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In both cases ring expansion is happening. What is Carbon tri chloride purpose?

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" What is Carbon tri chloride purpose?" is not answered in the link provided but the answer starts with a carbene attack on furan .

Trichloromethane in presence of a base undergoes 1,1-elimination (or an alpha elimination) because both groups are removed from the same carbon.The electron-withdrawing effect of the halogens makes the hydrogen acidic enough to be removed by a strong base such as hydroxide or alkoxide ion.

A halide ion then acts as a leaving group from the conjugate base, producing the carbene.1

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Carbenes find a pair of electrons to complete their valence shell of electrons.carbenes attack a lone pair, a C=C double bond (electron-rich or -poor), or even a C–H bond.In its reactions ,

  • It is an electrophilic reagent and is known to give ring expansion of aromatic rings.
  • Electrophillic attack takes place on pyrrole where the intermediate is most stable.

This leeds to normal Reimer–Tiemann reaction product pyrrole 2-aldehyde.

Carbene attack of a C=C double bond leads to an ring expansion leading to 3-chloropyridine.

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The reaction of bicyclic compounf acts on similar lines of dichloro carbene attack.2

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References

1 ORGANIC CHEMISTRY ,JOSEPH M. HORNBACK ,UNIVERSITY OF DENVER

2 MARCH’S ADVANCED ORGANIC CHEMISTRY REACTIONS, MECHANISMS,AND STRUCTURE ,SIXTH EDITION

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  • $\begingroup$ The second diagram is quite loose with the hydrogen on the nitrogen atom. $\endgroup$ – Zhe May 22 at 19:16
  • $\begingroup$ I missed this the first time, but the arrow pushing is backwards in the first step of the pyrrole example. $\endgroup$ – Zhe Jun 21 at 19:35
  • $\begingroup$ @Zhe yes it is pyrrole that donates to dichloro carbene and not other way around.I will corect shortly. $\endgroup$ – Chakravarthy Kalyan Jun 22 at 0:54

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