Wat is een geconcerteerd mechanisme?

In reactions where the distribution of electrons is not symmetrical, the direction in which curved arrows are drawn may be significant. One such case is the thermal decarboxylation of acetoacetic acid shown below. A concerted mechanism consisting of cyclic shifts of electron pairs may be drawn, and two such examples are shown below. Although both mechanisms lead to the products in the light blue box, the mechanism on the left (green arrow) is a more plausible representation than that on the right, because it reflects the acidic character of the carboxylic acid and the basic character of a carbonyl oxygen.



This distinction is clearly seen in the two-step mechanism written in the second row of the diagram. The initial proton transfer proceeds in the expected fashion, and the remaining electron shifts are the same as those in the green arrow concerted mechanism. There is no evidence supporting a two-step pathway for this decarboxylation, but it is possible that the proton transfer stage leads the concerted cyclic sequence.

The preceding example is nevertheless a warning that many reactions do take place in a stepwise fashion, and the mechanisms we write must recognize such cases. The conversion of an ester to an amide by reaction with ammonia, as shown below, is such an example. At first glance this seems to be a substitution reaction, and it would be possible to draw curved arrows for a concerted mechanism, as depicted in the box on the far right. In fact, this reaction, and other acyl transfer reactions, take place by an addition-elimination sequence, as drawn in the boxed space beneath the overall reaction equation. The concerted mechanism on the right is therefore an erroneous conjecture. Thus we have a useful warning that "arrow pushing" does not always lead to plausible interpretations of reaction mechanisms. Experimental support for any mechanism is highly desirable if not essential.

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http://www2.chemistry.msu.edu:80/faculty/reusch/VirtTxtJml/suppmnt2.htm