7.15: Additional Problems

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Visualizing Chemistry

PROBLEM7-22 Name the following alkenes, and convert each drawing into a skeletal structure:

(a) the= (b) The ball-and-stick model has a cyclohexene ring. C1 is bonded to an ethyl group. C3 is bonded to two methyl groups.

PROBLEM7-23 Assign E or Z stereochemistry to the double bonds in each of the following alkenes, and convert each drawing into a skeletal structure (red = O, green = Cl): (a) the= (b) The ball-and-stick model has 5-carbon chain with alternate double bonds. C2 and C3 are each bonded to methyl. C4 is bonded to methoxy. C5 is carboxylic acid.

PROBLEM7-24 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 ball-and-stick model has a cyclopentane ring. C1 is bonded to an ethyl group. The gray and white spheres represent carbon and hydrogen atoms, respectively.

PROBLEM7-25 The following alkyl bromide can be made by HBr addition to three different alkenes. Show their structures.

the=

Mechanism Problems

PROBLEM7-26 Predict the major product and show the complete mechanism for each of the following electrophilic addition reactions.

(a) an=

(b) An incomplete reaction shows 4-methyl-1-pentene reacting with hydrogen bromide to form unknown product(s) indicated by a question mark.

(c) An incomplete reaction shows 1-methylcyclopentene reacting with K I and phosphoric acid to form unknown product(s) indicated by a question mark.

PROBLEM7-27 Each of the following electrophilic addition reactions involves a carbocation rearrangement. Predict the product and draw the complete mechanism of each using curved arrows. (a)

An incomplete reaction shows 3-methyl-1-pentene reacting with hydrogen chloride to form unknown product(s) indicated by a question mark. (b)

An incomplete reaction shows 3,6-dimethylcyclohexene reacting with hydrogen bromide to form unknown product(s) indicated by a question mark. (c)

An incomplete reaction shows vinylcyclobutane reacting with K I and phosphoric acid to form unknown product(s) indicated by a question mark. Text says two different rearrangement products.

PROBLEM7-28 When 1,3-butadiene reacts with 1 mol of HBr, two isolable products result. Propose mechanisms for both.

A reaction shows 1,3-butadiene reacting with hydrogen bromide to form 3-bromo-1-butene and trans-1-bromo-2-butene.

PROBLEM7-29 When methyl vinyl ether reacts with a strong acid, H⁺ adds to C2 instead of C1 or the oxygen atom. Explain.

the=

PROBLEM7-30 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.

cyclohexene=

PROBLEM7-31 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.

cyclopentane=

PROBLEM7-32 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.)

PROBLEM7-33 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

PROBLEM7-34 Calculate the degree of unsaturation in the following formulas, and draw five possible structures for each:

(a) C₁₀H₁₆ (b) C₈H₈O (c) C₇H₁₀Cl₂ (d) C₁₀H₁₆O₂ (e) C₅H₉NO₂ (f) C₈H₁₀ClNO

PROBLEM7-35 How many hydrogens does each of the following compounds have?

(a) C₈H_?O₂,has two rings and one double bond

(b) C₇H_?N, has two double bonds

PROBLEM7-36 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.)

The structure of Loratadine.

Naming Alkenes

PROBLEM7-37 Name the following alkenes:

(a) (b) A double bond with 2-methylheptane connected by C 5 (up) and methyl (down) substituents on the left and methyl (up) and hydrogen (down) substituents on the right. (c) A double bond withtwo hydrogen substituents on the left and two ethyl substituents on the right.

(d) A double bond with H (up) and 3-methyl-1-pentene connected by C 4 (down) substituents on the left and methyl (up) and hydrogen (down) substituents on the right. (e) An 8-carbon chain with double bonds between C2-C3 and C4-C5. C4 and C5 are each bonded to a methyl group (opposite sides). (f)

PROBLEM7-38 Draw structures corresponding to the following systematic names:

(a) (4E)-2,4-Dimethyl-1,4-hexadiene (b) cis-3,3-Dimethyl-4-propyl-1,5-octadiene

(c) 4-Methyl-1,2-pentadiene (d) (3E,5Z)-2,6-Dimethyl-1,3,5,7-octatetraene

(e) 3-Butyl-2-heptene (f) trans-2,2,5,5-Tetramethyl-3-hexene

PROBLEM7-39 Name the following cycloalkenes:

(a) (b) A five-membered ring with one double bond. There are methyl groups on a double-bonded carbon and on the single-bonded carbon next to it. (c) A four-membered ring with two double bonds. There is one ethyl substituent. (d) A six-membered ring with two double bonds opposite one another on the ring. There are methyl groups at each end of one double bond. (e) (f) An eight-membered ring with two double bonds opposite one another on the ring.

