I am doing a general overview of different kinds of dualities present in science. I am quite well versed with the dualities in physics and mathematics. I was searching for dualities in chemistry, biology and other sciences. In that process, I encountered "mechanistic duality" in chemistry. For instance, see [1] and [2].

But I am not able to find a good introduction or description about "mechanisitic duality". It would also be nice if someone can briefly explain what this duality mean. Does someone have clue about its history (who coined it first and for what purpose)? Also, please point me to relevant book/paper which discuss this.

  1. Denmark, S. E.; Su, X.; Nishigaichi, Y. The Chemistry of Trichlorosilyl Enolates. 6. Mechanistic Duality in the Lewis Base-Catalyzed Aldol Addition Reaction. J. Am. Chem. Soc. 1998, 120 (49), 12990–12991. DOI: 10.1021/ja982993v.

  2. Ross, S. P.; Baire, B.; Hoye, T. R. Mechanistic Duality in Tertiary Amine Additions to Thermally Generated Hexadehydro-Diels–Alder Benzynes. Org. Lett. 2017, 19 (20), 5705–5708. DOI: 10.1021/acs.orglett.7b02888.


This just means that the same type of reaction may proceed by two different mechanisms depending on the exact chemical being used. It's not a particularly widely used term (it is not a "standard" phrase such as wave-particle duality) and there are many other ways of saying the same thing ("divergence", "pathway", etc.).

As an extremely simple example, alkyl bromides can react with nucleophiles via either an SN1 or SN2 pathway, depending on the degree of substitution of the carbon bonded to bromine. This is an example of how a different substrate may follow a different mechanism, although the end result is the same (replacement of Br with the nucleophile).

The papers you cited are just slightly more sophisticated examples of the same concept.

  • $\begingroup$ Thanks! So, if I have got you right, "mechanistic duality" is the presence of two different "mechanisms" through which a given reactants produce the same end product. Here, I have few clarifications 1) are there only two pathways? and if so why? 2) In other words, can there be cases where for a given reactants, there can be n number of pathways to end with the same outcome? 3) Is there any general theory which decides the number of pathways for the particular kind of inputs? Excuse! I am completely new to this area. Also, any reference you can point to will be of great help. $\endgroup$ – werunom Oct 4 '18 at 11:01
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    $\begingroup$ (1) It depends on the exact case. Two is not a special number in and of itself. (2) Yes, hydrolysis of esters is a good example, with at least three possible mechanisms that I can think of. (3) Yes, it's called chemistry. Generally there are some mechanisms which are considered plausible and some which are considered implausible. To find and identify these, you need to apply principles of chemistry which you have learnt - not just one principle, but many of them, accrued over years of study. Unfortunately, there is no real shortcut. $\endgroup$ – orthocresol Oct 4 '18 at 11:06
  • $\begingroup$ Thanks for your (1) and (2). Regarding your (3), probably I was not clear with my question. Consider for instance the duality in mathematics (in set theory, groups, vector-spaces, etc.). After repetitive occurrence of duality, mathematicians and philosophers of mathematicians have attempted to make sense of duality for mathematics. (See doi.org/10.1016/j.shpsb.2015.07.004) Similar to this, I was asking have there been accounts (or theories) which explain why there are two (or n) possible mechanisms for given reactants to give rise to the same products. $\endgroup$ – werunom Oct 4 '18 at 12:45
  • $\begingroup$ So, basically, they could have just said "two mechanisms" and everyone would have known what they meant, but they said "mechanistic duality" and people said, "Uh, what's that?" $\endgroup$ – David Richerby Oct 4 '18 at 13:34
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    $\begingroup$ @DavidRicherby, there is definitely a general trend towards flowery language in chemistry, but I think this particular case isn't really an issue. Just saying "two mechanisms" isn't enough for a cogent title. It would be something like: "Tertiary amine addition to thermally generated hexahydro-Diels–Alder benzynes occurs via two mechanisms", but that's not really a title, that is more like the first line of an abstract. I also think that most chemists would have no trouble understanding what "mechanistic duality" means. $\endgroup$ – orthocresol Oct 4 '18 at 15:22

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