# Cracking of dicyclopentadiene - was my simple Liebig condenser distillation setup adequate?

I recently did a distillation of dicyclopentadiene (bp 166 °C) to get cyclopentadiene (bp 40 °C). The setup as instructed by my supervisor consisted of a simple water-cooled Liebig condenser. Oil bath temp. at 180 °C, receiving flask held at -78 °C by a dry ice-acetone bath:

Everything seemed to work fine, and I got plenty of cyclopentadiene after distilling for about 4 hours.

Now (writing the report) I went hunting for some procedure references, and the first hit was this one (http://www.orgsyn.org/demo.aspx?prep=CV4P0238):

My interpretation of these instructions look like this:

My questions (too many I know, sorry):

• Is my interpretation of that setup correct?
• What is the Friedrichs condenser for?
• Why should the water temp. be 50 °C? (and how important is it to get this temp. correct?)
• Where would I attach the "calcium chloride drying tube" and what is it anyway, what's it for and how important is it to have one?
• Other procedures I found attached Nitrogen to the setup. Why would I do that and how important is it?
• How would you interpret "efficient water-cooled condenser"?
• Finally, can you put me at ease and tell me that my simple distillation setup was just fine?
• I remember that one time I used cyclopentadiene. We just took the bottle whose contents were solid wax (dimer) and put it into the rotavap’s water bath at $40~\mathrm{^\circ C}$ for the reverse reaction to happen … It was liquid (monomer) after a few minutes … You had to be quick, though, with measuring and adding because it would turn solid again quickly.
– Jan
Nov 8 '15 at 15:05
• The melting point of dicyclopentadiene is around 20 - 30 °C. So to me that sounds like you just used the liquid form of dicyclopentadiene, not cyclopentadiene. Nov 8 '15 at 15:25
• Hm, checking Wikipedia it sounds like you are right and our TA had no clue about the differences either xD. But then we were creating a metallocene so probably the equilibrium amount of monomer was enough (we used it as a solvent iirc).
– Jan
Nov 9 '15 at 8:40
• Your setup probably works perfectly well. the extra complexity in the sketch looks like it is intended to improve the purity of the product (by avoiding too much contamination with the dimer). This often doesn't matter much so is often avoided to keep things simple. Apr 14 '18 at 23:41

The Friedrichs condenser is for refluxing dicyclopentadiene (bp ~170 C). It's kept at 50 C to let the cyclopentadiene (bp ~40 C) pass without condensing.

• I see. Since I had my oil bath temp. at 180 °C, I ran the risk of distilling some "uncracked" dicyclopentadienes together with the cyclopentadiene. I speculate that it probably worked because the head temperature in all likelihood was below the bp of dicyclopentadiene (can't say for sure, as I didn't measure temperature there). - Edit: looking at my own foto, I notice that I actually did have a thermometer there, but I never read it :-( Nov 18 '15 at 21:15

I agree the setup is a bit much. The way I viewed the above diagram the receiving flask is on the left, hence the Friedrich's condenser is used to rapidly condense the hot, cracked cyclopentadiene rapidly. As such it gets the cold water, and the other condenser gets the hot water to help the reflux. Its a good idea to keep it cold at this point since low temp slows the dimerization. When Ive done it in the past I've had my setup under flowing N2 to keep it dry, but the drying tube could work depending on how pure you need it. Also I would freeze the collected cyclopentadiene during collection and storage. Things typically react slower in the solid phase. J. Chem. Educ., 1987, 64 (10), p 898

• You've got it wrong. Read the description above the sketch: "technical Dicyclopentadiene ...placed in two-necked flask equipped with thermometer and Friedrichs condenser (..50°C)". Also why would you put a thermometer in the receiving flask?
– Karl
Mar 29 '16 at 12:08
• Your right I just looked at the picture and wondered why someone would put a thermometer above the condenser on the receiving flask side. Perhaps a thermometer In the receiving flask is to make sure the cracked product stays sufficiently cooled. Mar 29 '16 at 18:08
• You put a thermometer in the educt to make sure you're not overheating it, most importantly. Does that publication you cite give a rate constant for the dimerisation (and at what temperature)? I somehow doubt that extra cooling is really necessary.
– Karl
Mar 29 '16 at 23:07

That setup in the drawing is way over the top, and you'll never get a drop of destillate with it. The Cyclopendadiene will fully condense in the bridge and all flows back into the Friedrichs. Ten degrees temperature difference to the dew point will never get it over 20 centimeters of glass tube.

However you should attach a drying tube to the exit near the receiving flask to avoid condensing water into it, especially if you do that nonsense with the dry ice/acetone. Typical for organic chemists, I'm sorry to say. A simple ice bath would help as much. Just keep the receiving flask as cold as the water in the cooler.

And 180 dC is a bit much, I'd just stay under the boiling point of the dimer. I remember adding some iron powder was supposed to speed up the retro-Diels-Alder reaction, i guess by lowering the effective concentration of the monomer through complexion.