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Following mechanism is commonly proposed for the Wurtz-Fittig-Reaction:

enter image description here

Picture from Wikipedia

The red marked reaction steps is a nucleophilic substitution (SN). I suppose that this is a SN1 reaction, with an intermediate carbocation, rather than a SN2 reaction. In my opinion the resulting carbocation is good enough stabilized throughout the aromatic ring and therefore SN1 should be preferred, moreover because of the steric hindrance of the aromatic ring.

Is this assumption right? Thanks a lot for your help.

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    $\begingroup$ I’m kind of torn between upvoting this question because you actually thought about it and downvoting because your thoughts are entirely in the wrong direction. Notice that the phenyl cation is not stabilised by resonance and not delocalised over the ring. Quite contrary to your expectations, it is very reactive. $\endgroup$
    – Jan
    Aug 31, 2017 at 9:31
  • $\begingroup$ @Jan Ok thanks, the orientation and type of the orbitals I totaly forgot. This article explains this problem further. Therefore it cannot be a Sn1. But a Sn2 neither (massive steric problems). So I assume that there is a total different mechanism. Maybe nevertheless a formation of a Phenyl cation, but through a different mechanism (abstraction the Br as a Bromid through the carban-Anion)? $\endgroup$
    – Nilsfrank99
    Aug 31, 2017 at 11:14
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    $\begingroup$ My best guess is that it’ll be akin to an $\mathrm{S_NAr}$ reaction. $\endgroup$
    – Jan
    Aug 31, 2017 at 11:25
  • $\begingroup$ Note that the picture you linked to isn't present on the corresponding English Wikipedia article as of today. $\endgroup$
    – Gaurang Tandon
    Feb 14, 2018 at 5:50

1 Answer 1

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Aryl carbons cannot go through SN1 nor SN2 substitution reactions. SNAr involves the creation of benzyne intermediate. The mechanism is this:

enter image description here

The benzyne has sp hybridized carbons, which should be linear, but forced to be bent. Therefore it can be attacked by a nucleophile, and become sp2 which isn't linear.

Notice that the carbanion can also attack the adjacent carbon. It's a special phenomena for this type of reaction. The mechanism was confirmed by carbon-14 labeling (Organic Chemistry/Paula Bruice, sixth edition, page 692).

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  • $\begingroup$ "The mechanism was probably confirmed by carbon-13 labeling." You should have a source for this, imo - otherwise it just sounds like speculation. $\endgroup$
    – orthocresol
    Sep 1, 2017 at 16:38
  • $\begingroup$ I added a source. $\endgroup$
    – Unknown
    Sep 1, 2017 at 17:24
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    $\begingroup$ Correct me if this is wrong, but isn't this the mechanism for elimination-addition of benzene, not $\ce{S_NAr}$? $\endgroup$
    – Teoc
    Sep 2, 2017 at 5:22
  • $\begingroup$ The title of page 690 is: "Nucleophilic Aromatic Substitution: An Addition-Elimination Mechanism", and in page 691 it clearly says that this type of reaction is called SNAr ("the reaction is called an SNAr reaction" [substitution nucleophlic aromatic]"). In page 692 the title is "Nucleophlic Aromatic Subsitution: An Elimination Addition Mechanism That Forms A Benzyne Intermediate". It's simply a unique case of SNAr. $\endgroup$
    – Unknown
    Sep 2, 2017 at 5:39

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