[bryct]

Hello,

I was wondering how one can determine which solvent is better for crystallization without trial and error.  Instead, determine it by comparing the polarity,melting and boiling points?  For example, if I want to recrystallize benzoic acid, and my choices of solvents are petroleum ether, 95% ethanol, or water, which one would be better??  I realize that all 3 solvents are polar and so is the solute.  I've narrowed it down to water and ethanol in this case, since their structures are more similar to the solute than an ether, but deciphering between the two solvents when they both have similar structures to the solute seems difficult without trial and error.  
Any thoughts would be greatly appreciated.  Thanks!!

[Alberto_Kravina]

Isn't benzoic acid soluble in hot water and less soluble in cold water ? Please check me on this...

[mike]

What other information are you given?

I would have said that trial and error was the way to do it

[bryct]

I'm not given any information.  I'm supposed to look up the melting and boiling point values and determine the best solvent without trial and error.

[Hbond]

Benzoic acid uses both both polar (acid) and van der Waals (benzene) bonding. The water is a good polar solvent. The petroleum ether is a good organic solvent, while the ethanol is a blend of the two. The ethanol would be my choice because it can break both types of bonds and therefore its removal should allow the crystals to form with both bonding charateristics forming together. The water is too polar and may result in hydrated water remaining. While the ether may result in polar bonds forming before van der Waals, thereby making a more amorphous solid.

[movies]

By the way, petroleum ether is actually not an ether; it's a mixture of low boiling hydrocarbons.  Its solvent properties are similar to that of hexane.

[Hima Potturi]

Benzoic acid is not very soluble in cold water but it's highly soluble in hot water. So, water should be the best choice.

[Alberto_Kravina]

Yo! Thanks for the answer to my question!
Which hydrocarbons are mixed in petroleum ether? I suppose C₅ and C₆ isomers...
 If yes, why is it called ETHER, ther's no --O-- functional group  

[mike]

Which hydrocarbons are mixed in petroleum ether? I suppose C5 and C6 isomers...
If yes, why is it called ETHER, ther's no --O-- functional group  

Ether is just an old/outdated name for a mixture of hydrocarbons with a specific boiling point range. I have also heard it called benzin, petroleum benzin, naptha, and pet ether, and here at my new job they call it X5 I guess because as you say C5 and C6 hydrocarbons are present (?)

[movies]

I think the "ether" refers to it's ethereal nature, that is to say, very low boiling liquid.  Back in the day scientists thought that the world was all held together by an intangible ether (and not just a lot of empty space), so when very low boiling liquids were isolated it was a natural assumption that they were a concentrated form of this ether, and thus got their name with no understanding of their chemical structure.

Petroleum ether is usually sold based on the boiling range which it is collected at.  Typically pet. ether boils around 30-60 degrees C, which means that it's mostly pentanes and some hexanes.  Higher boiling alkane mixtures were mentioned by Mike.  Another common one is ligroin, which has a boiling range of 60-80 degrees C and contains alkanes from hexanes to some nonanes, typically.