[ajam]

can any body tell me while doing the reaction where sodium hydride is suppose to be used, why it is suggested, that DMSO is not to be used as a solvent.

[movies]

Sodium hydride is a strong enough base that it can deprotonate DMSO to make what is sometimes called "sodium dimsylate."  It was a relatively common base back in the day because it was so easily accessible, but today amide bases like LDA are much preferred.

When I use NaH, my first choice solvent is always THF.  Others like toluene are okay as well if you need to heat more.

[ajam]

hi
thank you very much for such satisfying answer. my another question is while doing reactions we are using no. of solvents such as EA, ether, acetone. after removing these solvents under high vacuum, then also some trace of solvent would be there which can interfere in your spectra (specially NMR ) is there any remedy on this problem.

[PRIYA1022]

I would suggest you leave your compound for drying on high vacuum for a longer period ... (and anyway  you should be pretty sure about the proton signals for acetone, Ethylacetate or ether..just rule them out from the spectra of your compound)

[movies]

Another trick to get a nice clean NMR is to strip off the NMR solvent (usually CDCl₃) under vacuum, then redisolve it and take the NMR again.  That tends to get rid of traces of other solvents pretty effectively.  Be aware that if you want to get the % yield of your reaction, you must ensure that your entire sample is free of solvent before you trust the mass of product that you measure!

[russellm72]

Ah the old removing the CDCl3 and re-running the spectrum, brings back memories of my PhD.

I commonly azeotrope solvents such as ethylacetate off by spining down your residue from DCM and also DCM-hexane which often gives nice solids (assuming solid product). Then a quick pump in a dessicator using a Pirani guage so you know when full vacuum has been obtained.

[movies]

You can also azeotrope EtOAc with benzene, which is nice sometimes!

[ajam]

russellm72 i don't understood your meaning please will you explain it again for me?
thanks

[russellm72]

I assume you mean the second paragraph. Ethyl acetate has a reasonably high boiling point of 78 oC and as a consequence solids can hold on to it. If you spin your residue down from EtOAc then re-dissolve in DCM and spin down again you will have removed the last traces of EtOAc. Sometimes spining down fron DCM-hexane mixtures is good because as the DCM distills the product can crash out of soloution and when all the hexane has gone you are left with a nice solid. Also a little bit of hexane in your sample is generally Ok if you have another reaction to do as its pretty inert!

A Pirani guage simply fits within your vacuum line and gives you an indication when the hardest vacuum has been applied and thus no more solvent is being trapped in the -196 oC trap.

Hope that helps.

[ajam]

Thanks to all who replied and gave such a nice explanation. thanks once again this information is very helpful to me. bye