17 • Additional Problems
17 • Additional Problems
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.
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.
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.
Carvacrol is a naturally occurring substance isolated from oregano, thyme, and marjoram. What is its IUPAC name?
Reactions of Alcohols
The following 1H NMR spectrum is that of an alcohol, C8H10O. Propose a structure.
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
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.
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 SN2 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.
p-Nitrophenol and 2,6-dimethyl-4-nitrophenol both have pKa = 7.15, but 3,5-dimethyl-4-nitrophenol has pKa = 8.25. Why is 3,5-dimethyl-4-nitrophenol so much less acidic?
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)
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.
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.
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.