[Assembler]

I got this question from the GRE practice test (#52):  Which is more basic: pyridine, pyrrole, benzene, furan, or theophene?  The answer is pyridine.  My question is why?  Obviously, benzene and pyrrole are not the most basic.  The other choices all have localized sp² heteroatoms with localized pi electrons.  WTF?  Thanks.

[PRIYA1022]

A base is a molecule that has the ability to donate a pair of electrons. Pyridine is the most basic in the given series as the lone pair of electrons on the nitrogen atom is not involved in aromaticity, and hence freely available.(The lone pair is in the sp2 hybrid orbital of nitrogen)
In pyrrole, the lone pair is involved in the aromatic sextet, so it is not free, hence a weaker base.

[Assembler]

Pyridine is the most basic in the given series as the lone pair of electrons on the nitrogen atom is not involved in aromaticity, and hence freely available.(The lone pair is in the sp2 hybrid orbital of nitrogen)

Isn't this true for the sulphur and oxygen heteroatoms, too?

[PRIYA1022]

Yes , it is

[greengiant]

With regard to furan, try drawing a resonance form of the protonated molecule that delocalizes the positive charge without violating the octet rule. Secondly, ask yourself what is more basic, diethyl amine or diethyl ether?

The octet rule doesn't apply to S, but think about the hybridization and ring strain you'd need to delocalize the charge. Moving down a period (replacing O w/ S for example) usually gives a stronger acid so it should be harder to protnate thiophene than furan.

[PRIYA1022]

Niitrogen can accomodate a positive charge more easily compared to oxygen( O is more EN than N)
Regarding thiophene, i would say that the bonds and the l.p's are present in the third shell, So there is less attraction from the nucleus on these electrons, making them more suseptible for protonation, compared to Oxygen in Furan.Hence I would say that Thiophene is more basic than furan

[greengiant]

The pKa' of tetrahydrofuran is -2.1. The pKa' of tetrahydrothiophene is -4.4 meaning THF is more basic (more stable conjugate base) than THT.

http://www.chem.wisc.edu/areas/reich/pkatable/ka-water.gif

In comparing thiophene and furan, you also have to consider if the thermodynamic position of protonation is on the heteroatom or C so you make the electronegativity vs. disruption-of-aromaticity argument.

[PRIYA1022]

Do you mean to say that an acid( proton source) is Sufficient to disrupt the aromaticity?

[greengiant]

Of course if you protonate furan or pyrrole on carbon you generate a non-aromatic species. Anywhere you put a proton on pyrrole will give a non-aromatic species.