[VA]

Can anyone recommend a reaction where a product X is only formed within a narrow temperature range (say 20K or less). If the temperature goes above or below this range, other products are formed. Ideally the experiment should be done at a convenient temperature (say not more than 100C and not less than 0C).

I am an engineer rather than a chemist (hence the poor chemistry knowledge) I have developed a method for monitoring selectivity in real time and I would like to test it on live chemistry.

If anyone can help, I will be very happy to share the data with them. 

[xiankai]

althought i have no idea about specifics, how about settling for an equilibrium reaction, because most reactions do not form a product sharply in a narrow temperature range. at least a moderate change in reactants/products should be enough of an indicator.

when u mentioned 20K, did u mean 20^oC instead, because

say not more than 100C and not less than 0C

is kind of misleading.

[lavoisier]

I agree with xiankai, the selectivity will probably be a continuous function of the temperature: f(T).

If you talk about selectivity it means that you have at least 2 independent reactions competing.
A simple example is the base-catalyzed aldolic condensation of acetophenone with benzaldehyde, where the competing reaction is Cannizzaro disproportionation of benzaldehyde. I think there may be a significant temperature effect in this system.

It would be interesting to know how the reaction kinetics is reflected in f(T), e.g. if f(T) is monotonic or if it can have stationary points.

Stuff for physical chemists!

[Dude]

Are you referring to the organic chemistry concept of thermodynamic control versus kinetic control.  If so, here is one example.

http://www.molecules.org/experiments/Cullen/Cullen.html

[VA]

Thanks very much for these replies. The benzophenone reaction sounds very interesting. The reference to 20K should have read 20C (apologies for that).

I am interested in using calorimetry in combination with NIR. Calorimetry measures all process activity whilst NIR can be made product specific. Thus the point where the calorimetry and NIR curves are closest together should be the 'sweet spot'. This should allow the user to monitor and control yield in real time (on batch processes). 

Has anyone tried this? Would a technique of this sort be useful on full scale plants (which as a chemical engineer is my main interest)?

[lavoisier]

Sorry, it was not benzophenone but acetophenone, I've already corrected it.

Btw, I think you need something simpler to check the efficiency of your measurement. In the reaction I suggested you have 3 different final products, but the reactions occurring are probably too many:

PhCHO + PhCOCH3 <--> PhCH(OH)CH2COPh <--> PhCH=CHCOPh + H2O
PhCHO + HO^- <--> PhCH(OH)O^- <--> PhC^-(OH)2
PhC^-(OH)2 + PhCHO <--> PhC(OH)2CH(O^-)Ph <--> PhC(OH)O^-CH(OH)Ph --> PhCOO- + HOCH2Ph

I guess the system proposed by Dude is better for what you're trying to prove.
There, you only have an initial regioselection depending on the temperature (e.g. deprotonation on either side of a ketone), and the reaction which follows blocks the product so that you can't have inverse reaction/equilibration.

[VA]

Many thanks Dude and Lavoisier.

My thoughts are that many reactions could be monitored by triangulating with two analytical techniques. The second technique doesn't have to be calorimetry (although the advantage of calorimetry is that you don't need to know what the impurities are in order to measure them).

Take for example a reaction like this:

(1)       A + B -> AB

Also

(2)       A + B ->ABB

If you measured this reaction with NIR (true yield) and observed a low yield of AB it could be due to a slow reaction or it could be due to the conditions favouring the formation of ABB (or something else). If you also monitored the reaction with calorimetry however, this would tell you whether the reaction was going slowly or that another reaction was going on at the same time (since apparant yield - true yield = other reaction). If you can make this distinction in real time, you can determine optimum conditions (pH, temperature, addition rate, agitation rate, pressure, accumulation etc) in a single experiment. Using a real time method like this might also give a better insight into processes where transient effects are of interest (e.g. was the yield low because the method was poor or because something else happened later). I suspect that this approach might also work with physical process change like crystallisation.

Are the experiments (acetophenone, HBr/1,3-butadiene) you quoted common in real life chemistry? I am keen to find an application of real practical value.

[minithin]

when u mentioned 20K, did u mean 20^oC instead, because

say not more than 100C and not less than 0C

is kind of misleading.

the OP was correct. he spoke of a 20K range