7 • Additional Problems
7 • Additional Problems
The following carbocation is an intermediate in the electrophilic addition reaction of HCl with two different alkenes. Identify both, and tell which C−H bonds in the carbocation are aligned for hyperconjugation with the vacant p orbital on the positively charged carbon.
The following alkyl bromide can be made by HBr addition to three different alkenes. Show their structures.
When 1,3-butadiene reacts with 1 mol of HBr, two isolable products result. Propose mechanisms for both.
When methyl vinyl ether reacts with a strong acid, H+ adds to C2 instead of C1 or the oxygen atom. Explain.
Addition of HCl to 1-isopropylcyclohexene yields a rearranged product. Propose a mechanism, showing the structures of the intermediates and using curved arrows to indicate electron flow in each step.
Addition of HCl to 1-isopropenyl-1-methylcyclopentane yields 1-chloro-1,2,2-trimethylcyclohexane. Propose a mechanism, showing the structures of the intermediates and using curved arrows to indicate electron flow in each step.
Limonene, a fragrant hydrocarbon found in lemons and oranges, is biosynthesized from geranyl diphosphate by the following pathway. Add curved arrows to show the mechanism of each step. Which step involves an alkene electrophilic addition? (The ion OP2O64− is the diphosphate ion, and “Base” is an unspecified base in the enzyme that catalyzes the reaction.)
epi-Aristolochene, a hydrocarbon found in both pepper and tobacco, is biosynthesized by the following pathway. Add curved arrows to show the mechanism of each step. Which steps involve alkene electrophilic addition(s), and which involve carbocation rearrangement(s)? (The abbreviation H—A stands for an unspecified acid, and “Base” is an unspecified base in the enzyme.)
Calculating a Degree of Unsaturation
Loratadine, marketed as an antiallergy medication under the brand name Claritin, has four rings, eight double bonds, and the formula C22H?ClN2O2. How many hydrogens does loratadine have? (Calculate your answer; don’t count hydrogens in the structure.)
Ocimene is a triene found in the essential oils of many plants. What is its IUPAC name, including stereochemistry?
α-Farnesene is a constituent of the natural wax found on apples. What is its IUPAC name, including stereochemistry?
Alkene Isomers and Their Stability
Tamoxifen, a drug used in the treatment of breast cancer, and clomiphene, a drug used in fertility treatment, have similar structures but very different effects. Assign E or Z configuration to the double bonds in both compounds.
Carbocations and Electrophilic Addition Reactions
Retin A, or retinoic acid, is a medication commonly used to reduce wrinkles and treat severe acne. How many different isomers arising from E,Z double-bond isomerizations are possible?
Fucoserratene and ectocarpene are sex pheromones produced by marine brown algae. What are their systematic names? (Ectocarpene is difficult; make your best guess, and then check your answer in the Student Solutions Manual.)
tert-Butyl esters [RCO2C(CH3)3] are converted into carboxylic acids (RCO2H) by reaction with trifluoroacetic acid, a reaction useful in protein synthesis (Section 26.7). Assign E,Z designation to the double bonds of both reactant and product in the following scheme, and explain why there is an apparent change in double-bond stereochemistry:
Vinylcyclopropane reacts with HBr to yield a rearranged alkyl bromide. Follow the flow of electrons as represented by the curved arrows, show the structure of the carbocation intermediate in brackets, and show the structure of the final product.
The isobutyl cation spontaneously rearranges to the tert-butyl cation by a hydride shift. Is the rearrangement exergonic or endergonic? Draw what you think the transition state for the hydride shift might look like according to the Hammond postulate.
Aromatic compounds such as benzene react with alkyl chlorides in the presence of AlCl3 catalyst to yield alkylbenzenes. This reaction occurs through a carbocation intermediate, formed by reaction of the alkyl chloride with AlCl3 (R−Cl + AlCl3 ⟶ R+ + AlCl4−). How can you explain the observation that reaction of benzene with 1-chloropropane yields isopropylbenzene as the major product?