# Step-by-Step Solution: Detailed Mechanism for Bromopropylation of Benzene ## **Reaction Overview** You are asked for the mechanism of the following reaction: **Benzene** → **para-bromo-propylbenzene** (Product: para-bromo-n-propylbenzene) This is a **Friedel–Crafts alkylation** reaction, where a propyl group (–CH₂CH₂CH₃) and a bromine atom are introduced onto benzene. ## **Step 1: Generation of Electrophile** The usual reagents for this transformation are: - **1-bromopropane (CH₃CH₂CH₂Br)** - **AlCl₃ (Lewis acid catalyst)** **Mechanism:** 1. **Activation of the alkyl halide:** \[ CH_3CH_2CH_2Br + AlCl_3 \rightarrow CH_3CH_2CH_2^+ + AlCl_3Br^- \] The AlCl₃ reacts with the Br atom, generating a **propyl carbocation** (electrophile). --- ## **Step 2: Electrophilic Aromatic Substitution (EAS)** ### **Arenium Ion Formation (Sigma Complex)** 2. **Benzene attacks the propyl cation:** - The π electrons from benzene attack the propyl cation, forming a carbocation intermediate (the arenium ion). - This can occur at the para position if para-selectivity is directed by a substituent already present (e.g., if Br is introduced first and then the propyl group, or vice versa). - The arenium ion intermediate is resonance stabilized. #### **Resonance Structures of the Arenium Ion** The arenium ion formed after attack at the para position can be represented as three resonance structures: - Positive charge delocalized around the ring: ``` + + + / \ / \ / \ | | | | | | | | | | | | \ / \ / \ / // \\ // \\ // \\ Br Pr Br Pr Br Pr ``` (Where Br = bromine and Pr = propyl at para positions.) #### **Resonance Hybrid** - The resonance hybrid would show delocalization of the positive charge over the ortho and para positions relative to the site of attack. --- ## **Step 3: Deprotonation** 3. **Restoration of Aromaticity:** - A base (often AlCl₄⁻ or another Cl⁻) abstracts the proton from the arenium ion, restoring aromaticity. \[ \text{Arenium ion} + \text{Base} \rightarrow \text{bromo-propylbenzene} + H^+ \] --- ## **Step 4: Final Product** The final product is **para-bromo-n-propylbenzene**. --- ## **Summary Table** | Step | Description | |-----------------------|-----------------------------------------------------------| | 1. Electrophile gen. | CH₃CH₂CH₂Br + AlCl₃ → CH₃CH₂CH₂⁺ + AlCl₄⁻ | | 2. EAS (Arenium ion) | Benzene attacks propyl cation → arenium ion intermediate | | 3. Resonance | Draw 3 resonance forms for arenium ion | | 4. Deprotonation | Loss of H⁺, aromaticity restored | | 5. Product | para-bromo-n-propylbenzene | --- ## **Mechanism Diagram** Below is a simplified representation (please draw arrows and resonance forms for your answer): 1. **Benzene + propyl cation → arenium ion** 2. **Arenium ion (show 3 resonance structures)** 3. **Arenium ion + base → para-bromo-n-propylbenzene** --- ## **Final Answer** ### **The detailed stepwise mechanism:** 1. **Generation of propyl cation:** \( CH_3CH_2CH_2Br + AlCl_3 \rightarrow CH_3CH_2CH_2^+ + AlCl_4^- \) 2. **Benzene attacks propyl cation:** Formation of the arenium ion (sigma complex). 3. **Resonance structures:** Draw all three resonance contributors for the arenium ion intermediate. 4. **Deprotonation:** Loss of a proton restores aromaticity. 5. **Final product:** para-bromo-n-propylbenzene. --- ### **Contributing Resonance Structures for the Arenium Ion** (Show positive charge delocalized at ortho and para positions relative to the site of attack.) --- **If you need hand-drawn resonance structures or a resonance hybrid diagram, let me know!**