CH 331 Unit 3 Study Guide This study guide covers all concepts from Homework 5–6, Quizzes 5–6, and the Unit 3 Assessment. Unit 3 continues the exploration of alkene reactions and introduces stereochemistry, focusing on chirality, enantiomers, and optical activity. ### 1. Addition Reactions of Alkenes #### Core Concepts - Alkenes and Addition Reactions: Alkenes undergo addition reactions where $\pi$ bonds (double bonds) are replaced by $\sigma$ bonds (single bonds). - Regiochemistry: Determined by the nature of the reagent, can follow: - Markovnikov's Rule: The more substituted carbon receives the positive part of the reagent. - Anti-Markovnikov: The less substituted carbon receives the positive part of the reagent. - Stereochemistry: Refers to the spatial arrangement of the substituents during addition, can be: - Syn Addition: Both substituents add to the same face of the double bond. - Anti Addition: Substituents add to opposite faces of the double bond. #### Key Reaction Types - Halogenation - Reagent: X₂ (e.g., Br₂ or Cl₂) in CH₂Cl₂ or CCl₄. - Product: Vicinal dihalide. - Mechanism: Forms a halonium ion intermediate leading to anti addition. - Halohydrin Formation - Reagent: X₂ + H₂O. - Product: A halohydrin (one halogen and one hydroxyl group). - Mechanism: Anti addition; OH attaches to the more substituted carbon. - Hydrogenation - Reagent: H₂, with a catalyst (Pd/C or Pt). - Product: Alkane. - Mechanism: Syn addition of both hydrogens to the same face. - Hydration - Types: - Markovnikov Hydration: Acid-catalyzed; OH on more substituted carbon. - Anti-Markovnikov Hydration: Using BH₃/THF followed by H₂O₂/OH⁻; OH on less substituted carbon. - Epoxidation - Reagent: mCPBA or another peroxy acid. - Product: Epoxide (three-membered cyclic ether). - Mechanism: Stereospecific; oxygen adds from one face (syn addition). - Ozonolysis - Reagent: O₃ at –78 °C followed by a reductive workup. - Product: Carbonyl compounds (aldehydes or ketones). - Distinguishing: Can differentiate between linear and cyclic structures. #### Practice Tasks - Identify the product(s) of halogenation, halohydrin, hydration, epoxidation, and ozonolysis reactions. - Problem-Solving Steps: 1. Identify the alkene structure and the specific reagent(s) being used. 2. Determine the type of addition reaction based on the reagents. 3. Apply the appropriate regiochemistry rule (Markovnikov or anti-Markovnikov) to determine where the new atoms will attach. 4. Consider the stereochemical outcome (syn or anti addition) if new chiral centers are formed or if stereoisomers are possible. 5. Draw the final product(s), including all stereochemical details if applicable. - Predict the regiochemistry (Markovnikov or anti-Markovnikov) of each addition. - Problem-Solving Steps: 1. Locate the double bond in the starting alkene. 2. Identify the two carbons forming the double bond and determine which one is more substituted (bonded to more carbons). 3. Based on the reagent, decide if the reaction follows Markovnikov's rule (positive part of reagent to more substituted carbon) or anti-Markovnikov's rule (positive part to less substituted carbon). - Recognize reaction intermediates (e.g., halonium ion, carbocation). - Problem-Solving Steps: 1. Review the mechanism for the specific addition reaction. 2. Recall if the mechanism involves a discrete carbocation (e.g., acid-catalyzed hydration, hydrohalogenation) or a cyclic halonium ion (e.g., halogenation, halohydrin formation). - Choose appropriate reagents to produce a desired product. - Problem-Solving Steps: 1. Examine the given starting material (alkene) and the desired product. 2. Identify the functional group change (e.g., addition of X₂, OH, H, O). 3. Determine the regiochemical and stereochemical requirements of the desired product. 4. Select the reagent combination that achieves these specific functionalization, regiochemical, and stereochemical outcomes. - Predict when a rearrangement or ring opening may occur. - Problem-Solving Steps: 1. Recognize reactions that proceed through carbocation intermediates (e.g., HX addition, acid-catalyzed hydration). Carbocations can undergo hydride or alkyl shifts to form a more stable carbocation. 2. For cyclic intermediates like halonium ions, understand that nucleophilic attack can lead to ring opening from the opposite face of the electrophile. ### 2. Reaction Mechanisms and Intermediates #### Core Concepts - Carbocation vs. Halonium Ion - Halogenation proceeds through a cyclic halonium ion rather than a free carbocation, which prevents rearrangements and leads to anti stereochemistry. - Nucleophilic Attack in Halohydrin Formation - Water or another nucleophile opens the halonium ring from the opposite side, leading to anti addition. - Reaction Coordinate Interpretation - Understand energy profiles indicating activation energy and intermediates in reactions. #### Practice Tasks - Identify whether a reaction proceeds via a carbocation or halonium ion intermediate. - Problem-Solving Steps: 1. For reactions involving hydrogen halides (HX) or acid-catalyzed hydration (H₂O/H⁺), expect a carbocation intermediate. 2. For reactions involving halogens (X₂) or halogens with water (X₂/H₂O), expect a cyclic halonium ion intermediate. - Predict the stereochemical outcome (syn vs. anti addition). - Problem-Solving Steps: 1. Recall the mechanism for the specific reaction. 2. Reactions proceeding through concerted pathways or cyclic intermediates often have specific stereoselectivity: - Syn addition: Hydrogenation (H₂/Pd); Epoxidation (mCPBA). - Anti addition: Halogenation (X₂); Halohydrin formation (X₂/H₂O). 3. Reactions with carbocation intermediates typically result in mixtures due to planar carbocation intermediate. - Relate reaction intermediates to observed products. - Problem-Solving Steps: 1. Draw the intermediate structure (carbocation or halonium ion). 2. Visualize how the nucleophile would attack the intermediate, considering steric hindrance and electronic factors. 3. For halonium ions, remember the nucleophile attacks from the opposite face of the original electrophilic addition, leading to anti products. - Label transition states and intermediates on a reaction coordinate diagram. - Problem-Solving Steps: 1. Intermediates correspond to valleys (energy minima) between hills. 2. Transition states correspond to peaks (energy maxima) on the reaction coordinate diagram. 3. Identify the number of steps in the reaction; each step has one transition state and may form an intermediate. ### 3. Reagent–Product Relationships #### Core Concepts - Each reagent combination leads to characteristic transformations: - Br₂ or Cl₂ → Halogenation → Dihalide. - Br₂/H₂O → Halohydrin formation → Halohydrin (anti addition). - H₂/Pd or Pt → Hydrogenation → Alkane (syn addition). - BH₃/THF, H₂O₂/OH⁻ → Anti-Markovnikov hydration → Alcohol. - H₂O/H⁺ → Markovnikov hydration → Alcohol. - mCPBA → Epoxidation → Epoxide. - O₃, reductive workup → Ozonolysis → Carbonyl compounds. #### Practice Tasks - Match reaction types to reagent sets. - Problem-Solving Steps: 1. Review the