I'm trying to use hexanes as the nonsolubilizing recrystallization solvent in a multisolvent recrystallization (benzene is the solubilizing solvent) that requires an oxygen free atmosphere and hexanes is present in the glovebox. I am using it in place of light petroleum ether (not in glovebox) and was wondering what the differences between hexanes and light petroleum ether were to determine if I can adequately substitute the two.
$\begingroup$
$\endgroup$
1
-
2$\begingroup$ My understanding is that they're fairly similar, but hexanes is purified to be mostly hexanes, whereas for petroleum ether, you get a mixture of hydrocarbons that boil in a given range. For recrystallization purposes, the substitution is probably fine. I guess it depends on how finicky your recrystallization is. $\endgroup$– ZheOct 19, 2016 at 2:53
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
3
Petroleum ether: This is a mixture of a lot of different compounds (aliphatic hydrocarbons in this case) that as a mixture have a boiling point within a specified range. The contents are dealt with as having low amounts of aromatic compounds and high amounts of this and that, not one specific formula. It's like a blackbox to me and can be used for crystallisations that are usually easy to do.
Hexane: In the synthesis laboratory I worked in, hexane was dealt with as we would deal with petroleum ether, since it was a mixture the many possible constitutional isomers and not pure n-hexane. We bought the technical grade hexane in our chemical store and purified it with a solvent purification system to have it dry and compatible for glove box use. Since they appeared so clean in our Schlenk flasks I thought in the beginning that it was pure n-hexane but a simple 1H NMR spectrum taught me otherwise. The hexanes mixture was used for medium difficulty crystallisations.
Pentane: The pentane we bought and purified, however, was n-pentane and was used for difficult crystallisations that were not easy to reproduce. Already among organometallic compounds there are examples in which obtaining crystals at all is an enormous struggle and having single crystal quality is simply a matter of hope.
Therefore, by going from petroleum ether to hexane to n-hexane (or n-pentane) is a matter of going from a wide mixture to a compound to a pure chemical.
-
$\begingroup$ Any rationale for this finding, that stuff crystallises better from pentane than hexane or petrol ether? Btw., the use of n-hexane is frowned upon, because it is supposed to be rather more toxic than e.g. heptane. A mixture named "isohexane" seems to be widely used as substitute, where cyclohexane is regarded as too expensive. $\endgroup$– KarlOct 19, 2016 at 15:37
-
$\begingroup$ Ah, thank you! I knew about benzene being frowned upon but not n-hexane. $\endgroup$ Oct 22, 2016 at 9:30
-
$\begingroup$ The difficulty in crystallisation comes from highly symmetric clusters, large solvent spaces in the crystal lattice, or sometimes the compound crystallises only once and cannot be recrystallised because it decomposes. To find the optimal crystallisation conditions, it's best to rely on solvents with high purity and always the same properties. This cannot be found in petroleum ether. Petroleum ether is different in laboratories all over the world and pure compounds have a higher chance of leading to a reproducible experimental procedure. $\endgroup$ Oct 22, 2016 at 9:32