12.13: Additional Problems

17 • Additional Problems 17 • Additional Problems

Visualizing Chemistry

Give IUPAC names for the following compounds: (a)

(b)

(c)

(d)

2, and (3) Dess–Martin periodinane. (a)

(b)

Problem 17-22

Predict the product from reaction of the following substance (reddish brown = Br) with:

(a) PBr₃ (b)

Aqueous H₂SO₄

(c) SOCl₂ (d) Dess–Martin periodinane (e) Br₂, FeBr₃

Predict the product from reaction of the following substance with: (a)

NaBH₄; then H₃O⁺

(b) LiAlH₄; then H₃O⁺ (c) 2 CH₃CH₂MgBr; then H₃O⁺

Name and assign R or S stereochemistry to the product(s) you would obtain by reaction of the following substance with ethylmagnesium bromide. Is the product chiral? Is it optically active? Explain.

Mechanism Problems

Evidence for the intermediate carbocations in the acid-catalyzed dehydration of alcohols comes from the observation that rearrangements sometimes occur. Propose a mechanism to account for the formation of 2,3-dimethyl-2-butene from 3,3-dimethyl-2-butanol.

Acid-catalyzed dehydration of 2,2-dimethylcyclohexanol yields a mixture of 1,2-dimethylcyclohexene and isopropylidenecyclopentane. Propose a mechanism to account for the formation of both products.

Epoxides react with Grignard reagents to yield alcohols. Propose a mechanism.

Treatment of the following epoxide with aqueous acid produces a carbocation intermediate that reacts with water to give a diol product. Show the structure of the carbocation, and propose a mechanism for the second step.

Reduction of 2-butanone with NaBH₄ yields 2-butanol. Is the product chiral? Is it optically active? Explain.

(b)

(c)

N1, S_N2) involved in the conversion of the following alcohols into the corresponding alkyl halide. (a)

(b)

(c)

Problem 17-32

The conversion of 3° alcohols into alkenes under acidic conditions involves two cationic intermediates. For each reaction, draw the complete mechanism using curved arrows. (a)

(b)

(c)

Problem 17-33

For each reaction, write the mechanism using curved arrows for the conversion of the alcohol into the corresponding alkene with POCl₃. In each case, explain the regiochemistry of the elimination. (a)

(b)

(c)

Problem 17-34

The trimethylsilyl (TMS) protecting group is one of several silicon protecting groups for alcohols. For each reaction, draw the mechanism for the protection of (R)-3-bromo-1-butanol with the following silyl chlorides, using triethylamine as the base: (a)

tert-butyldimethylsilyl chloride (TBS-Cl)

(b)

triisopropylsilyl chloride (TIPS-Cl)

(c) triethylsilyl chloride (TES-Cl)

When the following alcohol is treated with POCl₃ and pyridine, the expected elimination product is formed. However, when the same alcohol is treated with H₂SO₄, the elimination product is 1,2-dimethylcyclopentene. Propose a mechanism for each pathway to account for these differences.

K_a’s than alcohols because of resonance stabilization with the aromatic ring. Draw all of the resonance contributors for the following phenolate ions. (a)

(b)

(c)

Naming Alcohols

(b)

(c)

(d)

(e)

(f)

Problem 17-38

Draw and name the eight isomeric alcohols with formula C₅H₁₂O.

Trans-3-Chlorocycloheptanol

(b)

2-Ethyl-2-buten-1-ol

(c) o-(2-Hydroxyethyl)phenol (d) 3-Methyl-1-phenyl-1-butanol

Bombykol, the sex pheromone secreted by the female silkworm moth has the formula C₁₆H₂₈O and the systematic name (10E,12Z)-10,12-hexadecadien-1-ol. Draw bombykol, showing the correct geometry for the two double bonds.

Carvacrol is a naturally occurring substance isolated from oregano, thyme, and marjoram. What is its IUPAC name?

Synthesizing Alcohols

(b)

(c)

(d)

(e)

(f)

(b)

(c)

Problem 17-44

What carbonyl compounds might you start with to prepare the following compounds by Grignard reaction? List all possibilities. (a)

2-Methyl-2-propanol

(b)

1-Ethylcyclohexanol

(c) 3-Phenyl-3-pentanol (d) 2-Phenyl-2-pentanol (e)

(f)

(b)

(c)

(d)

Reactions of Alcohols

PBr₃

(b)

SOCl₂

(c) Dess–Martin periodinane

Styrene (PhCH$\text{=}CH$₂)

(b) Phenylacetaldehyde (PhCH₂CHO) (c) Phenylacetic acid (PhCH₂CO₂H) (d) Benzoic acid (e) Ethylbenzene (f) Benzaldehyde (g) 1-Phenylethanol (h) 1-Bromo-2-phenylethane

Acetophenone (PhCOCH₃)

(b) Benzyl alcohol (c) m-Bromobenzoic acid (d) 2-Phenyl-2-propanol

Cyclopentanone

(b) Cyclopentene (c) 1-Methylcyclopentanol (d) trans-2-Methylcyclopentanol

HBr

(b) NaH (c) H₂SO₄

Spectroscopy

The following ¹H NMR spectrum is that of an alcohol, C₈H₁₀O. Propose a structure.

