Chapter 15

What type of infrared absorption bands due to hydroxyl groups would you expect for trans-cyclobutane-1,2-diol and butane-1,2-diol (a) in very dilute solution, (b) in moderately concentrated solution, and (c) as pure liquids? Give your reasoning.

Pure, dry ethanol has a triplet NMR resonance for its OH proton and a quintet resonance for its \(\mathrm{CH}_{2}\) protons. If \(5 \%\) by weight of water is added to the ethanol, a new single peak is observed about \(0.8\) ppm upfield of the ethanol OH triplet. If \(30 \%\) by weight of water is added, there is only a single large OH resonance, and the \(\mathrm{CH}_{2}\) resonance becomes a quartet. Explain the changes produced in the NMR spectrum by adding water.

What order of basicity would you predict for water, methanol, isopropyl alcohol, and tert-butyl alcohol in the gas phase? Give your reasoning.

An alternative and plausible mechanism for esterification of carboxylic acids is shown by the following steps: This mechanism corresponds to an \(S_{\mathrm{N}} 2\) displacement of water from the methyloxonium ion by the acid. How could you distinguish between this mechanism and the addition-elimination mechanism using heavy oxygen \(\left({ }^{18} \mathrm{O}\right)\) as a tracer?

Formation of tert-butyl ethanoate by direct esterification goes very poorly: Explain why the reaction fails, and indicate the products you actually expect to form on heating a mixture of ethanoic acid and tert-butyl alcohol with sulfuric acid as a catalyst.

A suitable method of preparing tert-butyl esters is to add the carboxylic acid to 2-methylpropene. Good yields can be obtained if a strong acid catalyst is used, if water is excluded, and if the temperature is kept low: Write a mechanism for the reaction that accounts for the need for a strong acid catalyst, and why anhydrous conditions and low temperatures are necessary.

The diethyl ester of cis-butenedioic acid can be prepared by heating the corresponding anhydride with ethanol and concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) in benzene in a mole ratio of perhaps \(1: 2.5: 0.25 .\) Write the steps that occur in this reaction and explain how the use of benzene and more than a catalytic amount of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) makes the formation of the diethyl ester thermodynamically more favorable than with just a catalytic amount of \(\mathrm{H}_{2} \mathrm{SO}_{4}\).

Hemiacetal formation is catalyzed by both acids and bases, but acetal formation is catalyzed only by acids. Write the steps involved in the formation of 1 -methoxyethanol from ethanal in methanol containing sodium methoxide: $$ \mathrm{CH}_{3} \mathrm{CHO}+\mathrm{CH}_{3} \mathrm{OH} \stackrel{\mathrm{NaOCH}_{3}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{OCH}_{3} $$ Explain why 1,1-dimethoxymethane cannot be prepared from ethanal and methanol with a basic catalyst.

Ketals are not always capable of being made in practical yields by the direct reaction of alcohols with ketones because of unfavorable equilibria. Satisfactory preparations of \(\mathrm{RO}-\mathrm{C}-\mathrm{OR}\) with \(\mathrm{R}=\) methyl or ethyl are possible through the reactions of ketones with trimethoxy- or triethoxymethane. This process requires an acid catalyst: Write the mechanistic steps involved in this acid-induced methoxy exchange reaction.

Write the steps that could plausibly take place in the reaction of a primary alcohol with phosphorus tribromide in the presence of the weak base pyridine to give an alkyl bromide.

Methylmagnesium iodide with 2-butenal gives an addition product that, when hydrolyzed with dilute sulfuric acid and extracted with ether, yields an ether solution of impure 3 -penten-2-ol. Attempted purification by distillation of the ether extract gives only 1,3-pentadiene and di(1-methyl-2-butenyl) ether. Write equations for each of the reactions involved. How could you avoid ether and diene formation in the preparation of 3-penten-2-ol by this method?

What are the expected products on heating the following alcohols with strong sulfuric acid? Give your reasoning. a. cyclohexanol b. a mixture of methanol and 1 -methylcyclohexanol c. \(\left(\mathrm{CH}_{2}\right)_{2} \mathrm{C}=\mathrm{CHCH}_{2} \mathrm{OH}\)

It is possible to prepare amides from tertiary alcohols and alkanenitriles, \(\mathrm{RCN}\), in concentrated sulfuric acid as the catalyst (Ritter reaction), as illustrated in the equation for the synthesis of N-tert- butylethanamide: $$ \left(\mathrm{CH}_{3}\right)_{2} \mathrm{COH}+\mathrm{CH}_{3} \mathrm{CN} \stackrel{\mathrm{H}_{2} \mathrm{SO}_{4}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{CONHC}\left(\mathrm{CH}_{3}\right)_{3} $$ Show the steps in the mechanism of this reaction, given the fact that nitrilium ions of structure \(\mathrm{R}-\mathrm{C} \equiv \stackrel{\oplus}{\mathrm{N}}-\mathrm{R}\) are likely to be intermediates.

In the conversion of 2-propanol to 2-propanone with chromic acid, which is the redox step, esterification or elimination? What is the change in oxidation level of carbon in this reaction?

Show the mechanistic steps you expect to be involved in the oxidation of benzenecarbaldehyde (benzaldehyde) to benzenecarboxylic (benzoic) acid in an alkaline solution of potassium permanganate.

Explain why oxidation of secondary alcohols with \({ }^{18} \mathrm{O}\) -labeled potassium permanganate produces an \({ }^{18} \mathrm{O}\) containing ketone in acidic solution, but not in basic solution.

