Sciencemadness Discussion Board

Ambiguities in paper nomenclature... stirring a solvent at a temperature above its boiling point?

Sidmadra - 22-3-2018 at 11:20

I've run into this ambiguity a few times in papers and I've always managed to work around it. I'm looking at a paper discussing a Lithiation of an aromatic compound, and it reads as follows:
Quote:
To a flame dried 2 neck flask were added 3g of Compound A, and 30ml of dry hexane under Argon. After that, n-butyllithium (5ml, 2.4M in hexane) was added at room temperature. The mixture was allowed to stir at 85 C for 2 hours, and then worked up according to literature.


I'm confused how they are able to stir hexane, with a boiling point of ~68c at 85c. Typically these issues can be worked around, but due to the delicacy of working with pyrophoric reagents, I thought it wise to seek clarification. I've read that Lithiation reactions of this nature that involve directing groups often take place rapidly, even at room temperature.

In this excerpt, do they just mean that the reaction was placed in an oil bath of 85c, and that the temperature of the reaction itself was probably far less? They make no mention of a condenser or anything. Occasionally when papers state a reaction is stirred above it's boiling point, I assume it is done so inside of a pressure flask, but this excerpt clearly states a 2 neck flask.

[Edited on 22-3-2018 by Sidmadra]

LearnedAmateur - 22-3-2018 at 11:25

‘Allowed to stir’ to me implies that it was under a rolling boil, since mixtures like that stir themselves. If it were mechanical stirring, then it would be along the likes of ‘the mixture was stirred’ instead. If they didn’t want it disturbed during the reaction but keeping it above hexane’s boiling point then pressure would’ve been increased somewhere along the line. Can’t offer any info about the reaction itself.

DraconicAcid - 22-3-2018 at 11:29

If it was actually at 85oC, they would have said it was refluxed or heated under pressure. I'd be inclined to assume it was a typo for 25oC. "Allowed to stir" doesn't imply a rolling boil to me- just that they left it on the stirring plate. If the idiom seems weird to you, the author was probably thinking "allowed to stand" but then remembered they had a stirrer in it.

Sidmadra - 22-3-2018 at 11:49

Edit:

I just overlooked this... there was a diagram above the reaction that specified they used an Oil Bath which was at 85 degrees. Oops. So I'm guessing the internal reaction temperature doesn't need to come close to that.

[Edited on 22-3-2018 by Sidmadra]