6. i. What is the function of Lewis acid in alkylation of benzene? ii. Write the chemical reaction equation for the formation of toluene from benzene. (3 marks)

Question

6. i. What is the function of Lewis acid in alkylation of benzene? ii. Write the chemical reaction equation for the formation of toluene from benzene. (3 marks)

Answer 1

So, Benzene is aromatic, which means it has a very unique type of resonance stability. In plain terms, those pi electrons of the double bonds are not as available to take part in the typical alkene reactions we are previously used to (i.e. adding H₂O or Br₂), making Benzene a very poor nucleophile due to the fact that it does not want to give up its aromaticity just to add a substituent. With that being said, if we have a poor nucleophile, we just "super charge" the electrophile (or we increase the positivity of the electrophile), making it more attractive to the weak nucleophile. By using a Lewis acid like AlCl₃ or FeBr₃, it makes a very strong electrophile (Cl⁺ or Br⁺) so that the Benzene is driven by this attraction to react. In your example specifically, the electrophile would be something like H₃C-Cl⁺-Al^-Cl₃ or ⁺CH₃. (Note: I'm saying Cl+ and Br+ to simplify the complexes, in books it will be more like Cl-Cl⁺-Al^-Cl₃ and Br-Br⁺-Fe^-Br₃).