[nan]

Does anyone know the lit. of titrating LDA.
Or how to titrate nBuLi I guess. Does that have to be done at -78C, because I'm thinking nBuli is going to deprotonate THF...
Thanks

[lavoisier]

Hi nan,
there are several methods for doing this titration.

Many are based on the double deprotonation of weak acids (phenylacetic acid, some toluamides...). BuLi or LDA quantitatvely abstract the most acidic proton first; the endpoint is then immediately detected when a small quantity of dianion is formed (because it's coloured - although sometimes it's faint yellow, just the same as your BuLi or LDA).

I tend to prefer a different method (recommended by my supplier of BuLi, by the way). It is also described in many papers, which specifically criticise some aspects of the double deprotonation methods. Unfortunately I don't have the references here: I will briefly describe it for you.

First you need to prepare a 1.0 M solution of sec-butanol in anhydrous toluene.
Then you dissolve, say, 1 mg of 1,1'-dipiridyl in 15 mL diethyl ether at room temperature (under inert and anhydrous conditions of course)
Add your BuLi until you get a stable, red solution.
Add sec-BuOH until the red colour disappears.
Add just enough BuLi to get the red colour back. Remember the intensity of the colour you see at this point, because it will be your endpoint.

Now you can start the actual titration.
Add a measured volume of sec-BuOH solution, then titrate with your BuLi (until you reach the endpoint you previously observed). The molarity of BuLi will be the volume of sec-BuOH divided by the volume of BuLi.
The authors suggest to repeat this three times for better accuracy.

It sounds more complicated than other methods, but note that here you can repeat the titration many times without having to weigh the reagents again and again. You can even titrate several bottles of BuLi, LDA in one flask.

[nan]

Thanks a lot Lavoisier,
Some Questions regarding: What is 1,1'-dipyridyl to be exact? Also the red colour is from? Why use sec-BuOH?
Can't wait to try it

[movies]

The method I usually use is to take a briquette of menthol (usually about 80-100 mg), dissolve that in dry THF (~2 mL) and then add 1-2 mg (you don't need to really weight it) of triphenylmethane.  You can roughly calculate how much of your lithium reagent you will need, and then add it dropwise to the solution of menthol/Ph₃CH/THF.  When all the menthol is deprotonated the triphenylmethane will be deprotonated abnd the solution will turn pink (eventually dark red if you add to much), then you can back calculate the concentration based on the volume of alkyl lithium you added.  I usally carry this procedure out in triplicate ad average the results.  It really only takes about 30 mins start to finish and it doesn't take much material either.

[Mitch]

good preps movies and lavoisier. +3 scooby snacks to both of you.

[lavoisier]

Thanks a lot Lavoisier,
Some Questions regarding: What is 1,1'-dipyridyl to be exact? Also the red colour is from? Why use sec-BuOH?
Can't wait to try it

1,1'-dipyridyl is a commercial reagent, it's two pyridine rings held together by a bond between their alpha carbons (the carbon next to N).

I don't remember what the red colour is exactly due to, it should be a lithium complex of 1,1'-dipyridyl anion.

I don't know why they use sec-BuOH. It may be because it's less prone to side reactions, but here I'm just guessing. As long as I don't know the reason I tend to stick with the literature procedure.

[movies]

I would watch out for your n-BuOH getting wet.  That is why I use menthol, it always stays quite dry, even when stored on the bench.  I think n-BuOH is fairly hygroscopic.

[nan]

 Thanks all of you guys.
 Haven't tried either one yet, a fellow in the lab next door was titrating LDA himself, and he got the prep to dissolve nBuLi in water and to titrate LiOH with HCl.
 Interesting to see the results from different preps.

[Custos]

1,1'-dipyridyl is a commercial reagent, it's two pyridine rings held together by a bond between their alpha carbons (the carbon next to N).

Then that would be 2,2'-dipyridyl wouldn't it? The nitrogen is numbered as one.

[lavoisier]

Yes, of course, sorry.

[lavoisier]

Thanks all of you guys.
 Haven't tried either one yet, a fellow in the lab next door was titrating LDA himself, and he got the prep to dissolve nBuLi in water and to titrate LiOH with HCl.
 Interesting to see the results from different preps.

Sorry nan, but this method won't work! It's fundamentally wrong and also dangerous!

Think about it: why do you titrate LDA? Because it may be partially hydrolysed to diisopropylamine and LiOH, and you want to know what remains of the original nitrogen anion.
If you pour LDA in water (a very exothermic reaction, btw) you convert the remaining LDA exactly to diisopropylamine and LiOH, making the whole thing pointless. You don't give LDA the chance of behaving differently from its decomposition products.
In addition to that, HCl is such a strong acid that it will titrate both LiOH and diisopropylamine...

If you use a weak acid (such as sec-BuOH) instead, it will only titrate the strong base LDA.

So please tell your friend that he's getting it horribly wrong.
Besides, if people use such complicated methods instead of a silly HCl titration, there must be a reason, they are not making their life needlessly difficult.