Chapter 15 — Exercises

Fifty problems on alkene structure, stability, and electrophilic addition. Drawing required wherever a structure or mechanism is asked for. ∗ marks problems with full worked solutions in Appendix Answers to Selected Exercises.

Section A — Alkene structure and naming

15.1∗ (routine) Draw the structure of each alkene and name using IUPAC nomenclature: (a) (E)-2-butene (b) (Z)-2-pentene (c) 2-methyl-2-butene (d) (E)-3-methyl-2-pentene (e) 1,3-butadiene (f) 2,3-dimethyl-2-butene

15.2 (routine) Why does the C=C of an alkene have restricted rotation? Give the energy cost of rotation.

15.3∗ (routine) Assign E or Z to each: (a) Cl-CH=CH-Br (with Cl and Br on opposite sides) (b) CH₃-CH=CH-CH₃ (cis) (c) CH₃-C(=CH₂)-CH₂CH₃ (terminal alkene with methyl at C2; consider as monosubstituted at the terminal)

15.4 (moderate) A compound has the formula C₅H₁₀. List all possible alkene structures. How many constitutional isomers? How many stereoisomers?

15.5 (challenge) Why is cis-cyclooctene more stable than trans-cyclooctene, but cis-cyclohexene is the only possible isomer of cyclohexene? Connect to ring strain.

Section B — Stability

15.6∗ (routine) Rank the following alkenes by stability (most stable first): (a) 1-butene (b) cis-2-butene (c) trans-2-butene (d) 2-methyl-1-butene (e) 2-methyl-2-butene (f) 2,3-dimethyl-2-butene

15.7 (routine) Why is 2,3-dimethyl-2-butene more stable than 2,3-dimethyl-1-butene? Use hyperconjugation arguments.

15.8 (moderate) Hyperconjugation: define it. Show how it works for an alkyl-substituted alkene with an electron-rich C-H σ orbital interacting with the C=C π* orbital.

15.9 (challenge) A cyclohexene has 6 π electrons in a 6-atom system; benzene also has 6 π electrons. Why is benzene aromatic but cyclohexene not? Connect to Hückel's rule.

Section C — Electrophilic addition mechanism

15.10∗ (routine) Predict the product of: 2-methyl-2-butene + HBr → ?

15.11 (routine) Predict the product: propene + HCl → 2-chloropropane (Markovnikov). Why?

15.12∗ (moderate) Draw the full mechanism for: propene + HCl → 2-chloropropane. Show each step explicitly.

15.13 (moderate) Why does the cation form at the more-substituted carbon? Use Hammond postulate + carbocation stability arguments.

15.14 (moderate) Predict the product of: 3,3-dimethyl-1-butene + HCl → ? Account for any rearrangements.

15.15 (challenge) Predict the product: cyclopentadiene + HCl → ? Note the 1,4 vs 1,2 addition possibilities (Ch 19 preview).

Section D — Markovnikov vs anti-Markovnikov

15.16∗ (routine) Apply Markovnikov rule: (a) propene + HCl → ? (b) 2-methyl-1-butene + HBr → ? (c) 1-pentene + HI → ?

15.17 (routine) Why does HBr + alkene + peroxide give anti-Markovnikov product? Identify the mechanism (radical chain).

15.18 (moderate) Why does HBr alone give Markovnikov but HBr + peroxides give anti-Markovnikov? Connect to mechanism difference.

15.19 (moderate) Hydroboration-oxidation gives anti-Markovnikov hydration. Sketch the principle: BH₃ adds with B on the less-substituted C (steric).

15.20 (challenge) Use HBr + peroxide on a complex alkene like 4-methyl-1-hexene. Predict the product. Identify the radical intermediate and why it forms at the less-substituted C.

Section E — Bromine addition

15.21∗ (routine) Predict the stereochemistry of: cis-2-butene + Br₂ → ? trans-2-butene + Br₂ → ?

15.22 (routine) Draw the bromonium ion intermediate of Br₂ + alkene addition.

15.23∗ (moderate) Sketch the mechanism of Br₂ + propene → 1,2-dibromopropane. Show the bromonium ion and the backside attack.

15.24 (moderate) Why does Br₂ addition give anti-1,2-dibromide stereospecifically?

15.25 (challenge) A cyclic alkene + Br₂ gives anti-1,2-dibromocycloalkane. Sketch the chair vs. half-chair conformations of the product.

