Greetings,
I have a question about reported Yields. Is there a reason that people sometime publish isolated yields in optimisation tables instead of obtaining NMR yields?
I was under the impression that in most cases, acquiring the NMR yield of a reaction is easier compared to working up the reaction and isolating the product.
Many thanks in advance,
-Manos
Hi,
NMR- and isolation- yields are completely different terms, and therefore they should not be used interchangeably. The 1H NMR spectrum says nothing about "yield"- it only confirms the organic purity of the product.
In some areas that is enough for eg. to compare the "quality" of the catalysts. In this case, products are not isolated because usually they are really simple and known for 100 years, so they are not important in the paper.
However, in the real world the synthesis without isolation is not the synthesis, because (in many cases) the isolation is the most difficult and the least efficient stage of the whole process.
Keep in mind that there are thousands of compounds that have been recorded on NMR but no one was ever able to isolate them in pure form.
In conclusion, wherever possible, report isolation yield, and leave "NMR yield" only for specific occasions, when its really merit to publish such thing.
Good Luck !
This depends on a few factors and both ways to report yield have their merit. In my experience NMR yields are more commonly reported when there is not enough product to easily isolate and accurately measure. While isolated yields are used when you can easily weigh it. I find isolated yields more practical and easier, but I think it really just comes down to personal preference and the type of experiment.
The isolated yield should always be given preference. NMR yields are preferred in exceptional cases as pointed out by Baillie DeHaven
Isolated yield demonstrated that you could really get the stated compound. For example, if your method will be pick up by some one to generate the stated molecule. However, for optimization of the method everything (up to me) is OK, if for optimized conditions the isolated yield will be stated.
Hi,
NMR- and isolation- yields are completely different terms, and therefore they should not be used interchangeably. The 1H NMR spectrum says nothing about "yield"- it only confirms the organic purity of the product.
In some areas that is enough for eg. to compare the "quality" of the catalysts. In this case, products are not isolated because usually they are really simple and known for 100 years, so they are not important in the paper.
However, in the real world the synthesis without isolation is not the synthesis, because (in many cases) the isolation is the most difficult and the least efficient stage of the whole process.
Keep in mind that there are thousands of compounds that have been recorded on NMR but no one was ever able to isolate them in pure form.
In conclusion, wherever possible, report isolation yield, and leave "NMR yield" only for specific occasions, when its really merit to publish such thing.
Good Luck !
Dear Emmanouil,
I feel that "NMR yield" as a very confusing term. I`d rather recommend to use term "NMR conversion". Because on NMR spectra you can only observe ratio of some two characteristic peaks originating from substrate and product. Nothing more. This is "conversion". You may use these data for preliminary optimisation of your reaction conditions, however, most of the people go for HPLC studies for that purpose.
No mater if you use NMR or HPLC assay with optimized catalyst/reaction conditions you should go for preparative scale reaction to check what is a real yield of your reaction. Because it is factor of two issues. One is efficiency of catalyst (or reaction conditions) and another is efficiency of product isolation. Therefore, as someone mentioned before, you can have very high conversion, but reaction workout will be so difficult that will make your reaction not much useful.
With best,
In synthetic organic chemistry only the isolated yield makes sense. You may use NMR or LS-MS for monitoring your reaction and optimizing conditions, but unless you have isolated a pure material, the numbers you got from NMR or HPLC are not reliable and could be misleading.
The term "NMR yield" should be avoided as much as possible, perhaps even disallowed in respectful journals.
I use to consider that NMR yield (conversion) is a more accurate measure for evaluating the merit of a reaction condition during optimization because there could be some product loss during isolation resulting in lower isolated yields. Losses during isolation mostly when there are undesired compounds of similar property, can induce variable error. However, after reading the above answers, I agree that isolated yield provides more practical information than what is observed through NMR spectroscopy. As Dr. Markiewicz said, isolation can indeed be the most challenging and least efficient step, isolated yields should therefore be preferred over conversions observed through other techniques.
After the completion of reaction, if you got enough amount of product(Above 10 mg ) it is better to calculate the isolated yield with percentage of conversion. calculated using the formula
Yield = (actual number of moles/theoretical number of moles)*100%
But if your product is very low in amount, then go for NMR yield, you can get a better idea by following the link
http://www.organ.su.se/bo/Gruppfiler/NMR%20yield%20calculation.pdf.
For the synthetic chemist, especially for methodology development, I prefer isolated yield, It will give better idea.
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