Chapter 26

Predict which one of the following pairs of compounds will be the stronger acid. Give your reasoning. a. 3-nitrobenzenol or 4 -nitrobenzenol b. 3,5 -dimethyl-4-nitrobezenol or 2,6 -dimethyl-4-nitrobenzenol c. 4 -methoxybenzenol or 3-methoxybenzenol d. azabenzen-4-ol or azabenzen-3-ol

Treatment of 1,3-benzenediol (resorcinol) with an ammonia-ammonium chloride solution under pressure at \(200^{\circ}\) (no sulfite) gives 3 -aminobenzenol. Write a reasonable mechanism for this transformation. Would you expect benzenol itself to react similarly? Why?

Explain why benzenol with bromine gives tribromobenzenol readily in water solution but 2- and 4monobromobenzenol in nonpolar solvents. Notice that \(2,4,6\) -tribromobenzenol is at least a 300 -fold stronger acid than phenol in water solution.

The herbicide "2,4-D" is (2,4-dichlorophenoxy)ethanoic. Show how this substance could be synthesized starting from benzenol and ethanoic acid.

1,3-Benzenediol (resorcinol) can be converted to a carboxylic acid with carbon dioxide and alkali. Would you expect 1,3-benzenediol to react more, or less, readily than benzenol? Why? Which is the most likely point of monosubstitution? Explain.

Explain how you would use proton NMR spectra to show that the product of oxidation of \(2,4,6\) -tri-tert-butylbenzenol in the presence of butadiene links the aromatic rings at the 4 -position, not at the 2 -position.

Work out the course of hydrolysis and decarboxylation of \(2,4,6\) -triaminobenzoic acid to \(1,3,5\) -benzenetriol.

Reduction of the carbonyl group of quercetin gives a substance that loses water readily to give a brilliantviolet compound. This compound, when treated with hydrochloric acid, is converted reversibly to a red salt. Consider possible ways that the reduction product could dehydrate to give a violet substance and show how addition of a proton to it could occur in such a way as to give a substantial color change.

Natural usnic acid is optically active but racemizes on heating without need for acid or base catalysts. Write a mechanism involving a reversible electrocyclic reaction for this racemization that also accounts for the fact that when usnic acid is heated in ethanol, an optically inactive ethyl ester of a carboxylic acid is formed.

Acidification of a solution containing semiquinone radicals such as 1 tends to cause the radicals to disproportionate to the arenediol and arenedione. Why should acid cause changes in the relative stabilities of the semiquinones and the corresponding diol-dione pairs?

You will see that the natural quinones, vitamin \(\mathrm{K}_{1}\), plastoquinone, and \(\mathrm{CoQ}\), all have three or four groups on the quinone ring. What kind of possible destructive side reactions would ring substituents tend to prevent? Give your reasoning.

Explain why 9-phenylfluorene is a stronger acid than triphenylmethane.

a. Why should 3-phenyl-1-propene be appreciably more reactive than methylbenzene in hydrogen-abstraction reactions? b. Would you expect 1 -phenyl-1-propene \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CHCH}_{3}\right)\) to be more, or less, reactive than 3-phenyl-1-propene \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\right)\) if account is taken of the stabilization of the ground state as well as the stabilization of the radicals?

The ionization constants of 3- and 4-cyanobenzoic acids at \(30^{\circ}\) are \(2.51 \times 10^{-4}\) and \(2.82 \times 10^{-4}\), respectively. Benzoic acid has \(K_{a}\) of \(6.76 \times 10^{-5}\) at \(30^{\circ} .\) Calculate \(\sigma_{\text {meta }}\) and \(\sigma_{\text {para }}\) for the cyano substituent.

The magnitudes and signs of the \(\sigma\) constants associated with meta and para substituents can be rationalized in terms of inductive and electron- delocalization influences. Show how it is possible, within this framework, to account for the following facts: a. Fluorine has a sizable positive \(\sigma\) constant when meta but almost zero when para. b. The \(\sigma\) constant of the methoxy group \(\left(-\mathrm{OCH}_{3}\right)\) is positive in the meta position an negative in the para position. c. The \(-\stackrel{\oplus}{\mathrm{N}}\left(\mathrm{CH}_{3}\right)_{3}\) group has a slightly larger positive \(\sigma\) constant in the meta position than in the para position, but the reverse is true for the \(-\mathrm{N}_{2}^{+}\) group. d.* The \(\sigma\) constant of the \(-\mathrm{CF}_{3}\) group is more positive when para than when meta.

Predict whether the meta and para \(\sigma\) constants for the following groups would be positive or negative, and large or small. Give your reasoning. a. \(-C \equiv N\) b. \(-\mathrm{CH}_{2} \stackrel{\oplus}{\mathrm{N}}\left(\mathrm{CH}_{3}\right)_{3}\) c. \(-\mathrm{OCF}_{2} \mathrm{H}\) d. \(-\mathrm{CO}_{2}^{-}\)

The \(\rho\) constant for the ionization of benzoic acid is \(1.000\) for water solutions at \(25^{\circ}\). Would you expect \(\rho\) for acid ionization to increase, or decrease, in going to a less polar solvent such as methanol? Explain.

For each of the following pairs of compounds give a chemical test, preferably a test-tube reaction, that will distinguish between the two compounds. Write a structural formula for each compound and equations for the reactions involved. a. benzenol and cyclohexanol b. methyl 4-hydroxybenzoate and 4-methoxybenzoic acid c. 1,4 - and 1,3-benzenediol d. 1,4 -benzenediol and tropolone e. 9,10 -anthracenedione and 1,4 -anthracenedione

Write structural formulas for substances (one for each part) that fit the following descriptions: a. an arenol that would be a stronger acid than benzenol itself b. the dichlorobenzenol isomer that is the strongest acid c. the Claisen rearrangement product from 1,3-dimethyl-2-(1-methyl-2-propenyloxy)benzene d. a quinone that would not undergo Diels-Alder addition e. a quinone that would be a better charge-transfer agent than 1,4 -benzenedione f. the expected product from addition of hydrogen cyanide to 2 -cyano-1,4-benzenedione

Predict the positions to which coupling would occur (or whether coupling would occur at all) with benzenediazonium chloride in slightly alkaline solution for the following compounds. Give your reasoning. a. \(2,4,6\) -trimethylbenzenol b. 2-naphthalenol c. 1 -methyl-2-naphthalenol d. 9-phenanthrenol

When 2-hydroxybenzoic acid (salicylic acid) is treated with excess bromine in aqueous solution, it forms 2,4,6-tribromobenzenol. Write a reasonable mechanism for this reaction. Would you expect the same type of reaction to occur with 3-hydroxybenzoic acid?

The important polymer intermediate "bis-phenol A" [2,2-bis-(4-hydroxyphenyl)propane] used, among other things, in epoxy resins, is made by an acid-induced condensation of 2 -propanone and benzenol. Write a stepwise mechanism for this reaction that is consistent with the nature of the reactants and the products. (Review Section \(15-4 \mathrm{E}\) on electrophilic reactions of carbonyl compounds, Section \(22-4 \mathrm{E}\), and Section 26-1E.)

Make an atomic-orbital model of benzenol, showing in detail the orbitals and electrons at the oxygen atom. From your model, would you expect one, or both, pairs of unshared electrons on oxygen to be delocalized over the ring? What would be the most favorable orientation of the hydrogen of the hydroxyl group for maximum delocalization of an unshared electron pair?