of the ortho hydrogens is replaced by deuterium, the rate of cyclization drops from $1.56 \times 10^{-4}$ to $1.38 \times 10^{-4} \mathrm{~s}^{-1}$. Calculate the kinetic isotope effect. The product from such a reaction contains $60 \%$ of the original deuterium. Write a mechanism for this reaction that is consistent with both the magnitude of the kinetic isotope effect and the deuterium retention data.

Allison Krajewski

Numerade Educator

02:41

Problem 9

Reaction of $3,5,5$-trimethylcyclohex-2-en-1-one with $\mathrm{NaNH}_{2}$ ( 3 equiv) in THF generates an enolate. When bromobenzene is added to this solution and stirred for $4 \mathrm{~h}$, a product 9-A is isolated in $30 \%$ yield. Formulate a mechanism for this reaction.

Niamat Khuda

Numerade Educator

03:14

Problem 10

Several phenols can be selectively hydroxymethylated at the ortho position by heating with paraformaldehyde and phenylboronic acid in propanoic acid. An intermediate $\mathbf{1 0}-\mathbf{A}$ having the formula $\mathrm{C}_{14} \mathrm{H}_{13} \mathrm{O}_{2} \mathrm{~B}$ can be isolated in the case of 2-methylphenol. Propose a structure for the intermediate and indicate the role of phenylboronic acid in the reaction.

Ian Kaigh

Numerade Educator

09:19

Problem 11

When compound 11-A is dissolved in $\mathrm{FSO}_{3} \mathrm{H}$ at $-78^{\circ} \mathrm{C}$, the NMR spectrum shows that a carbocation is formed. If the solution is then allowed to warm to $-10^{\circ} \mathrm{C}$, a different carbocation is formed. When the acidic solution is quenched with $15 \% \mathrm{NaOH}$, the first carbocation gives product $11-\mathbf{B}$, whereas the second gives 11-C. What are the likely structures of the two carbocations?

Zubair Abdulla

Numerade Educator

09:19

Problem 12

Zubair Abdulla

Numerade Educator

02:36

Problem 13

Acylation of 1,4 -dimethoxynaphthalene with acetic anhydride (1.2 equiv) and $\mathrm{AlCl}_{3}$ (2.2 equiv) in dichloroethane at $60^{\circ} \mathrm{C}$ leads to two products, as shown below. Suggest a rationalization for the formation of these two products. What might account for the demethylation observed in product 13-B?

Temi Ajayi

Numerade Educator

View

Problem 14

The solvolysis of 4-arylbutyl arenesulfonates in nonnucleophilic media leads to formation of tetralins. Two $\sigma$ intermediates, 14-A and 14-B, are conceivable.

Victor Salazar

Numerade Educator

04:00

Problem 15

The kinetic expression for chlorination of anisole by HOCl given on p. 799 becomes simpler for both less reactive and more reactive reactants. For benzene the expression is
$$
\text { Rate }=k[\text { benzene }][\mathrm{HOCl}]\left[\mathrm{H}^{+}\right]
$$
and for 1,4 -dimethoxybenzene it is
$$
\text { Rate }=k[\mathrm{HOCl}]\left[\mathrm{H}^{+}\right]
$$
Why does the form of the rate expression depend on the reactivity of the aromatic compound? What conclusions can be drawn about the mechanism of chlorination of benzene and 1,4 -dimethoxybenzene under these conditions?

Ian Kaigh

Numerade Educator

02:37

Problem 16

Relative reactivity and product distribution data for nitration of the halobenzenes is given below. Calculate the partial rate factors for each position for each halogen. What insight into the substituent activating/directing effects of the halogens can you draw from this data?
$$
\begin{array}{llccc}
\hline \text { Halogen } & \text { Rel rate } & \text { \%ortho } & \text { \%meta } & \text { \%para } \\
\hline \mathrm{F} & 0.15 & 13 & 0 & 87 \\
\mathrm{Cl} & 0.033 & 30 & 1 & 69 \\
\mathrm{Br} & 0.03 & 37 & 1 & 62 \\
\mathrm{I} & 0.18 & 38 & 2 & 60 \\
\hline
\end{array}
$$

