Wednesday, July 22, 2009
Lithium Aluminium Hydride Reductions - Rochelle's Salt
Haha I'm still (barely) alive. Thanks for sticking around. Been busier than usual sorting my private and professional life out. Wrote a ton of grant proposals, published some papers (here, here, here and here), writing a book chapter, trying to be productive in the lab (fat chance) as well as having a life after work and some time off. So Curly Arrow got down prioritised for a while. Hopefully that is changing now.
Last week I did a lithium aluminium hydride reduction on large scale (see picture). This reminded me of the first time I had to work a reaction of this type up. My first experience (sometime last century) was a DIBAL reduction and if you haven't tried this stuff yet I can tell you that all these aluminium hydride reagents end up forming massive aluminium emulsions that are impossible to work with. The first time round I ended up making an utter mess and getting a very low yield. Realising that I couldn't possibly be the first chemist to encounter this problem I looked into things. The trick is obviously to break the emulsion up. There is a number of ways to do this. My favourite method is to use a saturated aqueous solution of Rochelle's salt (sodium potassium tartrate). Rochelle's salt is an excellent ligand for aluminium and breaks the aluminium emulsion. The procedure is simple. Cool your finished reduction down to 0 degrees C, or lower depending on the situation (For my large scale reduction I cooled it with acetone/dry ice) and quench excess reducing agent with something non-protic. For example ethyl acetate or acetone works well. Just remember to use something you can easily evaporate off when things are done. Don't be impatient and add it dropwise with vigorous stirring. Use a addition funnel for larger scale reactions. When the quench is complete remove the cooling bath.
I find it convenient to have a saturated aqueous solution of Rochelle's salt standing around. Please note that Rochelle's salt has a ridiculously high solubility in water so when preparing the aqueous solution go easy on the water and pick a small flask. When my reaction is quenched and everything looks like jelly I add some Rochelle's salt solution. Often I'll add it as a half saturated (or even more dilute) solution (a larger aqueous layer sometimes makes separation of the phases at the end easier). After pouring Rochelle's into your flask get the mixture stirring vigorously, have a cup of coffee and check your email. The better stirring and the more Rochelle's you use the faster it'll break up the emulsion. Ultimately you end up with two nice and clear phases that are simple to separate in a separatory funnel. D!