Cant form derivative of crotonic acid?

[samukm2]

Hi, I have been trying to form an analine derivative of crotonic acid, but after three trials a dont get any crystalls. I was guessing that the reasing may be that my acid has a vlow melting poing. Any advise? Thanks.

[hypervalent_iodine]

What do you need the crystals for? Often, compounds will exist as liquids at room temperature. It's just how it is really. If you were expecting crystals and you aren't getting them it may be that you have solvent contaminating your product. Some solvents can be quite tricky to get out. My advice would be to pump it down as much as you can on a rotary evaporator (you may need to put in a water bath at about 45 degrees C) and then put it under a high vacuum or lyophiliser.

 

A simple proton NMR should tell you if solvents are the problem. Otherwise, your product may just have a low melting point. Nothing wrong with that, unless of course you wanted to get an X-ray crystal structure.

 

Edit to add: your product may be hydroscopic (I've not looked into it for your exact case). If you leave it under high vac for a while, you may see condensation on your gooseneck connector. A quick treatment with a heat gun will get rid of that. Keeping water out will probably involve a super seal on your flask, parafilm and a fridge. Or transfer it to a dry, glass vial and parafilm around the lid before putting it in the fridge.

Edited April 2, 2011 by hypervalent_iodine

[mississippichem]

I once had a product that I couldn't make crystalize. NMR and mass spec showed it to be the pure target compound. We ran a molecular dynamics simulation and it turned out that this compound was an oil! It's frustrating because we wanted a neutron diffraction structure, but these things happen in chemistry.

[Horza2002]

Not all compounds will be solid or crystals. In fact, all of mine are oils. Alternatively, you have a miniscule amount of solvent trapped in the material and so it doesn't crystalise

[UC]

Not all compounds will be solid or crystals. In fact, all of mine are oils. Alternatively, you have a miniscule amount of solvent trapped in the material and so it doesn't crystalise

 

 

There were compounds that I had to leave on a high vac line for 2 weeks before they would give up enough EtOAc to decide to crystallize.

[hypervalent_iodine]

EtOAc is the devil. When I worked with sugar based compounds last year, I spent most of my time trying to get rid of the stuff. Working with sugars meant I also couldn't heat my product above 30 degrees C, lest I make toffee. Most of my products took a few days under high vac before they would even think about crystallizing out.

[Horza2002]

To be honest, I rarely bother getting all the last remaing of solvent out of my compounds (except final compounds). Unless of course that solvent is going to cause problems....like having chloroform in the presence of BuLi is really just asking for trouble.

 

You can also just flash freeze them....that works well on the rare occassion I actually get a solid

[hypervalent_iodine]

To be honest, I rarely bother getting all the last remaing of solvent out of my compounds (except final compounds). Unless of course that solvent is going to cause problems....like having chloroform in the presence of BuLi is really just asking for trouble.

 

You can also just flash freeze them....that works well on the rare occassion I actually get a solid

 

Yes, I mentioned lyophilising in my first post. We don't have a freeze drier in our lab I guess I'm just pedantic. Unless it's a carry through, I like to have all solvent goooonnnneee.

[Horza2002]

Another way I have found works very well is to azeotrope the sovlent off. Once you have the majority of the solvent off, add in some diethyl ether and it helps remove the last of solvent. Works very well for ethyl acetate.

 

You can also use this method in sequence to get rid of water from a sample. Dissolve the sample in toluene and remove under vacuum removes the water. Then added ethyl acetate to get the toluene off, then added DCm to get the ethyl acetate off and then add ether to get the DCM off. Its a rather long winded approach but it certainly works.