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Author Topic: Relative Basicity of Aromatics  (Read 7671 times)

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Assembler

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Relative Basicity of Aromatics
« on: October 07, 2006, 08:19:40 AM »

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.
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PRIYA1022

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Re: Relative Basicity of Aromatics
« Reply #1 on: October 07, 2006, 09:11:02 PM »

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.
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Assembler

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Re: Relative Basicity of Aromatics
« Reply #2 on: October 08, 2006, 07:49:22 AM »

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?
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PRIYA1022

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Re: Relative Basicity of Aromatics
« Reply #3 on: October 09, 2006, 01:20:32 PM »

Yes , it is
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greengiant

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Re: Relative Basicity of Aromatics
« Reply #4 on: October 09, 2006, 03:11:21 PM »

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.
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PRIYA1022

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Re: Relative Basicity of Aromatics
« Reply #5 on: October 09, 2006, 03:35:20 PM »

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
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greengiant

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Re: Relative Basicity of Aromatics
« Reply #6 on: October 10, 2006, 06:43:32 AM »

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.
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PRIYA1022

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Re: Relative Basicity of Aromatics
« Reply #7 on: October 10, 2006, 05:26:19 PM »

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

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Re: Relative Basicity of Aromatics
« Reply #8 on: October 11, 2006, 05:57:47 AM »

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. ::)
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