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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: jinclean on October 25, 2009, 07:51:34 AM

Title: The furan is more nucleophilic than the thiophen?
Post by: jinclean on October 25, 2009, 07:51:34 AM

The electronegativity is O>S,bue why in the heterocycle ,the O is more nucleophilic  than the S? Some people told me the reason is using 3p rather than 2p obital,so the conjugation isn't very well.but i think it  is secondary because the S can suffer more positive charge on it than O.So i think there should be a main reason create  this phenomenon
 

Title: Re: The furan is more nucleophilic than the thiophen?
Post by: Arctic-Nation on October 25, 2009, 08:26:07 AM

You forgot to take into account the aromaticity of the compounds. In the series thiophene - pyrrole - furan, thiophene has the largest amount of aromatic character (only slightly less than benzene), which makes it more resistant to electrophilic attack. Another effect is the character of the 3p-2p orbital overlap, which is weaker than 2p-2p overlap, thus resulting in less electron density within the ring and a smaller stabilization of the cationic intermediates in electrophilic substitution.
Furan, on the other hand, has a much larger electron density (remember: electronegativity rules single bonds, free electron pairs rule double bonds), making it much more reactive towards electrophiles.
A simple example to highlight the difference in reactivity is the Diels-Alder reaction between a diene (in this case furan or thiophene) and an electron-poor alkene, a dienophile. Thiophene only reacts with dienophiles under forcing conditions, while furan is so reactive that reactions can sometimes be carried out (or have to be carried out) at very low temperatures.

Title: Re: The furan is more nucleophilic than the thiophen?
Post by: jinclean on October 25, 2009, 08:47:13 AM

Thanks for your reply !But i think since the thiophen is 3p-2p weak overlap,so why thiophen has the most aramatic in the three five members heterocycle?And in the cation intermediate,although the furan has a larger eletron density ,the oxide onium have a small stabilization than the sulfur onium.

Title: Re: The furan is more nucleophilic than the thiophen?
Post by: jinclean on October 27, 2009, 06:21:15 AM

please reply for my post……

Title: Re: The furan is more nucleophilic than the thiophen?
Post by: movies on October 28, 2009, 11:30:27 AM

Think of the better overlap between the O and the C-pi system as an electron donating group within the ring.  That pushes the energy of the HOMO higher and therefore makes it more reactive.