Lottie Adams

Numerade Educator

01:04

Problem 17

Ipso substitution is relatively rare in electrophilic aromatic substitution and was not explicitly covered in Section $9.2$ in the discussion of substituent effects on reactivity and selectivity. Using qualitative concepts, discuss the effect of the following types of substituents on the TS and intermediate for ipso substitution.
a. A $\pi$-donor substituent that is more electronegative than carbon, e.g., methoxy.
b. A $\pi$-acceptor substituent that is more electronegative than carbon, e.g., cyano or nitro.
c. A very polar EWG that does not have $\pi$-conjugation capacity, e.g., $\mathrm{N}^{+}\left(\mathrm{CH}_{3}\right)_{3}$.
d. A group without strong $\pi$-conjugating capacity that is less electronegative than carbon, e.g., $\mathrm{Si}\left(\mathrm{CH}_{3}\right)_{3}$.

Grigoriy Sereda

Numerade Educator

04:22

Problem 18

The nitration of $2,4,6$-tris- $(t$-butyl)toluene gives rise to three products. The product distribution changes when the 3-position and the 5-position are deuterated, as shown by the data below. Indicate a mechanism for formation of each product. Show why the isotopic labeling results in a change in product composition. Calculate the isotope effect. Does this appear to be a primary isotope effect? Is an isotope effect of this magnitude consistent with your proposed mechanism?

Mercedes Mazza

Numerade Educator

01:35

Problem 19

Analyze the results of the studies of intramolecular electrophilic substitution that are described below. Write mechanisms for each of the cyclizations and comment on the relation between ring size and the outcome of cyclization.

Aadit Sharma

Numerade Educator

02:09

Problem 20

Explain the outcome of the following reactions by a mechanism showing how the product could be formed.
a. 2,6-Di- $(t$-butyl)phenoxide reacts with $o$-nitroaryl halides in $\mathrm{NaOH} / \mathrm{DMSO}$ at $80^{\circ} \mathrm{C}$ to give 2,6 -di- $(t$-butyl)-4-(2-nitrophenyl)phenol in $60-90 \%$ yield. Under similar conditions, 1,4-dinitrobenzene gives 2,6-di- $(t$-butyl)-4(4-nitrophenyl)phenol.
b. 2 -(3-Chlorophenyl)-4,4-dimethyloxazoline reacts with alkyllithium reagents to give 2-(2-alkylphenyl)-4,4-dimethyloxazolines.
c. Nitrobenzene reacts with cyanomethyl phenyl sulfide in $\mathrm{NaOH} / \mathrm{DMSO}$ to give a mixture of 2- and 4-nitrophenylacetonitrile.
d. The following transformation occurs:
e. Reaction of benzene with 3,3,3-trifluoropropene in the presence of $\mathrm{BF}_{3}$ gives 3,3,3-trifluoropropylbenzene.
f. 3-Chloronitrobenzene reacts with 4-amino-1,2,4-triazole in $\mathrm{K}^{+} .{ }^{-} \mathrm{O}-t-\mathrm{Bu} /$ DMSO to give 2-chloro-4-nitroaniline.
g. Good yields of tetralone can be obtained from 4-phenylbutanoic acid or the corresponding acyl chloride in the presence of the strongly acidic resin Nafion-H. With 3-phenylpropanonic acid, only the acyl chloride gives a cyclization product.

Alkendra Singh

Numerade Educator

01:56

Problem 21

Reaction of several 3-bromobenzoic acids with excess LDA at $-70^{\circ} \mathrm{C}$, followed by addition of benzyl cyanide and warming, gives the product mixtures shown below. Suggest a mechanism for formation of products 21-A and 21-B under these conditions.
$$
\begin{array}{llccc}
\mathrm{X} & \mathrm{Ar} & \text { 21-A } & \text { 21-B } & \text { 21-C } \\
\hline 4-\mathrm{CH}_{3} \mathrm{O} & \mathrm{Ph} & 56 & 9 & 11 \\
4-\mathrm{CH}_{3} \mathrm{O} & 4-\mathrm{CH}_{3} \mathrm{Ph} & 70 & 8 & 12 \\
4-\mathrm{CH}_{3} \mathrm{O} & 2-\mathrm{CH}_{3} \mathrm{Ph} & 44 & 5 & 10 \\
4-\mathrm{CH}_{3} & \mathrm{Ph} & 53 & <2 & 7 \\
4-\mathrm{CH}_{3} & 4-\mathrm{CH}_{3} \mathrm{Ph} & 43 & <2 & 8
\end{array}
$$

Bryce Werts

Numerade Educator