## Step-by-Step Mechanisms with Explanations --- ## **Part 1: Ethyl-Substituted Cyclohexene with HBr** ### **Step 1: Protonation of the Alkene** - The double bond in the starting alkene reacts with HBr. - The π electrons attack the proton (**H⁺**), leading to the formation of a carbocation. - The more stable carbocation forms at the carbon adjacent to the ethyl group (due to hyperconjugation and inductive effects). **Curved arrow:** Double bond → H of HBr Br⁻ leaves as an anion. **Result:** Carbocation at the more substituted carbon (next to ethyl group). **Explanation:** Carbocations are more stable when they are more substituted. The ethyl group stabilizes the carbocation via hyperconjugation and inductive effects. --- ### **Step 2: Nucleophilic Attack by Br⁻** - The bromide ion (**Br⁻**) attacks the carbocation. - The product is a **Markovnikov addition**: Br attaches to the more substituted carbon. **Curved arrow:** Lone pair on Br⁻ → carbocation. **Explanation:** Br⁻ always attacks the carbocation formed in the previous step, giving Markovnikov addition. --- ## **Part 1 Final Answer:** **Markovnikov addition** occurs; Br adds to the more substituted carbon (next to the ethyl group). --- ## **Part 2: SCH₃-Substituted Cyclohexene with HBr** ### **Step 1: Protonation of the Alkene** - The double bond reacts with HBr. - Protonation occurs, but the **SCH₃** group is an electron-donating group by resonance (lone pairs on S can stabilize positive charge via resonance). **Curved arrow:** Double bond → H of HBr Br⁻ leaves as an anion. **Result:** Carbocation forms at the carbon **further from** the SCH₃ group so that the positive charge can be stabilized by resonance with the SCH₃ group. **Explanation:** The SCH₃ group stabilizes a carbocation via resonance if the positive charge is adjacent to it (on the carbon next to SCH₃). --- ### **Step 2: Nucleophilic Attack by Br⁻** - Br⁻ attacks the carbocation. - The product is **anti-Markovnikov**: Br attaches to the less substituted carbon (further from SCH₃). **Curved arrow:** Lone pair on Br⁻ → carbocation. **Explanation:** Resonance stabilization by SCH₃ makes the carbocation at the less substituted position more stable, overriding the usual Markovnikov rule. --- ## **Part 2 Final Answer:** **Anti-Markovnikov addition** occurs; Br adds to the less substituted carbon (further from SCH₃), due to resonance stabilization of the carbocation by the SCH₃ group. --- ## **Summary Table** | Substituent | Carbocation stabilization | Product type | Br attaches to... | |-------------|-----------------------------|----------------------|---------------------------| | Ethyl | Inductive/hyperconjugation | Markovnikov | More substituted carbon | | SCH₃ | Resonance | Anti-Markovnikov | Less substituted carbon | --- **In essence:** - Markovnikov addition dominates unless a strong resonance-donating group (like SCH₃) stabilizes a different carbocation, leading to anti-Markovnikov addition. - The key is to analyze **how the substituent stabilizes the intermediate carbocation**.