Wittig Reaction
13. Add an additional 8-mL of dichloromethane to the separatory funnel, and shake vigorously again.
(Any yellow color is product, so where yellow is, more CH
2
Cl
2
rinse might be good….)
14. Pour the organic layer into the same 125-mL Erlenmeyer that has the other dichloromethane.
15. “Dry” the organic solution with sodium sulfate.
16. At this point or sooner, get a 400-mL beaker ~1/3 filled with hot water in it, and warm on hot
plate. Maybe target ~55º water bath; with the hot-plate around 5, maybe?)
17. Filter the organic solution into a separate ground-glass-neck125-mL Erlenmeyer, using a long-
stemmed funnel lightly-plugged with glass wool to filter off the sodium sulfate.
18. Rinse the original Erlenmeyer and the funnel (anything yellow) with additional dichloromethane.
19. Add a boiling stick to your organic solution, and then place the Erlenmeyer into the warm-water
bath to boil off the dichloromethane. (Be thorough…. Once it’s boiled down some, you can turn
up the hot plate to a higher temperature to facilitate a faster boil-off. No point in wasting lots of
time boiling the solvent off if we can do it faster. But, we don't it to go crazy and go boiling over
the top, either.)
• Note: How do you know when to quit? If you know what your theoretical yield is, it will
help you realize approximately how much stuff you should expect to have left once the
solvent is removed…
20. Once the solvent is pretty much gone, remove from hot bath, add a vacuum adaptor, turn on
vacuum, and continue under vacuum for 2 minutes to remove any last traces of CH
2
Cl
2
.
21. Remove your Erlenmeyer from the hot water bath.
• Does anything crystallize?
• At this point you have at least two things present: the desired alkene 4 and the undesired
phosphine oxide side product 7. If you also have some CH
2
Cl
2
solvent that hasn’t quite all
boiled away, that will reduce your eventual yield and prevent crystallization.
• Place your material into an ice bath, and scratch it with a boiling stick. If it crystallizes,
that confirms that you’ve done an adequate job of boiling off your dichloromethane. If it
doesn’t crystallize, you should probably boil some more off. (Jasperse has a quick way.)
• If you don’t get rid of your dichloromethane adequately, leftover dichloromethane will
keep product dissolved at the end of the recrystallization process, and your yield will be
compromised.
22. Purify your alkene by recrystallizing from 1-propanol solvent. (The water bath can now be boiling
hot, so turn up your hot-plate setting to ~5.) The concept here is that the triphenylphosphine oxide
is more soluble in the propanol than is the alkene product, because the phosphine oxide can use its
oxygen to hydrogen-bond to the solvent, whereas the alkene has no hydrogen-bonding capability.
• Do you remember the logic and procedure for a recrystallization? If not, try to review!
• A good starting guess may be about 6 mL.
• This recrystallization can be done right in the same 125-mL Erlenmeyer flask.
23. After Buchner funnel filtration, rinse with a very small amount (2-4 mL?) of ice-cold propanol.
We don’t want to add water and make the solvent much “worse” for fear that water will knock the
triphenylphosphine oxide out of solution and contaminate the product.
24. Let things dry thoroughly before getting your yield and mp. (Vacuum for at least 10 minutes.)
Once you have your mass, also calculate your % yield. (Don’t expect a very high yield. The
solvent good enough to host all of the triphenylphosphine oxide also hosted much product. )
Lab Report: Standard synthesis style lab report. Be sure to include detailed observations on some
of the things that happened. For product, include yield, mp, and % yield.
Questions: None assigned.