HW Question 4.27

[Grace Ramey 2K]

Can someone explain why 4.27 part A is polar molecule? It is similar to benzene (which I believe is a nonpolar molecule), except that one CH group is exchanged for a nitrogen. But why would one nitrogen atom cause the whole molecule to become polar?

Thanks for the help!

[Aijun Zhang 1D]

Benzene is nonpolar since it has a symmetrical structure and all the bond dipoles cancel.
But for pyridine, the C-H group is replaced by a nitrogen. And there is a lone pair on nitrogen. The molecule then becomes not symmetrical since C-N bond has different dipole then C-C bond. Also C-H bond dipole polar effect cannot be canceled now.
You can see the 3D structure by this url.

[Kelly Kiremidjian 1C]

By replacing the -CH- in Benzene with a Nitrogen atom the dipole moments no longer cancel so it becomes a polar molecule.

[Sabah Islam 1G]

The nitrogen atom is more electronegative than than the CH groups present in the rest of the molecule; thus, pulling the electrons shared between the nitrogen and the carbons it is bonded to, towards the nitrogen, having a different electronegativity difference than the difference between the carbons and hydrogens in the molecule. On the other hand, benzene has 6 CH groups bonded to every carbon molecule, which makes the molecule symmetrical and having the same electronegativity differences between carbon and hydrogen present throughout the entire molecule. Also, there is a lone pair present on the nitrogen atom, which further contributes to the polarity of the molecule.

[Dana Wilks 3I]

Would we be expected to know the arrangement of C5H5N without being given a diagram?