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This is what I found (source):

enter image description here

I wonder about several things here.

First, why is tetrahydrothiophene's dipole moment bigger than that of tetrahydrofuran, when considering that tetrahydrofuran has an atom of stronger electronegativity?

Secondly, why thiophene has weaker dipole moment when the S atom has larger p obitals, and therefore, has weaker conjugation ability than O to aromatic ring. The result seems that S in thiophene has better conjugation effect. (that's why it has less stronger dipole.) and I don't seem to explain why.

So what are the reasons for these dipole moment orders?

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As another (downvoted) answer points out, the key to this comparison involves the heteroatom donating an electron pair to the ring in the aromatic compound. This shifts the negative charge away from the heteroatom's corner into the ring. Thus we either do not get as much of a negative end at the electronegative heteroatom's corner, or with a stronger interaction we can reverse the dipole. In the examples shown, pay attention to the directions as well as the numbers of the given dipole moments!

Clearly nitrogen has this stronger interaction, while oxygen and sulfur give the weaker effect, with a reduced but not reversed dipole moment. Oxygen is so electronegative that even in forming the aromatic ring it donates an electron pair only weakly. The large sulfur atom has lower electronegativity, but overlaps poorly with the smaller carbon atoms. (In the nonaromatic tetrahydrothiophene, the larger size of the sulfur atom makes the dipole longer and thus may give a larger dipole moment with less charge separation.) Nitrogen in pyrrole has the moderate electronegativity and strong overlap which, together, enable the stronger interaction.

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    $\begingroup$ Thanks for your answer but my question is still not solved. I'll make it easy. Tetrahydrofuran has dipole 1.73D and furan has dipole 0.7D. So furan's overlaping effect offset 1.73-0.7=1.03D. On the other hand, Thiophene offset tetrahydrothiophene's dipole 1.9-0.51=1.39D. Isn't this mean thiophene donates electron better than furan? By the way, I checked furan is better at EAS than thiophene, which means furan donates electrons better. But what happened to dipoles??? (and thanks for explanation of tetrahydrothiophene's bigger dipole. That makes good sense.) $\endgroup$ – Zillai May 16 '18 at 16:31
  • $\begingroup$ 1.03 vs 1.39 vs ... 3.39, for pyrrole. It is pyrrole that srands out for dipole alteration, the others are just small potatoes. $\endgroup$ – Oscar Lanzi May 17 '18 at 0:50
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It depends on the extent of which the heteroaton can donate its lonepair into the ring and as N does it the most the dipole moment is upward and similar way it can be determined for the other two

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