Re: How do they make linseed-oil more reactive ?

Hi Jack,

I'm sorry I don't have a good answer for you. I did not find anything
connecting the use of linseed oil and cyclopentadiene. Here is what I do
know that might help you out some. 

Cyclopentadiene is a five carbon cyclic hydrocarbon containing two double
bonds in conjugation. This arrangement makes it an excellent Diels-Alder
diene and it readily reacts with itself at room temperature to form a
dimeric species aptly called dicyclopentadiene (a crystalline solid with
mp=32 degrees C). A bottle of cyclopentadiene left standing will be
completely dimerized in a few days.

Cyclopentadiene may be recovered by heating the dimer to >150 degrees C
causing the retro-Diels Alder to occur then distilling off the monomeric
cyclopentadiene (bp is around 30 degrees C I think) into a cooled receiver
flask. Material obtained in this way may be used as cyclopentadiene for a
day or so before significant amounts of dimer form. So my first answer to
your question is that cyclopentadiene is probably _not_ used for the
purpose you described because it is simply not practical. If it is used
then it is probably used in the dimeric form. In any case I cannot see how
it would facilitate drying of linseed oil and I was unable to find any
reference to it.

Linseed oil is obtained from linseeds, constituting 30-40 percent of the
total mass. The oil contains a mixture of glycerides of unsaturated acids:
linolenic, linoleic, oleic, stearic, palmitic, myristic. [Merck Index, 8th
ed.] The oil dries by air oxidation to solid (or viscous) products.

I can guess that if linseed oil was treated with cyclopentadiene (in the
monomeric form only) then Diels-Alder reaction would occur between the
double bonds in the unsaturated oils and the two double bonds in the
cyclopentadiene. The norbornene-like products would likely be solids and
therefore would not need to be oxidized in order to "dry"; however, this is
only an unsubstantiated guess. 

Well, I hope some of this information was helpful!

Jeremy.



DIELS-ALDER REACTION WITH CYCLOPENTADIENE:

	HC-CH          R R                  HC=CH
        // \\          | |    ------>       /   \
       HC   CH    +   HC=CH   <------     HC-CH2-CH
         \  /                               \   /
          CH2                               HC-CH
					     | |
   Cyclopentadiene    alkene                 R R

					Norbornene-like
					   structure