Pyrrole & Pyrrolidine - which is the better base ?

[Jan Vermaak]

Hi,

Which is the better base of pyrrole & pyrrolidine.
As I see it at the moment, pyrrole will be a better base than pyrrolidine
because the protonated structure can be resonance stabilized whereas
no delocalization of the +'ve charge on the N in pyrrolidine can occur.
Furthermore, the aromiticity of pyrrole ensures an electron rich compound,
more so than the sp3 hybridized orbitals found in pyrrolidine.
Is this rationale correct ? Does somebody have the pKa values to
confirm or deny my suspicions ?
Thanks
Jan

[Db lowe]

In article <2qh1ju$m...@inet.up.ac.za>, s935...@rkw-risc.cs.up.ac.za (Jan
Vermaak) writes:

>Which is the better base of pyrrole & pyrrolidine.

Pyrrolidine is the stronger base, I believe. Pyrrole's lone pair is involved in
the aromaticity of the ring already, while pyrrolidine's is free and ready to
soak up a proton.

Derek Lowe
dbl...@aol.com

[Douglas Meier]

Also, pyrrolidine's sp3 orbitals have less s character than pyrrole's sp2's,
making the electron pairs farther from the nucleus and thus more
nucleophilic.

--
Douglas C. Meier | Many words have been granted to me, and
Northwestern University, ACNS | some are wise, and some are false, but
Standard Disclaimer Applies | only three are holy: "I will it!"
dme...@casbah.acns.nwu.edu | -Ayn Rand, _Anthem_

[Richard Parkin]

In addition to this, I believe that the pKa of pyrrole is about 1. So you
can see how weak a base it really is.....

Richard

[dal...@saturn.wwc.edu]

In article <2qh1ju$m...@inet.up.ac.za> s935...@rkw-risc.cs.up.ac.za (Jan Vermaak) writes:
>From: s935...@rkw-risc.cs.up.ac.za (Jan Vermaak)
>Subject: Pyrrole & Pyrrolidine - which is the better base ?
>Date: 7 May 1994 21:35:58 GMT

>Hi,

I don't agree. Protonating the nitrogen in pyrrole ties up the pair of
electrons, thus interrupting the aromatic ring current. The resonance energy
is high enough to make this difficult to do. The pKa of the conjugate acid of
pyrrole is about zero while pyrrolidine is about ten.

[Jan Vermaak]

Thanks for the responses - i realized my mistake soon after posting.
I suppose my main fault was to ignore the aromaticity of pyrrole which means
any reaction leading to a loss of aromaticity is thermodynamically
unfavourable.
Cheers
Jan

[Stephen Miller]

dal...@saturn.wwc.edu writes:

>In article <2qh1ju$m...@inet.up.ac.za> s935...@rkw-risc.cs.up.ac.za (Jan Vermaak) writes:

>>Which is the better base of pyrrole & pyrrolidine.
>>As I see it at the moment, pyrrole will be a better base than pyrrolidine

Sorry, no. Pyrrole is not basic at all. The lone pair is completely delocalized
in the ring. If you put pyrrole in a stongly acidic environment, you won't
protonate the nitrogen but you may protonate the pi system, probably resulting
in polymerization.
Pyrrolidine has an sp3 hybridized nitogen, so it's fairly basic (pka 9-10 or
so). As a general rule, sp3 hybridized nitrogens are more basic than sp2
hybridized nitrogens, which are in turn more basic than sp hybridized
nitrogens.
Pyrrole is a different case since the lone pair is not in a
hybrid orbital on the nitrogen, but rather delocalized in the ring in a p
orbital. As such it is not available to pick up a proton.

>>Thanks
>>Jan

>I don't agree. Protonating the nitrogen in pyrrole ties up the pair of
>electrons, thus interrupting the aromatic ring current. The resonance energy
>is high enough to make this difficult to do. The pKa of the conjugate acid of
>pyrrole is about zero while pyrrolidine is about ten.

Right. But I'm not sure you can even isolate the 'conjugate acid' of
pyrrole for reasons explained above.

Steve.

[sardara...@gmail.com]

Correction:
pyrrole shows pkb of 14 while that of pyrrolidine is 3.This clearly shows that pyrrolidine is as strong base as alkylamines(pkb 3 to 4) than pyrrole.

[Mark Thorson]

Thanks for the quick reply!