Sciencemadness Discussion Board

SUPERBASE! (Well, what is it?)

BromicAcid - 4-8-2004 at 15:06

Today while I was looking though some of the items on eBay I ran across some Potassium t-butoxide and next to it, it said that it was a superbase component. Well, many of my inorganic text books and even my organic have covered superacids and their ability to protanate and their various mixtures and their use in the oil industry, etc. However I had never run across the term superbase. Although its existence makes sense. After some searching there are some exotic mixtures and some simple, e.g., I found a site that listed a mixture of KOH in DMSO as a superbase environment.

Regardless, a search for the exact properties of a superbase was fruitless. So can anyone give a good definition of a superbase and possibly more information on their chemical reactions?

Nick F - 4-8-2004 at 17:46

I doubt there's a specific definition of a superbase. They're just anything that's very good at deprotonating things! It's not a term I've ever heard before, either.

I'd guess many organometallics could be considered superbases. t-Butyl lithium, lithium diisopropylamide and such come to mind.
I'm surprised an alkoxide is a component of one, alcohols are fairly acidic (compared to amines and alkanes!).

BromicAcid - 4-8-2004 at 17:53

The mix involving the alkoxide is a 1:1 mixture of potassium t-butoxide and n-butyl lithium. There really isn't much on the internet involving superbases. One interesting thing that I found said they could deprotonate ethers leading to polymerization of ether chains. I really wish that I would have found this subject before my library run today so I could have done some more research. I always want to learn about the 'strongest' whatever and superbase just seems like something that I want to learn more about.

Of course the first thing I wanted to do was react a superbase with a superacid ;)

svm - 4-8-2004 at 20:04

i think the deal with "super bases" is that the alkyllithium exchanges the lithium with the potassium on the alkoxide base. the result is a alkylpotassium, which is a stronger base than an alkyllithium. there are a lot of theories about what actually happens in the solution, i don't think anyone knows, but i like the explanation above.

i suppose the grand poo-bah of bases would be an alkyl cesium (or francium, if it weren't so darned unstable).

atombum - 4-8-2004 at 20:24

In reply to Nick F:

Alkali metal alkoxides are actually strong bases in and of themselves. And I believe alkyl branching increases basicity. You combine that with the fact the cation is K, not Na, as with the more common alkoxide, i.e. sodium ethoxide, and you should have a pretty strong base.
With alkyl lithium in the mix, who knows how powerful a base the mix might be. :o

Nick F - 5-8-2004 at 07:41

I know alkoxides are strong bases, but they are not the strongest by a long way!

It shouldn't be too hard to make up a simple superbase similar to the one Bromic described. Alkoxides can be made from alcohols and hydroxides in the presence of CaO, and alkyl lithiums can be made from alkyl chlorides and lithium....

svm - 5-8-2004 at 08:03

the pKa of an alcohol is between 16 and 19 depending on the substitution (more substitution, higher pKa, stronger base). the pKa of an alkyllithium is around 44. big difference!

BromicAcid - 5-8-2004 at 14:15

Just a guess, the butyl lithium can deprotonate the butoxide and this leads to the superbase conditions. I did find this small piece of information:

"Superbases are capeable of deprotonating compounds of very low acidity, to about pKa 45."

From Here.

Interesting PDF mentioning some reaction specifics for superbase formation.

According to the latter link the potassium in the butoxide and the lithium on the butane swap and you end up with butyl potassium and lithium t-butoxide and butyl potassium is the superbase component.

[Edited on 8/5/2004 by BromicAcid]

Theoretic - 20-10-2004 at 01:40

The strongest base ever: CsH! The electrochemical potential of deprotonation is huge: -2.25 V for: H- + H+ => H2. But still, it's technically a deprotonation. :D

BromicAcid - 20-10-2004 at 15:30

Cesium hydride may be a good base but it isn't that great, just that the hydrogen leaving the solution constantly drives the equilibrium. It's better then cesium metal.

Hawley's Condensed Chemical Dictionary list's cesium hydroxide as the strongest base, what it doesn't say and it should say is, 'In aqueous solution'. I'd like to see any cesium salt deprotonate methane.

Theoretic - 21-10-2004 at 05:17

The hydride ion is a much stronger base than the hydroxide ion. One can deprotonate a water molecule to make the OH- ion with relative ease, while deprotonating a hydrogen molecule to make H- is quite hard.

BromicAcid - 21-10-2004 at 07:11

Any alkali metal, which would deprotonate water extremely easily, will somewhat easily deprotonate hydrogen, as they readily make hydrides at mild temperatues, some like cesium readily at room temperature. So there is really no excessive difficulty in forming a hydride.

Nice table of bases and links to some other information and superbases.

The 'Proton Sponge' sounds to be about as powerful as things get.

[Edited on 10/21/2004 by BromicAcid]

svm - 21-10-2004 at 08:28

I've seen the pKa of H2 listed around 28. I think hydride bases are generally thought to operate in a pKa range of 33-35, which is quite a bit more basic than the "proton sponge" (pKa around 14). I might be confusing the reference values though, as pKa is dependent on the solvent.

I know that some huge, highly conjugate phosphorus/nitrogen bases have been made and they have a pKa up around 50.

here are some other pKa tables:

http://daecr1.harvard.edu/pKa/pka.html
http://www.chem.wisc.edu/areas/reich/pkatable/index.htm

BromicAcid - 21-10-2004 at 15:24

Well, a proton sponge sounded strong ;)

I tried to find some information on them but found only a little, most of it from the google groups archives from Uncle Al.

About the phosphorous-nitrogen bases, that link I have above mentions them on one of the off-shoot pages:
Quote:
Verkade's bases, football-shaped proazaphosphatrane molecules of type 1, are very strong bases due to the extraordinary stability of 2 when 1 reacts with a proton.

Due to the stability of the protonated form 2, Verkade's bases are about eight orders of magnitude stronger as a Lewis base than any amine known, including the prominently used DBU, DBN and Proton Sponge. (14795).


Very interesting, if only there was a simple prep.. or is that too much to ask :D

They also have other nitrogen-phosphorous bases:
Quote:
Phosphazene bases are extremely strong and uncharged bases, built on a unit where a nitrogen basic center is double bonded to pentavalent phosphorus.[1,2] Through oligomerization of the peralkylated triaminoiminophosphorane unit, the basicity improves dramatically (Figure 1). In the case of the monomeric phosphazene base, its basicity is about a 2-3 units beyond the basicity range of DBU (MeCNpKBH+ 24.3) and DBN, but reaches a MeCNpKBH+ of over 40 in the case of a tetrameric P4 phosphazene base (DBU = 1,8-Diazabicyclo[5.4.0]undec-7-ene, DBN = 1,5-Diazabicyclo- [4.3.0]non-5-ene).


I still really wish I could find a book strictly covering super bases locally though.