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How to compare dipole moment in thiophene, furan, and pyrrole?
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Mike Mccall
How to compare dipole moment in thiophene, furan, and pyrrole?
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
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
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.
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.
Thanks for your answer but my question is still not solved. Ill make it easy. Tetrahydrofuran has dipole 1.73D and furan has dipole 0.7D. So furans overlaping effect offset 1.73-0.7=1.03D. On the other hand, Thiophene offset tetrahydrothiophenes dipole 1.9-0.51=1.39D. Isnt 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 tetrahydrothiophenes bigger dipole. That makes good sense.)More
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
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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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.
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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