help...Organic chemisty ques.
Alan "Uncle Al" Schwartz
>Hi...
>
>I wanted to know why is Z-Silbene not a planar compund? I also want to
>know In the "cracking of cyclopentadiene, why is it necessary to distll
>the product very slowly?
>
>Any help would be appreciated....
For the first: Consider steric interaction between the ortho-protons on
each benzene ring.
For the second: You cannot distill Cp which has not formed by thermal
cracking of the dicyclopentadiene (which is slow). Monomer and dimer are
close enough in boiling point that a sloppy job will deliver a lot of
dimer to the collection pot, which is a real pain to remove from many
Diels-Alder product mixtures.
Mohammad El-Mahdi
david jones
: Hi...
You mean dicyclopentadiene. Left to itself, cyclopentadiene does an auto
Diels-Alder to a dimer (viscous liquid, almost solid in a cold room).
The Diels-Alder is reversible, so you heat to get distill off the volatile
cyclopentadiene, which is then replenished by the dimer breaking down.
I imagine overheating it would drive it into polymerization and you wouldn't
get as much yield. However in my days as a chemist I was pretty liberal
about how I cracked the stuff - the starting material was cheap and
plentiful.
Z-Silbene? Stilbene? If biphenyls etc. are non planar its because the
delocalization offered by coplanar benzene rings is not enough to offset
the steric interference of the groups attached to the rings.
--
Dave Jones, Yorkshireman living in Brighton, but not the
one with the pebbly beach and crumbling piers.
Live from Rochester NY USA.
C++ will do for C what Algol-68 did for Algol
AF
el-m...@ix.netcom.com(Mohammad El-Mahdi ) wrote:
> Hi...
>
> I wanted to know why is Z-Silbene not a planar compund? I also want to
> know In the "cracking of cyclopentadiene, why is it necessary to distll
> the product very slowly?
>
> Any help would be appreciated....
Hi Mohammad:
Cis stilbene is not planar due to steric repulsion of the two internal
ortho hydrogen atoms on each of the phenyl rings. In other words, if you
make a model, it is not hard to imagine the edges of the phenyl rings
bumping into each other. The trans isomer does not have this situation.
As for the necessity for the slow cracking of cyclopentadiene, I believe
it's a compromise between obtaining pure cyclopentadiene (efficient slow
fractional distillation) and throughput (rate of collection of distilate).
Let me explain: cyclopentadiene (monomer) boils at 46 C and
dicyclopentadiene (dimer) boils at 170 C. The rate of monomer formation
(cracking) becomes reasonable at temperatures above 150 C and approaching
the boiling point of the dimer. If you were to attempt to distill the
monomer from the dimer at say 60 C it would probably take a very long time
due to the slow rate of monomer formation at that temperature.
Alternatively, if you were to attempt a quick simple distillation of the
dimer (short-path condenser), you would obtain a mixture of monomer and
dimer. Thus the combination of a fractional column and slow distillation
allows time for equilibration and efficient partitioning of monomer and
dimer while the ascending vapors contact the descending condensate in the
distillation column.
A co-worker once told me that a piece of copper wire placed in the
fractionation column would catalyze the cracking process. Give it a try!
Andy