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Topic: nucleophilic substitution addition in aromatic comp.  (Read 2279 times)

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Offline mana

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nucleophilic substitution addition in aromatic comp.
« 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

Offline pgk

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Re: nucleophilic substitution addition in aromatic comp.
« Reply #1 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.

Offline mana

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Re: nucleophilic substitution addition in aromatic comp.
« Reply #2 on: January 15, 2017, 01:44:44 AM »
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?

Offline pgk

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Re: nucleophilic substitution addition in aromatic comp.
« Reply #3 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. 

Offline pgk

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Re: nucleophilic substitution addition in aromatic comp.
« Reply #4 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.

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