# Ranking Alkenes by Reactivity with HCl Let's analyze and rank the following alkenes by their reaction rate with **HCl** (hydrochloric acid): - **A:** Cyclohexene - **B:** Methylenecyclohexane - **C:** 4-Methylenetetrahydropyran - **D:** 3-Methylenetetrahydropyran - **E:** 1,3-Dioxacyclohex-4-ene ## Step 1: Understanding the Reaction **Alkenes + HCl**: - Reaction proceeds via **electrophilic addition**. - The alkene donates electrons to H⁺, forming a **carbocation intermediate**. - The more stable the carbocation, the **faster** the reaction. **Key Factors**: - **Carbocation stability**: Tertiary > Secondary > Primary > Methyl - **Resonance stabilization**: More resonance = more stable carbocation - **Electron-withdrawing groups**: Destabilize carbocation, slow the reaction --- ## Step 2: Analyzing Each Alkene ### **A: Cyclohexene** - Simple 6-membered ring with one double bond. - Protonation gives a **secondary carbocation**. - No resonance stabilization. ### **B: Methylenecyclohexane** - Extra =CH₂ group attached to cyclohexane ring. - Protonation at exocyclic double bond gives **tertiary carbocation**. - No resonance, but **highly stable** due to hyperconjugation. ### **C: 4-Methylenetetrahydropyran** - 6-membered ring with one oxygen atom and an exocyclic methylene group at position 4. - Protonation gives a carbocation **adjacent to oxygen**; possible resonance with oxygen's lone pairs. - **Resonance stabilization** of carbocation. ### **D: 3-Methylenetetrahydropyran** - 6-membered ring with oxygen, exocyclic methylene at position 3. - Protonation gives carbocation **further from oxygen**; less resonance stabilization compared to C. ### **E: 1,3-Dioxacyclohex-4-ene** - 6-membered ring with **two oxygens** at positions 1 and 3, and a double bond at position 4. - Protonation gives a carbocation **flanked by two oxygens**. - **Strong resonance stabilization** due to two adjacent oxygens (both can donate electrons). --- ## Step 3: Ranking Carbocation Stability **Most stable carbocation = fastest reaction with HCl** - **E:** Resonance from two oxygens → most stabilized. - **C:** Resonance from one oxygen. - **B:** Tertiary carbocation (no resonance, but most hyperconjugation). - **D:** Less resonance (oxygen farther away). - **A:** Only secondary carbocation, no resonance. --- ## Step 4: Final Ranking **Highest to Lowest Reaction Rate:** 1. **E: 1,3-Dioxacyclohex-4-ene** (Most resonance stabilization from two oxygens) 2. **C: 4-Methylenetetrahydropyran** (Resonance from one oxygen) 3. **B: Methylenecyclohexane** (Tertiary carbocation, no resonance) 4. **D: 3-Methylenetetrahydropyran** (Oxygen less able to stabilize carbocation) 5. **A: Cyclohexene** (Secondary carbocation, no resonance) --- ## **Summary Table** | Alkene | Carbocation Stability | Rank | |-----------------------------------------|---------------------------|------| | E: 1,3-Dioxacyclohex-4-ene | Two oxygens (most stable) | 1 | | C: 4-Methylenetetrahydropyran | One oxygen (resonance) | 2 | | B: Methylenecyclohexane | Tertiary | 3 | | D: 3-Methylenetetrahydropyran | Weak resonance | 4 | | A: Cyclohexene | Secondary (no resonance) | 5 | --- ## **Final Answer** **E > C > B > D > A** (from fastest to slowest with HCl)