What is the major driving force for losing a proton as the last step in an electrophilic aromatic substitution reaction?

Explanation:

In the following reaction we have shown an example of aromatic substitution reaction .

C₆H₆ + RCl = C₆H₅R  + HCl

This  reaction takes place in the presence of catalyst like AlCl₃ which is a lewis acid .

First of all formation of carbocation is made as follows .

RCl + AlCl₃ = R⁺ + AlCl₄⁻

This R⁺ is carbocation which is also called electrophile . It attacks the ring to get attached with it .

C₆H₆ + R⁺  =   C₆H₅R⁺H.

The complex formed is unstable , though it is stabilized by resonance effect . In the last step H⁺ is kicked out of the ring . The driving force that does it is the steric hindrance due to presence of two adjacent group of H and R⁺ at the same place . Second driving force is attack by the base  AlCl₄⁻ that had been formed earlier . It acts as base and it extracts proton ( H⁺ ) from the ring .

C₆H₅R⁺H + AlCl₄⁻  = C₆H₆ + AlCl₃ + HCl .

The formation of a stable product C₆H₆ also drives the reaction to form this product .