Predict the products likely to be formed on cleavage of the following ethers with hydroiodic acid: a. \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}_{3}\) b. \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}=\mathrm{CH}_{2}\) c. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2}-\mathrm{O}-\mathrm{CH}_{3}\) d. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COCH}_{3}\)

a. Explain why trimethyloxonium salts will convert alcohols to methyl ethers at neutral or acidic \(\mathrm{pH}\), whereas either methyl iodide or dimethyl sulfate requires strongly basic reaction conditions. b. What products would you expect from the reactions of trimethyloxonium fluoroborate with (1) ethanethiol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{SH}\), (2) diethylamine, \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{NH}\), and (3) hydrogen bromide.

Draw structures for the following compounds and name each as an oxa-, aza-, or thiacycloalkane (cycloalkene, cycloalkadiene, and so on, as appropriate). a. aziridine b. thiirane c. oxetan-2-one d. 1,3 -diazole e. \(1,3,5\) -trioxan f. 3-phenyloxolane g. perhydroazepine

Either tert-butyl alcohol or 2-methylpropene treated with strong sulfuric acid and hydrogen peroxide \(\left(\mathrm{H}_{2} \mathrm{O}_{2}\right)\) gives a mixture of two reasonably stable liquid compounds (A and B), the ratio of which depends on whether the hydrogen peroxide or organic starting material is in excess. The molecular formula of A is \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}_{2}\), whereas \(\mathrm{B}\) is \(\mathrm{C}_{8} \mathrm{H}_{18} \mathrm{O}_{2}\). Treatment of A and B with hydrogen over a nickel catalyst results in quantitative conversion of each compound to tert-butyl alcohol. A reacts with acyl halides and anhydrides, whereas \(\mathrm{B}\) is unaffected by these reagents. Treatment of \(1 \mathrm{~mole}\) of A with excess methylmagnesium iodide in diethyl ether solution produces 1 mole of methane and 1 mole each of tert-butyl alcohol and methanol. One mole of \(\mathrm{B}\) with excess methylmangesium iodide produces 1 mole of 2 -methoxy-2-methylpropene and 1 mole of tert-butyl alcohol. When \(\mathrm{B}\) is heated with chloroethane, it causes chloroethane to polymerize. When \(\mathrm{B}\) is heated alone, it yields 2 -propanone and ethane, and if heated in the presence of oxygen, it forms methanol, 2 -propanone, methanal, and water. Determine the structure of A an \(\mathrm{dB}\) and write equations for all reactions involved, showing the mechanisms and intermediates that are important for each. Write at least one structure for A and for B that is isomeric with your preferred structures and show how these substances would behave in each of the given reactions.

The reaction of methyl ethanoate with water to give methanol and ethanoic acid is catalyzed by strong mineral acids such as sulfuric acid. Furthermore, when hydrolysis is carried out in water enriched in the rare oxygen isotope, \({ }^{18} \mathrm{O}\), the following exchange takes place faster than formation of methanol: No methanol- \({ }^{18} \mathrm{O}\left(\mathrm{CH}_{3}{\underline{\phantom{xx}}}^{18} \mathrm{OH}\right)\) is formed in hydrolysis under these conditions. a. Write a stepwise mechanism that is in harmony with the acid catalysis and with the results obtained in \({ }^{18} \mathrm{O}\) water. Mark the steps of the reaction that are indicated to be fast or slow. b. The reaction depends on methyl ethanoate having a proton-accepting ability comparable to that of water. Why? Consider different ways of adding a proton to methyl ethanoate and decide which is most favorable on the basis of structural theory. Give your reasoning. c. Explain why the reaction is slowed down in the presence of very high concentrations of sulfuric acid.

Write a mechanism for the reaction of trans-2-butene with trifluoroperoxoethanoic acid to give trans-2,3dimethyloxacyclopropane that is consistent with the fact that the reaction is first order in each participant and gives suprafacial addition.

How would you expect the fraction of elimination toward the methyl groups, as opposed to elimination toward the methylene group, to compare in \(E 1\) and \(E 2\) reactions of 2 -chloro-2-methylbutane and the corresponding deuterium- labeled chloride, 2 -chloro-2-methylbutane-3-D \(_{2}\) ? Give your reasoning.

Triethyloxonium fluoroborate can be prepared from 1-chloromethyloxacyclopropane and a \(\mathrm{BF}_{3}\) -etherate according to the equation The boron in the complex boron anion ends up as \(\mathrm{BF}_{4}^{-}\), but the details of this reaction need not concern you. Write the steps that you expect to be involved in the reaction to form \(\mathrm{R}_{3} \mathrm{O}^{\oplus}\) and that you can support by analogy with other reactions discussed in this chapter.

Support your explanation of each of the following facts by reasoning based on mechanistic considerations: a. \(D\) -1-Phenylethanol reacts with thionyl chloride, \(\mathrm{SOCl}_{2}\), in pyridine to give \(L\) - 1 -phenylethyl chloride by way of an intermediate chlorosulfite ester, b. 2 -Buten-1-ol and \(\mathrm{SOCl}_{2}\) in ether and a one-molar equivalent of tributylamine gives 1 -chloro-2-butene. In the absence of the base, the rearrangement product, 3-chloro-1-butene, is obtained.

1,2-Ethanediol (ethylene glycol) is a familiar "antifreeze". However, it also is used in automotive cooling systems in climates that rarely, if ever, reach temperatures at which water would freeze. What other function, as important as lowering the freezing point, does the diol serve when added to automotive cooling systems?