June 19, 2019, 05:18:42 PM
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Generic Discussion / Re: Introduce Yourself
« Last post by rolnor on Today at 04:01:39 PM »
I started at Astra-Zeneca -86, mistake!
I am attempting to convert an aldehyde into a vinylsulfone using Horner-Wadsworth-Emmons chemistry with lithium hexamethyldisilazane as the base (28.7%).  The number in parentheses is the yield of a closely related, purified product, as reported in the paper I am following.  With respect to the HWE reaction, the protocol specified stirring for an hour at 0 °C  followed by two hours at room temperature.  The other reactant is a diethyl β-sulfonylphosphonate.

Although the important thing is just to get the product, I am somewhat concerned about the low yield in the HWE reaction, given that this is a multistep synthesis.  The only idea that I can think of as a possible improvement is to monitor the HWE reaction by TLC or by P-31 NMR (there should be a difference in chemical shifts of roughly 10 ppm).  I have previously found P-31 NMR to be useful in that it is relatively sensitive yet ideally there are only two molecules present, reactant and product.  However, I do not presently know whether or not incompleteness of the reaction was the reason for the relatively low yield in the step.  I have not yet surveyed other similar reactions for yield, although I have seen a variety of bases used.
Ha--I agree on the 3%. I work a lot with ocean chemists and they demand drift rates around 0.01%/day in recent times! Apparently it is doable, at least according to the claims of some commercial salinity probe manufactures. Putting it in perspective, evaporation of 0.1cc in 1 day will lead to this drift and I have seen it in a few poorly sealed flasks.
  Cleaning the probe does not work here-the slightest perturbation to the electrodes changes the whole value and it needs calibration.
  Regarding comparing two probes--yes-this works so apparently excitation of the probe is causing the drift. -leaving it in the flask with the power off shows no change upon repeating the measurement.
  No odor but I have not checked pH. Are you suspicious of oxidation here leading to H+ ions?
  The use of different sized flasks to evaluate possible build-up of reactants was done earlier with spurious results due to my original circuit design. I would like to try this again some time. Regardless I think this would imply some type of reaction at the electrodes and I do not see why this might happen at such a low voltage. Wouldn't we have to exceed the electropotental of at least one of the ions present?

   I have found an apparent correlation of drift with frequency of measurement--taking measurements every minute for 1 week leads to the drift---taking a measurement every hour does not.

High School Chemistry Forum / Re: Photo Chemsitry
« Last post by Corribus on Today at 03:37:24 PM »
To be pedantic, fluorescence and phosphorescence are not photochemical processes, since they involve no chemical reaction. Photochemistry refers to processes like, e.g., photo-oxidation or photosynthesis.
I am following a literature protocol, and the yield is 60% after column chromatography.  I am inclined to try DIBAL and if I also obtain 60%, to stay with DIBAL.
alright thanks guys that clarifies everything :)
Yes, you're right I did misread it. It is asking to make up a 5% NaCl solution then adding a 0.04 M acetic acid solution to it.
@OP, I interpret the question to mean that the final concentration of NaCl is 5%.  I also interpret the question to mean that 99.9% acetic acid is something that one starts with.  One must use the final pH to decide how much acetic acid to use.
I think you are misreading the question. You can make solution that is either 99.9% acetic acid or a solution that has pH of 3.2, it can't be both at the same time.

Not to mention the fact that solution can't be both 5% in NaCl and 99.9% in acetic acid, fractions must sum up to 100%.
Yes, final strength is 5% NaCl and it is an aqueous solution prepared from concentrated acetic acid, water and NaCl. So far I have converted percent into molar. I calculated that the 99.9% concentrated acetic acid is 17.34 M and the 5% NaCl solution to be 0.855 M. Right now I'm having difficulty figuring out how to calculate the volume I would need of both the 17.34 M acetic acid and 0.855 M NaCl solution so that if I mixed them together I would end up with 2.5 L of a 0.04 M solution. My plan was to make out the NaCl solution first, which I know how to do I just don't know how much of it to make up, and then add the concentrated acetic acid to this until mixture reached 2.5 L.
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