# Step-by Solution and Mechanism The reaction shown is **addition of HBr to sty (phenylethene)**, you are asked to show the mechanism **in the absence and presence of peroxide (ROOR)**. The outcome depends on the conditions: --- ## 1. **Absence of Peroxide (Normal Addition, Markovnikov Rule)** ### **Stepwise Mechanism:** #### **Step 1: Protonation of Alkene (Formation of Carbocation)** - The alkene π electrons attack the proton (H⁺) from HBr. - The most stable carbocation forms (Markovnikov rule: H adds to the carbon with more hydrogens). **Curved arrows:** - π electrons of the double bond attack the H of HBr. - The Br– takes the electrons from the H–Br bond. **Intermediate:** - Benzyl carbocation (resonance-stabilized). #### **Step 2: Nucleophilic Attack by Br⁻** - The Br⁻ attacks the carbocation to give the Markovnikov product. --- #### **Structures:** **Step 1:** ```markdown Ph-CH=CH2 + HBr → Ph-CH⁺-CH3 + Br⁻ | | (styrene) (benzyl carbocation) ``` **Curved arrow:** from alkene to H, and from H–Br bond to Br. **Step 2:** ```markdown Ph-CH⁺-CH3 + Br⁻ → Ph-CH(Br)-CH3 ``` **Product:** **1-bromo-1-phenylethane (Markovnikov product)** --- ## 2. **Presence of Peroxide (ROOR, Radical Addition, Anti-Markovnikov Rule)** ### **Stepwise Mechanism:** #### **Step 1: Initiation (Radical Formation)** - ROOR decomposes to form alkoxy radicals. - Alkoxy radical abstracts H from HBr to generate Br• (bromine radical). #### **Step 2: Propagation** 1. **Addition of Br• to Alkene** - Br• adds to the less substituted carbon of the alkene (to form the more stable benzyl radical). - **Anti-Markovnikov addition:** Br adds to the terminal carbon. 2. **Radical abstracts H from HBr** - The new radical abstracts H from HBr, generating the product and another Br• radical (which continues the chain). --- #### **Structures:** **Initiation:** ```markdown ROOR → 2 RO• (by heat/light) RO• + HBr → ROH + Br• ``` **Propagation Step 1:** Curved arrow: Br• attacks terminal carbon of double bond, forming benzyl radical. ```markdown Ph-CH=CH2 + Br• → Ph-CH•-CH2Br ``` **Propagation Step 2:** Curved arrow: Radical abstracts H from HBr. ```markdown Ph-CH•-CH2Br + HBr → Ph-CH2-CH2Br + Br• ``` **Product:** **1-bromo-2-phenylethane (Anti-Markovnikov product)** --- ## **Summary Table** | Condition | Product | Mechanism Type | Regioselectivity | |---------------------|---------------------------------|---------------------|-------------------| | No peroxide | 1-bromo-1-phenylethane | Ionic (carbocation) | Markovnikov | | With peroxide (ROOR)| 1-bromo-2-phenylethane | Radical | Anti-Markovnikov | --- ## **Electron Flow (Curved Arrows):** - **Ionic Addition:** - Arrow from double bond to H of HBr. - Arrow from H–Br bond to Br. - Arrow from Br⁻ to carbocation. - **Radical Addition:** - Arrow from Br• to terminal carbon. - Arrow from radical carbon to H of HBr. - Arrow from H–Br bond to Br•. --- ## **Final Answers:** ### **Markovnikov Product (No Peroxide):** - **Product:** 1-bromo-1-phenylethane ### **Anti-Markovnikov Product (With Peroxide):** - **Product:** 1-bromo-2-phenylethane --- Let me know if you need the **curved arrow diagrams** or any **structural drawings** for further clarification!