1H NMR spectra: (a)

C₅H₁₂O

(b)

C₈H₁₀O

Propose a structure consistent with the following spectral data for a compound C₈H₁₈O₂:

• IR: 3350 cm^–1
• ¹H NMR: 1.24 δ (12 H, singlet); 1.56 δ (4 H, singlet); 1.95 δ (2 H, singlet)

The ¹H NMR spectrum shown is that of 3-methyl-3-buten-1-ol. Assign all the observed resonance peaks to specific protons, and account for the splitting patterns.

A compound of unknown structure gave the following spectroscopic data:

• Mass spectrum: M⁺ = 88.1
• IR: 3600 cm^–1
• ¹H NMR: 1.4 δ (2 H, quartet, J = 7 Hz); 1.2 δ (6 H, singlet); 1.0 δ (1 H, singlet); 0.9 δ (3 H, triplet, J = 7 Hz)
• ¹³C NMR: 74, 35, 27, 25 δ
(a) Assuming that the compound contains C and H but may or may not contain O, give three possible molecular formulas. (b) How many hydrogens does the compound contain? (c) What functional group(s) does the compound contain? (d) How many carbons does the compound contain? (e) What is the molecular formula of the compound? (f) What is the structure of the compound? (g) Assign peaks in the molecule’s ¹H NMR spectrum corresponding to specific protons.

Propose a structure for a compound C₁₅H₂₄O that has the following ¹H NMR spectrum. The peak marked by an asterisk disappears when D₂O is added to the sample.



General Problems

(b)

(c)



Benzoquinone is an excellent dienophile in the Diels–Alder reaction. What product would you expect from reaction of benzoquinone with 1 equivalent of 1,3-butadiene? From reaction with 2 equivalents of 1,3-butadiene?

Rank the following substituted phenols in order of increasing acidity, and explain your answer:



Benzyl chloride can be converted into benzaldehyde by treatment with nitromethane and base. The reaction involves initial conversion of nitromethane into its anion, followed by S_N2 reaction of the anion with benzyl chloride and subsequent E2 reaction. Write the mechanism in detail, using curved arrows to indicate the electron flow in each step.



Reaction of (S)-3-methyl-2-pentanone with methylmagnesium bromide followed by acidification yields 2,3-dimethyl-2-pentanol. What is the stereochemistry of the product? Is the product optically active?



Testosterone is one of the most important male steroid hormones. When testosterone is dehydrated by treatment with acid, rearrangement occurs to yield the product shown. Propose a mechanism to account for this reaction.



(b)

(c)

(d)



Problem 17-64

p-Nitrophenol and 2,6-dimethyl-4-nitrophenol both have pK_a = 7.15, but 3,5-dimethyl-4-nitrophenol has pK_a = 8.25. Why is 3,5-dimethyl-4-nitrophenol so much less acidic?



Compound A, C₁₀H₁₈O, undergoes reaction with dilute H₂SO₄ at 25 °C to yield a mixture of two alkenes, C₁₀H₁₆. The major alkene product, B, gives only cyclopentanone after ozone treatment followed by reduction with zinc in acetic acid. Write the reactions involved, and identify A and B.

Compound A, C₅H₁₀O, is one of the basic building blocks of nature. All steroids and many other naturally occurring compounds are built from compound A. Spectroscopic analysis of A yields the following information:

• IR: 3400 cm^–1; 1640 cm^–1
• ¹H NMR: 1.63 δ (3 H, singlet); 1.70 δ (3 H, singlet); 3.83 δ (1 H, broad singlet); 4.15 δ (2 H, doublet, J = 7 Hz); 5.70 δ (1 H, triplet, J = 7 Hz)
(a)

How many double bonds and/or rings does A have?
(b)
From the IR spectrum, what is the identity of the oxygen-containing functional group? (c) What kinds of hydrogens are responsible for the NMR absorptions listed? (d) Propose a structure for A.

Dehydration of trans-2-methylcyclopentanol with POCl₃ in pyridine yields predominantly 3-methylcyclopentene. Is the stereochemistry of this dehydration syn or anti?

2,3-Dimethyl-2,3-butanediol has the common name pinacol. On heating with aqueous acid, pinacol rearranges to pinacolone, 3,3-dimethyl-2-butanone. Suggest a mechanism for this reaction.



As a rule, axial alcohols oxidize somewhat faster than equatorial alcohols. Which would you expect to oxidize faster, cis-4-tert-butylcyclohexanol or trans-4-tert-butylcyclohexanol? Draw the more stable chair conformation of each molecule.

Propose a synthesis of bicyclohexylidene, starting from cyclohexanone as the only source of carbon.



A problem often encountered in the oxidation of primary alcohols to carboxylic acids is that esters are sometimes produced as by-products. For example, oxidation of ethanol yields acetic acid and ethyl acetate:



Propose a mechanism to account for the formation of ethyl acetate. Take into account the reversible reaction between aldehydes and alcohols:



Identify the reagents a–f in the following scheme:



Galactose, a constituent of the disaccharide lactose found in dairy products, is metabolized by a pathway that includes the isomerization of UDP-galactose to UDP-glucose, where UDP = uridylyl diphosphate. The enzyme responsible for the transformation uses NAD⁺ as cofactor. Propose a mechanism.



1H NMR spectra: (a)

C₉H₁₂O


(b)

C₈H₁₀O₂



Compound A, C₈H₁₀O, has the IR and ¹H NMR spectra shown. Propose a structure consistent with the observed spectra, and label each peak in the NMR spectrum. Note that the absorption at 5.5 δ disappears when D₂O is added.