15.26 (challenge) Why is the bromonium ion 3-membered rather than open chain (carbocation + Br⁻)? Connect to charge stabilization and the C-Br bond character.

Section F — Hydration

15.27∗ (routine) Predict the product of: 2-methyl-2-butene + H₂O + dilute H₂SO₄ → ?

15.28 (routine) Hydration of propene + H₂O + acid → 2-propanol (Markovnikov). Mechanism?

15.29 (moderate) Compare acid-catalyzed hydration with hydroboration-oxidation. Both make alcohols from alkenes. What's different about regiochemistry and stereochemistry?

15.30 (challenge) Industrial production of ethanol uses acid-catalyzed hydration of ethylene. Sketch the process. Why isn't this used for higher alcohols (sec-butanol from 1-butene)?

Section G — Carbocation rearrangements

15.31∗ (routine) Predict the product of: 3,3-dimethyl-1-butene + HCl. Show the rearrangement (1,2-methyl shift to give a 3° cation).

15.32 (moderate) Predict the product of: 4,4-dimethyl-2-pentene + HBr. Identify potential rearrangements.

15.33 (challenge) Some alkenes give multiple products due to mixed rearrangements. Predict the products of: 3-methyl-1-butene + HCl. Identify the rearrangement (1,2-H shift to give a more-stable 3° cation).

15.34 (challenge) Pinacol rearrangement: 1,2-diol + acid → ketone. Identify the carbocation rearrangement involved.

Section H — Stereochemistry

15.35∗ (routine) Predict the stereochemistry of products from: (a) cis-2-butene + Br₂ → ? (b) trans-2-butene + Br₂ → ? (c) cis-2-butene + HBr → (mixture; explain)

15.36 (moderate) A racemic alkene + Br₂ gives a meso or racemic dibromide? Use stereochemistry arguments.

15.37 (challenge) A chiral alkene + Br₂ gives an enantiopure product? Why or why not?

Section I — Spectroscopy

15.38∗ (routine) Identify spectroscopy clues for an alkene: (a) IR (b) ¹H NMR vinyl region (c) Coupling constants for cis/trans

15.39 (moderate) A compound shows ¹H NMR triplet at δ 0.9 (3H), multiplet at δ 1.4 (4H), multiplet at δ 5.4 (1H), multiplet at δ 5.7 (1H). What is it? Identify cis or trans.

15.40 (challenge) Distinguish 1-pentene vs. (E)-2-pentene vs. (Z)-2-pentene by ¹H NMR. What patterns differentiate them?

Section J — Industrial / multistep

15.41 (routine) Polyethylene synthesis from ethylene: explain the radical mechanism (Ziegler-Natta or radical, Ch 18).

15.42 (moderate) Vulcanization of rubber: cross-linking of polymer alkene chains by sulfur. Sketch the chemistry.

15.43 (challenge) Design a synthesis of 2-bromopentane from pentene + HBr.

15.44 (challenge) Design a synthesis of meso-2,3-dibromobutane from butene + Br₂. Specify the alkene starting material.

15.45 (challenge) A natural product has a (Z)-alkene. Suggest a synthesis using a stereoselective alkene-forming reaction (e.g., Wittig with stabilized ylide, or Lindlar reduction of alkyne).

Section K — Industrial alkenes

15.46∗ (routine) Identify the industrial uses of: (a) ethylene (b) propylene (c) butadiene (d) styrene

15.47 (moderate) How is ethylene produced industrially? (Steam cracking of ethane.) Sketch the mechanism (free-radical thermal cracking).

15.48 (challenge) Discuss the role of alkenes in modern polymer industry. Connect to Chapter 16 (polymerization) and Chapter 37 (Ziegler-Natta).

Section L — Open-ended

15.49 (challenge) Compare alkene electrophilic addition (this chapter) with alkyl halide nucleophilic substitution (Ch 10-11). Identify the mechanistic similarities and differences.

15.50 (challenge) Predict the products of a complex alkene-electrophile combination with multiple possibilities. For example: 1,4-cyclohexadiene + HCl → ? (Identify 1,2 vs 1,4 product distribution.)

Notes for instructors: Common stumbling blocks for Chapter 15: (1) Confusing Markovnikov direction. (2) Forgetting bromonium ion mechanism for Br₂. (3) Not considering carbocation rearrangements. (4) Mismatching cis/trans with E/Z. Computational exercises: optimize cis-2-butene and trans-2-butene; calculate energy difference; verify the trans is ~1 kcal/mol more stable.