22 • Additional Problems
22 • Additional Problems
Unlike most β-diketones, the following β-diketone has no detectable enol content and is about as acidic as acetone. Explain.
For a given α hydrogen atom to be acidic, the C–H bond must be parallel to the p orbitals of the CO double bond (that is, perpendicular to the plane of the adjacent carbonyl group). Identify the most acidic hydrogen atom in the conformation shown for the following structure. Is it axial or equatorial?
In the Hell–Volhard–Zelinskii reaction, only a catalytic amount of PBr3 is necessary because of the following equilibrium. Propose a mechanism for formation of the equilibrium.
Nonconjugated β,γ-unsaturated ketones, such as 3-cyclohexenone, are in an acid-catalyzed equilibrium with their conjugated α,β-unsaturated isomers. Propose a mechanism for the isomerization.
2-substituted 2-cyclopentenones can be interconverted with 5-substituted 2-cyclopentenones under basic conditions. Propose a mechanism for this isomerization.
Using curved arrows, propose a mechanism for the following reaction, one of the steps in the metabolism of the amino acid alanine.
Using curved arrows, propose a mechanism for the following reaction, one of the steps in the biosynthesis of the amino acid tyrosine.
One of the later steps in glucose biosynthesis is the isomerization of fructose 6-phosphate to glucose 6-phosphate. Propose a mechanism, using acid or base catalysis as needed.
The Favorskii reaction involves treatment of an α-bromo ketone with base to yield a ring-contracted product. For example, reaction of 2-bromocyclohexanone with aqueous NaOH yields cyclopentanecarboxylic acid. Propose a mechanism.
Treatment of a cyclic ketone with diazomethane is a method for accomplishing a ring-expansion reaction. For example, treatment of cyclohexanone with diazomethane yields cycloheptanone. Propose a mechanism.
The final step in an attempted synthesis of laurene, a hydrocarbon isolated from the marine alga Laurencia glandulifera, involved the Wittig reaction shown. The product obtained, however, was not laurene but an isomer. Propose a mechanism to account for the unexpected results.
Amino acids can be prepared by reaction of alkyl halides with diethyl acetamidomalonate, followed by heating the initial alkylation product with aqueous HCl. Show how you would prepare alanine, CH3CH(NH2)CO2H, one of the twenty amino acids found in proteins, and propose a mechanism for acid-catalyzed conversion of the initial alkylation product to the amino acid.
Heating carvone with aqueous sulfuric acid converts it into carvacrol. Propose a mechanism for the isomerization.
Acidity of Carbonyl Compounds
Rank the following compounds in order of increasing acidity:
(a) (b) (c) (d) (e) (f)
Base treatment of the following α,β-unsaturated carbonyl compound yields an anion by removal of H+ from the γ carbon. Why are hydrogens on the γ carbon atom acidic?
Treatment of 1-phenyl-2-propenone with a strong base such as LDA does not yield an anion, even though it contains a hydrogen on the carbon atom next to the carbonyl group. Explain.
Which, if any, of the following compounds can be prepared by a malonic ester synthesis? Show the alkyl halide you would use in each case.
(a) Ethyl pentanoate (b) Ethyl 3-methylbutanoate (c) Ethyl 2-methylbutanoate
(d) Ethyl 2,2-dimethylpropanoate
Which, if any, of the following compounds can be prepared by an acetoacetic ester synthesis? Explain.
(a) (b) (c)
Which of the following substances would undergo the haloform reaction?
(a) CH3COCH3 (b) Acetophenone (c) CH3CH2CHO (d) CH3CO2H (e) CH3CN
How might you convert geraniol into either ethyl geranylacetate or geranylacetone?
Aprobarbital, a barbiturate once used in treating insomnia, is synthesized in three steps from diethyl malonate. Show how you would synthesize the necessary dialkylated intermediate, and then propose a mechanism for the reaction of this intermediate with urea to give aprobarbital.
Fill in the reagents a–c that are missing from the following scheme:
Although 2-substituted 2-cyclopentenones are in a base-catalyzed equilibrium with their 5-substituted 2-cyclopentenone isomers, the analogous isomerization is not observed for 2-substituted 2-cyclohexenones. Explain.
Attempted Grignard reaction of cyclohexanone with tert-butylmagnesium bromide yields only about 1% of the expected addition product along with 99% unreacted cyclohexanone. If D3O+ is added to the reaction mixture after a suitable period, however, the “unreacted” cyclohexanone is found to have one deuterium atom incorporated into it. Explain.
Ketones react slowly with benzeneselenenyl chloride in the presence of HCl to yield α-phenylseleno ketones. Propose a mechanism for this acid-catalyzed α-substitution reaction.
South American Incas chewed the leaves of the coca bush, Erythroxylon coca, to combat fatigue. Chemical studies of Erythroxylon coca by Friedrich Wöhler in 1862 resulted in the discovery of cocaine, C17H21NO4, as the active component. Basic hydrolysis of cocaine leads to methanol, benzoic acid, and another compound called ecgonine, C9H15NO3. Oxidation of ecgonine with CrO3 yields a keto acid that readily loses CO2 on heating, giving tropinone.
The key step in a reported laboratory synthesis of sativene, a hydrocarbon isolated from the mold Helminthosporium sativum, involves the following base treatment of a keto tosylate. What kind of reaction is occurring? How would you complete the synthesis?
Sodium pentothal is a short-acting barbiturate derivative used as a general anesthetic and known in popular culture as a truth serum. It is synthesized like other barbiturates (see the Chemistry Matters at the end of this chapter), using thiourea, (H2N)2CS, in place of urea. How would you synthesize sodium pentothal?