# Ranking Alkenes by Reaction Rate with HCl ## Step 1: **Understanding the Reaction** **Alkenes react with HCl via electrophilic addition:** 1. **Protonation of the double bond** forms a carbocation intermediate. 2. **Chloride ion attacks** the carbocation to form the alkyl halide. **The rate-determining step:** - Formation of the carbocation. - More stable carbocations form faster. ## Step 2: **Analyzing Each Alkene** Let’s examine the structure and the carbocation formed after HCl addition: --- ### **A: Cyclohexene** - Structure: Simple six-membered ring with a double bond. - **Carbocation formed:** Secondary (after Markovnikov protonation). - **No resonance stabilization.** --- ### **B: Methylenecyclohexane** - Structure: Cyclohexane with an exocyclic methylene group (=CH₂). - **Carbocation formed:** Tertiary (proton adds to exocyclic carbon, leaving the positive charge inside the ring). - **No resonance stabilization, but tertiary > secondary.** --- ### **C: 4-Methylenetetrahydropyran** - Structure: Six-membered ring with one oxygen atom and an exocyclic methylene at position 4. - **Carbocation formed:** Oxygen is NOT adjacent to the carbocation, so no resonance stabilization. - **Secondary or tertiary carbocation depending on the position, but not stabilized by O.** --- ### **D: 2-Methylenetetrahydropyran** - Structure: Six-membered ring with O at position 1, exocyclic methylene at position 2. - **Carbocation formed:** Positive charge at C2, adjacent to the oxygen atom. - **Strong resonance stabilization** (O donates electron density). --- ### **E: 2-methylene-1,3-dioxane** - Structure: Six-membered ring with oxygens at positions 1 and 3, exocyclic methylene at position 2. - **Carbocation formed:** Positive charge at C2, adjacent to two oxygens. - **Even more resonance stabilization** (both oxygens can share electron density). --- ## Step 3: **Ranking by Carbocation Stability** **Carbocation stability order:** 1. **Resonance stabilized by two O > resonance by one O > tertiary > secondary > primary** So, the expected ranking (from FASTEST to SLOWEST): 1. **E: 2-methylene-1,3-dioxane** (Best resonance stabilization: two adjacent oxygens) 2. **D: 2-methylenetetrahydropyran** (Resonance stabilization: one adjacent oxygen) 3. **B: Methylenecyclohexane** (Tertiary carbocation, no resonance) 4. **A: Cyclohexene** (Secondary carbocation, no resonance) 5. **C: 4-methylenetetrahydropyran** (No resonance, probably secondary or less stable tertiary) --- ## Step 4: **Final Answer** ### **Order from Highest to Lowest Rate:** **E > D > B > A > C** - **E:** 2-methylene-1,3-dioxane (most stabilized carbocation) - **D:** 2-methylenetetrahydropyran (next best stabilization) - **B:** Methylenecyclohexane (tertiary carbocation) - **A:** Cyclohexene (secondary carbocation) - **C:** 4-methylenetetrahydropyran (least stabilized carbocation) --- ## **Summary Table** | Alkene | Carbocation Stability | Rate | |------------------------------|--------------------------------------|------------| | 2-methylene-1,3-dioxane (E) | Resonance, 2 oxygens | Fastest | | 2-methylenetetrahydropyran(D) | Resonance, 1 oxygen | | | Methylenecyclohexane (B) | Tertiary carbocation (no resonance) | | | Cyclohexene (A) | Secondary carbocation | | | 4-methylenetetrahydropyran(C)| No resonance, less stable | Slowest | --- **Reason:** The more stabilized the carbocation intermediate, the faster the reaction proceeds with HCl. Resonance stabilization (especially from adjacent oxygens) is most effective.