Chemical Forums

Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: mana on January 14, 2017, 10:16:39 AM

Title: nucleophilic substitution addition in aromatic comp.
Post by: mana on January 14, 2017, 10:16:39 AM

hi all
what is the product of 1-chloro-3-nitrobenzene with NH₂^- and NH₃?
the electron withdrawing group is on the meta position, so does the reaction proceed from benzyne intermediate instead of addition-elemination mechanism? :P
as you know the withdrawing groups must be at ortho and para position in addition-elimination mechanism.
thanks

Title: Re: nucleophilic substitution addition in aromatic comp.
Post by: pgk on January 14, 2017, 03:02:41 PM

Indeed, the reaction will occur via benzyne formation.
Question: Can you predict the major product formed, meaning ortho-, meta or para- nitroaniline?
Hint: Firstly, take a look to the mechanism of nucleophilic substitution of chlorobenzene by NH2- in liquid ammonia.

Title: Re: nucleophilic substitution addition in aromatic comp.
Post by: mana on January 15, 2017, 01:44:44 AM

Quote from: pgk on January 14, 2017, 03:02:41 PM

Question: Can you predict the major product formed, meaning ortho-, meta or para- nitroaniline?
Hint: Firstly, take a look to the mechanism of nucleophilic substitution of chlorobenzene by NH2- in liquid ammonia.

so it must be ortho or meta because NO2 is an withdrawing group and nucleophile prefer to attack at a position which lacks electron. am I right?

Title: Re: nucleophilic substitution addition in aromatic comp.
Post by: pgk on January 15, 2017, 06:11:01 AM

Yes and No.
The explanation is right but you probably mean ortho- and para-, according your explanation.
Anyway, para- nitroaniline will be formed in preference, due to the steric hindrance of the nitro- group. Do not forget that this is a nucleophile attack that preferably occurs to the less hindered site.
Hint: NH2- attacking to benzyne, creates an electron rich carbon atoms, at the one of it's ortho- positions. 

Title: Re: nucleophilic substitution addition in aromatic comp.
Post by: pgk on January 15, 2017, 06:56:23 AM

Clarification:
It's not a "pure" nucleophile substitution. This reaction occurs in three steps. Firstly, a nucleophile attack that creates the "benzyne bond" by elimination of HCl, followed by NH2- attack to the so formed "benzyne bond" and then by "neutralization" of the carbanion by NH3. But the "benzyne bond" can equally be formed, either to the 2,3- or to the 3,4- position of the nitro group.