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The below is a synthetic protocol I need to perform:

N-trifluoro acetoxy succinimide Synthesis

Trifluoroacetic acid anhydride was added to N-hydroxysuccinimide at 0 °C. The reaction was allowed to warm up to r.t. over 24 h. Then toluene (50 mL) was added and the volatile was removed in vacuo. The residue was taken up in DCM (15 mL) and the remaining solvent was removed in vacuo.

I don't quite get the last sentence. What does it exactly mean by 'taking up a residue in solvent'? Does it mean I just need to add 15 mL DCM to the reacting mixture?

Also, the second bit also gets me confused — if I put my mixture under vacuum, wouldn't all the solvents get sucked up? what would be the point of adding DCM if I am placing my mixture under vacuum immediately after adding DCM?

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    $\begingroup$ Looked at one database on a website and toluene and DCM don't seem to form an azetrope. // Wouldn't trifluoroacetic acid be a side product? Would that evaporate better with DCM? $\endgroup$ – MaxW Apr 24 '19 at 6:34
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You add the DCM to the material left after removing the toluene and then evaporate it off under vacuum. The reason is to ensure that all the trifluoroacetic acid is removed as this would adversely affect the next reaction, and TFA is readily soluble in DCM.

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  • $\begingroup$ Thank you! but why do you need both Toluene and DCM to remove trifluoroacetic acid? Is it just to doubly make sure that TFA is all gone? $\endgroup$ – chemrese Apr 24 '19 at 9:58
  • $\begingroup$ Exactly so. TFA will really mess us the next stage (which I presume is adding the N-Trifluoroacetoxysuccinimide to a nucleophile) $\endgroup$ – Waylander Apr 24 '19 at 10:17

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