19 • Additional Problems
19 • Additional Problems
The following molecular model represents a tetrahedral intermediate resulting from addition of a nucleophile to an aldehyde or ketone. Identify the reactants, and write the structure of the final product when the nucleophilic addition reaction is complete.
What is the geometric relationship between the p orbitals of the double bond and the nitrogen orbital that holds the lone pair? Why do you think this geometry represents the minimum energy?
When α-glucose is treated with an acid catalyst in the presence of an alcohol, an acetal is formed. Propose a mechanism for this process and give the structure of the stereoisomeric acetal that you would also expect as a product.
One of the steps in the metabolism of fats is the reaction of an unsaturated acyl CoA with water to give a β-hydroxyacyl CoA. Propose a mechanism.
Aldehydes and ketones react with thiols to yield thioacetals just as they react with alcohols to yield acetals. Predict the product of the following reaction, and propose a mechanism:
Ketones react with dimethylsulfonium methylide to yield epoxides. Suggest a mechanism for the reaction.
Propose a mechanism for the following reaction.
Paraldehyde, a sedative and hypnotic agent, is prepared by treatment of acetaldehyde with an acidic catalyst. Propose a mechanism for the reaction.
The Meerwein–Ponndorf–Verley reaction involves reduction of a ketone by treatment with an excess of aluminum triisopropoxide, [(CH3)2CHO]3Al. The mechanism of the process is closely related to the Cannizzaro reaction in that a hydride ion acts as a leaving group. Propose a mechanism.
Propose a mechanism to account for the formation of 3,5-dimethylpyrazole from hydrazine and 2,4-pentanedione. What has happened to each carbonyl carbon in going from starting material to product.
In light of your answer to Problem 19-45, propose a mechanism for the formation of 3,5-dimethylisoxazole from hydroxylamine and 2,4-pentanedione.
Trans alkenes are converted into their cis isomers and vice versa on epoxidation followed by treatment of the epoxide with triphenylphosphine. Propose a mechanism for the reaction.
Treatment of an α,β-unsaturated ketone with basic aqueous hydrogen peroxide yields an epoxy ketone. The reaction is specific to unsaturated ketones; isolated alkene double bonds do not react. Propose a mechanism.
One of the biological pathways by which an amine is converted to a ketone involves two steps: (1) oxidation of the amine by NAD+ to give an imine and (2) hydrolysis of the imine to give a ketone plus ammonia. Glutamate, for instance, is converted by this process into α-ketoglutarate. Show the structure of the imine intermediate, and propose mechanisms for both steps.
Primary amines react with esters to yield amides: RCO2R′ + R″NH2 → RCONHR″ + R′OH. Propose a mechanism for the following reaction of an α,β-unsaturated ester.
When crystals of pure α-glucose are dissolved in water, isomerization occurs slowly to produce β-glucose. Propose a mechanism for the isomerization.
The Wharton reaction converts an epoxy ketone to an allylic alcohol by reaction with hydrazine. Review the Wolff–Kishner reaction in Section 19.9 and then propose a mechanism.
Naming Aldehydes and Ketones
Reactions of Aldehydes and Ketones
Compound A, C8H10O2, has an intense IR absorption at 1750 cm–1 and gives the 13C NMR spectrum shown. Propose a structure for A.
When 4-hydroxybutanal is treated with methanol in the presence of an acid catalyst, 2-methoxytetrahydrofuran is formed. Explain.
The amino acid methionine is biosynthesized by a multistep route that includes reaction of an imine of pyridoxal phosphate (PLP) to give an unsaturated imine, which then reacts with cysteine. What kinds of reactions are occurring in the two steps?
6-Methyl-5-hepten-2-one is a constituent of lemongrass oil. How could you synthesize this substance from methyl 4-oxopentanoate?
Tamoxifen is a drug used in the treatment of breast cancer. How would you prepare tamoxifen from benzene, the following ketone, and any other reagents needed?
The 1H NMR spectrum shown is that of a compound with the formula C9H10O. How many double bonds and/or rings does this compound contain? If the unknown compound has an IR absorption at 1690 cm–1, what is a likely structure?
The 1H NMR spectrum shown is that of a compound isomeric with the one in Problem 19-80. This isomer has an IR absorption at 1730 cm–1. Propose a structure. [Note: Aldehyde protons (CHO) often show low coupling constants to adjacent hydrogens, so the splitting of aldehyde signals is not always apparent.]
When glucose (Problem 19-51) is treated with NaBH4, reaction occurs to yield sorbitol, a polyalcohol commonly used as a food additive. Show how this reduction occurs.
The proton and carbon NMR spectra for each of three isomeric ketones with the formula C7H14O are shown. Assign a structure to each pair of spectra.
The proton NMR spectrum for a compound with formula C10H12O2 is shown below. The infrared spectrum has a strong band at 1711 cm–1. The broadband-decoupled 13C NMR spectral results are tabulated along with the DEPT-135 and DEPT-90 information. Draw the structure of this compound.