PROBLEM7-40 Ocimene is a triene found in the essential oils of many plants. What is its IUPAC name, including stereochemistry?

an=

PROBLEM7-41 α-Farnesene is a constituent of the natural wax found on apples. What is its IUPAC name, including stereochemistry?

twelve-carbon= Menthene, a hydrocarbon found in mint plants, has the systematic name 1-isopropyl-4-PROBLEM7-42 methylcyclohexene. Draw its structure.

PROBLEM7-43 Draw and name the six alkene isomers, C₅H₁₀, including E,Z isomers.

PROBLEM7-44 Draw and name the 17 alkene isomers, C₆H₁₂, including E,Z isomers.

Alkene Isomers and Their Stability

PROBLEM7-45 Rank the following sets of substituents according to the Cahn–Ingold–Prelog sequence rules:

(a) (b)

(c) (d)

(e) (f)

Problem 7-46 Assign E or Z configuration to each of the following compounds:

(a) (b) (c) (d)

Problem 7-47 Which of the following E,Z designations are correct, and which are incorrect?

(a) (b) (c)

(d) (e) (f)

Problem 7-48 Rank the double bonds according to their increasing stability.

(a)

(b)

(c)

Problem 7-49 trans-2-Butene is more stable than cis-2-butene by only 4 kJ/mol, but trans-2,2,5,5-tetramethyl-3-hexene is more stable than its cis isomer by 39 kJ/mol. Explain.

Problem 7-50 Cyclodecene can exist in both cis and trans forms, but cyclohexene cannot. Explain.

Problem 7-51 Normally, a trans alkene is more stable than its cis isomer. trans-Cyclooctene, however, is less stable than cis-cyclooctene by 38.5 kJ/mol. Explain.

Problem 7-52 trans-Cyclooctene is less stable than cis-cyclooctene by 38.5 kJ/mol, but trans-cyclononene is less stable than cis-cyclononene by only 12.2 kJ/mol. Explain.

Problem 7-53 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

Problem 7-54 Rank the following carbocations according to their increasing stability.

(b)

(c)

Problem 7-55 Use the Hammond Postulate to determine which alkene in each pair would be expected to form a carbocation faster in an electrophilic addition reaction. (a)

(b)

(c)

Problem 7-56 The following carbocations can be stabilized by resonance. Draw all the resonance forms that would stabilize each carbocation. (a)

(b)

(c)

Problem 7-57 Predict the major product in each of the following reactions:

(a) (b)

(c)

(d)

Problem 7-58 Predict the major product from addition of HBr to each of the following alkenes:

(a) (b) (c)

Problem 7-59 Alkenes can be converted into alcohols by acid-catalyzed addition of water. Assuming that Markovnikov’s rule is valid, predict the major alcohol product from each of the following alkenes.

(a) (b) (c)

Problem 7-60 Each of the following carbocations can rearrange to a more stable ion. Propose structures for the likely rearrangement products.

(a) (b) (c)

General Problems

Problem 7-61 Allene (1,2-propadiene), H₂C $\text{=}$ C $\text{=}$ CH₂, has two adjacent double bonds. What kind of hybridization must the central carbon have? Sketch the bonding π orbitals in allene. What shape do you predict for allene?

Problem 7-62 The heat of hydrogenation for allene (Problem 7-61) to yield propane is −295 kJ/mol, and the heat of hydrogenation for a typical monosubstituted alkene, such as propene, is −125 kJ/mol. Is allene more stable or less stable than you might expect for a diene? Explain.

Problem 7-63 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?

Problem 7-64 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.)

Problem 7-65 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:

Problem 7-66 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.

Problem 7-63

(a) Cholesterol, C₂₇H₄₆O (b) DDT, C₁₄H₉Cl₅ (c) Prostaglandin E₁, C₂₀H₃₄O₅ (d) Caffeine, C₈H₁₀N₄O₂ (e) Cortisone, C₂₁H₂₈O₅ (f) Atropine, C₁₇H₂₃NO₃

Problem 7-68 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.

the=

Problem 7-69 Draw an energy diagram for the addition of HBr to 1-pentene. Let one curve on your diagram show the formation of 1-bromopentane product and another curve on the same diagram show the formation of 2-bromopentane product. Label the positions for all reactants, intermediates, and products. Which curve has the higher-energy carbocation intermediate? Which curve has the higher-energy first transition state?

Problem 7-70 Sketch the transition-state structures involved in the reaction of HBr with 1-pentene (Problem 7-69). Tell whether each structure resembles reactant or product.

Problem 7-71 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?

Problem 7-72 Reaction of 2,3-dimethyl-1-butene with HBr leads to an alkyl bromide, C₆H₁₃Br. On treatment of this alkyl bromide with KOH in methanol, elimination of HBr occurs and a hydrocarbon that is isomeric with the starting alkene is formed. What is the structure of this hydrocarbon, and how do you think it is formed from the alkyl bromide?

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