[ons121]

I am an undergraduate research assistant in an Organic Chemistry laboratory. I'm working on this experiment that requires me to make LDA.  I have the steps and procedure to make LDA, but somehow and somewhere I am messing up.

My LDA looks clear, sometimes slightly turbid, but never a clear yellowish color.

1. I flame dry and use a full vacuum for the round bottom flask
2. I let it cool
3. I add distilled diisoproylamine  under inert N2  (.7 ml)
4. Add THF (6.6 mL)
5. let it cool to 0 C (using regular ice and water), wait 10 mins
6. add n-BuLi dropwise (.4 ml)
7. stir for 20 mins at 0 C
8. Cool to -78 C using dry ice and acetone

then from here, I follow the other steps my prof has asked me.

Please can someone give me some pointers or guidance. I"ve been working on this project the last 4 months and I can't seem to get it right. My LDA always looks clear like water, or slightly turbid. Please help. Thank you in advance.

[Doc Oc]

That procedure looks correct, but there are a couple things you need to clarify.  That amount of pure diisopropylamine is about 5 mmol.  What is the molarity of the n-BuLi that you're using?  The mmols obviously need to match.

One big pitfall with lithium reagents is that they do go bad and need to be titrated to determine true concentration if they've been sitting for a while or the bottle is old.  This can be done fairly simply using diphenylacetic acid, I believe you can find a good procedure for this on the internet.

[ons121]

Sorry about not clarifying properly!

The n-BuLi is 2.36M in Hexanes, and yes everything has been calculated in mmoles.  I was reading online that the normal appearance of LDA is a light yellow to brownish color. I keep getting solution that is clear or slightly turbid. Whenever my professor does that reaction with me, he tends to get a yellowish color.

[discodermolide]

Sorry about not clarifying properly!

The n-BuLi is 2.36M in Hexanes, and yes everything has been calculated in mmoles.  I was reading online that the normal appearance of LDA is a light yellow to brownish color. I keep getting solution that is clear or slightly turbid. Whenever my professor does that reaction with me, he tends to get a yellowish color.

Why don't you buy it from Aldrich? Titrate it as described above and off you go.

[ons121]

I can't! My prof wants me to make LDA in the lab because it's cheaper and a good learning tool. At this point I'd like to. Haha. Maybe I can sneak it in without him knowing! =) 

[discodermolide]

I can't! My prof wants me to make LDA in the lab because it's cheaper and a good learning tool. At this point I'd like to. Haha. Maybe I can sneak it in without him knowing! =) 

Is it really cheaper if the experiments don't work well?

[Doc Oc]

The n-BuLi is 2.36M in Hexanes, and yes everything has been calculated in mmoles.  I was reading online that the normal appearance of LDA is a light yellow to brownish color. I keep getting solution that is clear or slightly turbid. Whenever my professor does that reaction with me, he tends to get a yellowish color.

This is your issue.  As I said, the amount of pure diisopropylamine you listed is about 5 mmol.  0.4 mL of a 2.36M solution is 0.94 mmol.  You should be adding approximately 2 mL of the n-BuLi, and only if that concentration is accurate.  You may need to add even more if it's gone bad.

[orgopete]

… My LDA always looks clear like water, or slightly turbid. Please help. Thank you in advance.

From your description, that sounds like the results I have gotten. If it is turbid, I think that is coming from the butyllithium. If you want yellow, you need to be sloppier or just let it sit around for a while. It will slowly become yellow. If you use very pure reagents, it won't be yellow. If you buy it, its yellow (or mine has been).

[g-bones]

Are you sure the LDA is the problem? Is the reaction you are using it for from the literature? If the BuLi is good and the amine is distilled and everything is under nitrogen, it sounds like you should be good to go.  What kind of reaction is it?  Typically, I would make a known concentration of LDA in THF and add that to whatever I am trying to deprotonate, then add in my electrophile after about 20 mins (depending on the acidity). 

PS Don't buy LDA, it decomposes readily at rt and wouldn't survive very long after you get it.

[ons121]

Sorry guys, I've had exams last week and a few more this week. My apologies for getting back late.

This experiment that I am working on is new. We're working on new methodology.   it's actually a 5 step process (I initially said 4, sorry.); which eventually will require hydrosilyation.

So really, there is no literature to follow. Just the steps my supervisor has written out. I don't want to say too much incase my boss gets on this site. Hopefully not.
He's not very helpful. He basically threw me out there without much training. I finally got the LDA to work after many trial and errors- now my problem is getting a higher yield after the workup, and figuring out how to use the MPLC machine.

I'm able to get 78% yield. I think that's not bad for a beginning chemist. I know it's not good but I'm literally out there on my own trying to figure things out.

Thank you everyone for your help. 

[OC pro]

78% is a fairly good yield when its based upon isolated material and reproducable. Not every reaction can be performed with >95% yield (although KCN and others want to make us believe this stupidity). 
You should proceed further in the synthesis.

[orgopete]

…So really, there is no literature to follow. Just the steps my supervisor has written out. I don't want to say too much incase my boss gets on this site. Hopefully not.
He's not very helpful. He basically threw me out there without much training. I finally got the LDA to work after many trial and errors- now my problem is getting a higher yield after the workup, and figuring out how to use the MPLC machine.

I'm able to get 78% yield. I think that's not bad for a beginning chemist. I know it's not good but I'm literally out there on my own trying to figure things out.

There is no easy answer here and no exemplar of how to. Imagine you are the hiring manager of a top secret organization seeking to hire chemists. Your project is really avant garde. You have two candidates, both with very good accomplishments and recommendations. One guy had extensive training and the other guy taught himself everything. Which guy should you hire?

[Halogenator]

I am an undergraduate research assistant in an Organic Chemistry laboratory. I'm working on this experiment that requires me to make LDA.  I have the steps and procedure to make LDA, but somehow and somewhere I am messing up.

My LDA looks clear, sometimes slightly turbid, but never a clear yellowish color.

1. I flame dry and use a full vacuum for the round bottom flask
2. I let it cool
3. I add distilled diisoproylamine  under inert N2  (.7 ml)
4. Add THF (6.6 mL)
5. let it cool to 0 C (using regular ice and water), wait 10 mins
6. add n-BuLi dropwise (.4 ml)
7. stir for 20 mins at 0 C
8. Cool to -78 C using dry ice and acetone

then from here, I follow the other steps my prof has asked me.

Please can someone give me some pointers or guidance. I"ve been working on this project the last 4 months and I can't seem to get it right. My LDA always looks clear like water, or slightly turbid. Please help. Thank you in advance.

Pure LDA is colorless. Your boss may be decomposing the THF by adding too rapidly. Doing this at 0C is common but unnecessary and invites decomposition of the THF because LDA starts to react with THF around or just above 0. Just form it at -78, proton transfer to BuLi is really fast. If you don't believe me, stick a thermometer in it and watch the exotherm. Once it reaches -78C again, it is ready to use.