Re: Why is pipendine more basic than pyridine

Dionne,

I am not familiar with pipendine.  Possibly your question has to do with 
piperidine ("ri" can look like "n", if you push the letters together) and 
pyridine.  I will answer it on that basis.

The basicity of these substances has to do with the fact that each has a 
nitrogen atom with a lone pair of electrons. It is the availability of 
this pair that makes each a base.  There are some similarities and some 
differences in their structures.  Each is composed of a six membered ring -
 five carbon atoms and one nitrogen.  In pyridine all atoms can be thought 
of as being sp2 hybridized.  It's an aromatic ring with a nitrogen atom 
included.  In piperidine, all atoms are sp3 hybridized.  Piperidine is a 
saturated ring.

Thus, the lone pair in pyridine is in an sp2 orbital.  In piperidine, the 
pair is in an sp3 orbital.  The sp2 orbital has more "s character" (33%) 
than the sp3 orbital (25% "s character").  The orbital with more "s 
character" (sp2)has a lobe that is larger around, BUT does not extend out 
from the nucleus as far as does the lobe that has less "s character", 
which is thinner but extends our further.

The piperidine is a stronger base, because the lone pair is more available 
to acids since it extends out further from the nitrogen nucleus.  
Stretching out further means that it can be more easily encountered and 
used by an acid; thus it is a better base, more basic.  One definition 
(Lewis)of a base is "an electron pair donor."

There may be a separate part of this comparison that has to do with the 
differing shapes and electron distributions of the rings, but I believe 
that the above explanation is the one most often given.

I hope that this has helped.  If you need more information or 
clarification, you may get in touch with me directly.

May You Have Joyous Holidays,

Dr. Jerry Franzen
Chemistry Department
Thomas More College
333 Thomas More Parkway
Crestview Hills, KY 41017


Dan Berger adds:
See also this MadSci answer.