From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Tue Feb 29 2000 - 08:16:14 EST
Hello,
IMO "ab initio" yield estimation without the help of hindsight, is an
academically interesting subject, but is a hopeless endeavor from
practical point of view. Just to mention a few of the hurdles one has to
jump over:
1. An ideal reactive system may be under kinetic control. We may then
accelerate the rate of reaction with catalysts. This is fine if we do not
accelerate concurrent or successive reactions.
2. In certain reactive system we may approach an equilibrium situation that
allows the easiest estimation of the product yield before purification.
3. In certain cases we are in the diffusion-controlled domain, where the
unit process is very fast but the rate of production is low, because the
reagents fail to meet each other.
4. In a practical situation we have to take into account tranfer phenomena,
namely momentum, heat and mass transfer that will affect phase
mixing/separating, cooling/heating, dividing/uniting, and other operations.
Usually but not always transfer phenomena can be ignored in the lab scale,
however, they may strongly affect scaling up, even in the very reduced
scale mentioned below by Jack.
5. Items 1-5 apply to the raw yields in reaction mixture; and we have still
to estimate the yields of the purification operations, that outside of
chromatographic methods, are really untraceable in the organic laboratory.
6. The situation changes radically on passing to a much higher scale of
production, where operational control is more sophisticated, on applying
the principles of chemical engineering.
All the best,
Jacob
These considerations apply to the reaction mixture. We still
>It would be interesting in general to compare pre-workup yields to
>post-purification yields in order to determine loss and thus, whether
>workup/purification conditions can be changed so as to minimize the loss (if
>any). But a yield reported in a journal should reflect what the next
>experimenter can hope to obtain in a similar reaction. IMHO, the only
>meaningful yield is what you have in your hands after purification. As to
>inflated yields, I've heard this before also. But one should keep in mind
>that generally the reported yield is the best yield obtained in several
>different reactions. I've personally had the experience of a small-scale
>reaction (~50 mg s.m.) result in a much higher yield than the same reaction
>repeated on a 1 gm scale.
>
>Jack Kellum
>Department of Chemistry and Biochemistry
>University of California, San Diego
>
>----- Original Message -----
>From: "antonio regla" <areglac$##$yahoo.com>
>To: "Multiple recipients of list orglist" <orglist$##$dq.fct.unl.pt>
>Sent: Monday, February 28, 2000 11:16 AM
>Subject: ORGLIST: Re: Pre-Work-Up Yields
>
>
>Dear List Members:
>Does anyone know of a good literature reference on
>ways to determine pre-work-up yields in organic
>reactions? Usually it is very hard to reproduce
>literature yields, they are too high most of the
>times, which makes some of us think if the yield
>reported is the isolated yield or before work-up. Some
>journals are very strict and require authors to
>specify which yields they are referring to, but others
>are not strict about it. The nature of reaction
>mixtures is usually quite complex, as one finds
>multiple phases, a mixture of by-products of unknown
>origin, ionic and covalent products. Are there any
>guidelines in the literature on how to analyze such
>mixtures as to determine the yield of the desired
>product. I have been told by some grad students that
>in order to please their research advisors sometimes
>they exaggerate the yields, which in my view is
>nothing more than cheating. I would like to recommend
>the practice of determining pre-work up yields, and I
>will appreciate any information you may have regarding
>this issue.
>Thanks in advance for your help.
>Sincerely.
>Antonio Regla
>__________________________________________________
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Prof. Jacob Zabicky Tel. 972-7-6461271/6461062/6472754
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