Reaction of furan with chloroform and hydroxide

This reaction looks like the Riemer-Tiemann reaction. The only difference is the reactant. First step is the formation of carbene. Second step involves the attack of π-electrons of furan on the vacant orbital of carbene which results in two possible type of products:

Two possibilities for electrophilic attack of carbene

The answer given is of type (B) (That is, dichlorocarbene is attached like product B). But both results in the same position of carbocations. Aren't both products produced in equal quantity?

Also, in the case of pyrrole, the opposite happens. That is, the major products are:

Reaction of pyrrole with chloroform and base


The key to this question lies in realising that the Reimer–Tiemann reaction is not an electrophilic aromatic substitution but it is in fact

addition of a carbanion to a strongly electron-deficient carbene.

This was discussed by @KlausWarzecha and @ron in answers to a previous question.

Therefore the major product will be the one where the attack comes from the carbon with the greatest electron density. Using MolCalc (partly developed by @JanJensen) I generated an electron density plot of furan which clearly shows that there is more electron density on the 3 and 4 carbons than on 2 and 5. This explains the given major product.

enter image description here

  • 1
    $\begingroup$ Is there a reason why there is a difference in electron density? $\endgroup$
    – Aditya Dev
    Nov 30 '15 at 16:40
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    $\begingroup$ This answer by @Ron says otherwise... chemistry.stackexchange.com/questions/26743/… $\endgroup$
    – Aditya Dev
    Nov 30 '15 at 17:15
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    $\begingroup$ @AdityaDev How does it say otherwise? The Reimer-Tiemann reaction is not an electrophilic aromatic substitution so the reasoning given in that answer does not apply. $\endgroup$
    – bon
    Nov 30 '15 at 19:06
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    $\begingroup$ I could not get why reaction of furan is a reimar teimar type rection and not pyrole. $\endgroup$
    – Pink
    May 24 '17 at 19:09

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