Reasons for dry conditions in substituition reactions with benzene


> 
> All substitution reactions of benzene must be carried out in dry conditions with a catalyst that produces a powerful electrophile.
> 
> 
> 


This was a statement from my book. My question is, why must it be carried out in ***dry conditions***? 


What will occur if moisture is present?




With electrophilic aromatic substitution (EAS) you use a strong lewis acid to create an electrophile.
$$\ce{AlCl3 + R-X \rightarrow AlCl3X- + R+} $$
$$\mathrm{R = Cl,\ Br,\ I,\ alky,\ aryl,\ RC(O),\ etc...}$$
$$\ce{X = Cl,\ Br,\ I, R-C(O)O,\ etc}...$$
These lewis acids (not the benzene) are incredibly sensitive to water which will irreversably react with the acid to form a hydrate.
$$\ce{ACl3 + X H2O \rightarrow ACl3.X H2O}$$
$$\mathrm{For}\ \ce{AlCl3}: \ \mathrm{1\leq X \leq 9}$$
This hydrate is not as strong of a lewis acid as the anhydrous acid and no longer able to form electrophiles. That is why you must use anhydrous conditions with substitution reactions.


 1

An electrophile goes after electron density, as in the two lone pairs in $\ce{H2O}$ plus poalrization. You then require conditions wherein water is inert (hypohalite halogenations, maybe samarium triflate), micelles exclude water, or the substituted benzene is so activated (dimethylaniline) that it doesn't care.


 0

Electrophiles
are reactive cations, they will react with water to form 
eg an alcohol (or some weak acid etc) and protons. 
No reaction with benzene. 


 0

Benzene and other aromatic hydrocarbons are immisible with water. So, probably there is no problem with benzene if we there is moisture. 


In most of the substitution reactions which benzene undergo, requires Lewis acid catalyst, like ferric halide or aluminium halide. Most commonly used catalysts are Anhyd. $\ce{AlCl3}$ and $\ce{FeBr3}$, which become inactivated if they react with water. So, this requires dry conditions. 


 -1