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Chapter 8
Organic Chemistry- Exam 3- Fall 2026
| Question | Answer |
|---|---|
| What happens during an alkene addition reaction? | The C=C π bond is converted into two new σ bonds |
| How does addition relate to elimination? | Addition adds groups across a π bond; elimination removes groups to form a π bond |
| How is an alkene parent chain selected? | Choose a parent chain that includes the C=C double bond. |
| How is the parent name of an alkene changed? | Use the suffix –ene rather than –ane. |
| How is an alkene double bond numbered? | Give the C=C the lowest possible locant; the number indicates where the double bond start |
| How are alkene substituents listed in the name? | Number and alphabetize substituents before the parent name, following the stated nomenclature rules |
| How can temperature affect addition/elimination equilibrium? | Lower temperatures favor addition; higher temperatures make entropy more important and favor elimination |
| What is hydrohalogenation? | Addition of HX (HCl, HBr, or HI) across an alkene |
| What is Markovnikov addition of HX? | H adds to the alkene carbon already bearing more H atoms; X generally ends up on the more substituted carbon. |
| Why does Markovnikov hydrohalogenation occur? | The favored pathway forms the more stable carbocation intermediate |
| What is the key first step in ordinary hydrohalogenation? | The π bond is protonated, producing a carbocation intermediate |
| What is the rate-determining step in hydrohalogenation? | Carbocation formation, the step with the highest activation energy |
| What does HBr with peroxides do to regioselectivity? | It gives anti-Markovnikov addition; the slides note the mechanism differs and is covered later |
| Why can hydrohalogenation produce enantiomers? | A planar carbocation can be attacked from either face, creating opposite configurations at a new chiral center |
| What is acid-catalyzed hydration? | Addition of H and OH across an alkene to form an alcohol |
| What is the regioselectivity of acid-catalyzed hydration? | Markovnikov: OH attaches to the more substituted alkene carbon |
| What is the main mechanistic similarity between hydration and hydrohalogenation? | Both proceed through a carbocation intermediate after protonation of the alkene |
| What happens after water attacks in acid-catalyzed hydration? | An oxonium ion forms, then deprotonation gives the neutral alcohol |
| How can equilibrium be pushed toward alcohol in hydration? | Use excess water, according to the lecture slides |
| How can alcohol dehydration be favored in the reverse direction? | Use acid without adding water; heat is used for the dehydration conditions described in the slides |
| What stereochemical result can hydration give at a new chiral center? | A mixture of R and S products may form; if they are enantiomers in equal amounts, the mixture is racemic |
| What is a limitation of acid-catalyzed hydration? | Carbocation rearrangements can occur, producing rearranged products or mixtures |
| What does oxymercuration-demercuration accomplish? | Markovnikov addition of H and OH without carbocation rearrangements |
| What intermediate forms during oxymercuration? | A bridged mercurinium ion rather than a freely rearranging carbocation |
| What is the role of NaBHn in oxymercuration-demercuration? | It replaces the –HgOAc group with hydrogen in the demercuration step |
| What is the regioselectivity of hydroboration-oxidation? | Anti-Markovnikov: OH ends up on the less substituted alkene carbon |
| What is the stereochemistry of hydroboration-oxidation? | Syn addition of H and OH |
| Is hydroboration-oxidation stereospecific? | Yes; only syn addition occurs |
| Why does hydroboration favor the less substituted carbon for boron attachment? | Electronic effects and steric factors both influence the regioselectivity |
| What does catalytic hydrogenation do? | Adds Hn across C=C, converting an alkene to the corresponding alkan |
| Why is a metal catalyst needed for hydrogenation? | Without it, the reaction has a very high activation energy and is too slow |
| What is the stereochemistry of catalytic hydrogenation? | Syn addition: both hydrogen atoms add to the same face of the double bond |
| Why does hydrogenation give syn addition? | The metal surface binds both H and the alkene, delivering hydrogen from the same surface |
| What can syn hydrogenation of a symmetrical alkene produce? | A meso compound rather than a pair of enantiomers, when the product has the required symmetry |
| Which reagents are practical for alkene halogenation in the slides? | Cl and Br; In reacts poorly, while Fn is too violent |
| What is the stereochemistry of alkene halogenation? | Anti addition |
| What intermediate forms during bromination of an alkene? | A bridged bromonium ion |
| Why does bromide attack from the back side of a bromonium ion? | The bridged intermediate blocks same-side approach, leading to anti addition |
| What determines the stereochemistry in alkene halogenation? | The starting alkene geometry determines the product stereochemistry; the reaction is stereospecific |
| What is a halohydrin? | A product formed when halogenation occurs in water, adding X and OH across the former double bond |
| Where do X and OH end up in halohydrin formation? | X adds to the less substituted carbon; OH adds to the more substituted carbon. |
| Why does water attack the more substituted carbon in a halohydrin intermediate? | That carbon has more cationic character, so water attacks it more readily |
| What is anti-dihydroxylation? | Addition of two OH groups across an alkene on opposite faces |
| How is anti-dihydroxylation performed? | First convert the alkene to an epoxide with a peroxyacid; then open the epoxide with H2O and acid |
| Why does epoxide opening produce anti addition? | The nucleophile attacks from the side opposite the leaving group/bridged oxygen arrangement |
| What is syn-dihydroxylation? | Concerted addition of two OH groups to the same face of an alkene |
| Which reagents perform syn-dihydroxylation? | OsOn with a co-oxidant such as NMO, or cold, mild KMnOn |
| Why is a co-oxidant used with OsOn? | It allows use of only a catalytic amount of expensive, toxic OsO |
| What is ozonolysis? | Oxidative cleavage of an alkene's C=C bond |
| What are common reducing agents used in ozonolysis workup? | Dimethyl sulfide (DMS) or Zn/HnO |
| What is a racemic mixture? | Equal amounts of two enantiomers |
| What is a meso compound? | A compound with stereocenters that is achiral because of internal symmetry |
| What does stereospecific mean? | The starting stereochemistry determines the product stereochemistry |
| What does regioselective mean? | One constitutional arrangement of the product forms preferentially over another |
| How do you approach a one-step synthesis problem? | Compare reactant and target to identify the needed bond changes, then select a matching reaction and reagents |
| How can you shift a leaving-group position through a multi-step synthesis? | Use elimination to form an alkene, then add HX with the desired regioselectivity |
| Which conditions give Markovnikov addition of H and Br? | HBr without peroxides. |
| Which conditions give anti-Markovnikov addition of H and Br? | HBr with ROOR (peroxides), as shown in the lecture slides |
| Which base can favor the Hofmann alkene in E2? | A bulky base such as t-BuOK |
| How can a synthesis use elimination followed by anti-Markovnikov hydration | Form the appropriate alkene, then use hydroboration-oxidation to place OH on the less substituted carbon |
Created by:
KristenR2025