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In my book is given the mechanism of $\ce{HBr}$ forming and I understand every step of it. We did the calculations of the rate of each reaction, which is easy depending on the sign (-,+) of compounds formed or spent, and knowing those rates we were finally able to find the rate of $\ce{HBr}$ which is well defined by the empirical equation of Bodenstein and Lind. If you know all those reactions, its not hard to find the equations you need, but my questions are :

Is there any way we can know which exactly are those reactions (mechanism) for other examples?

How would we know if any reaction is a branched one ?

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As far as I know, the short answer is no; in general, it is not possible to know the reaction mechanism a priori.

In general, the approach is:

  1. Postulate a reaction mechanism/pathway
  2. Use this to calculate the relevant reaction rate(s)
  3. Compare these rate expressions with the results of experiments

If your proposed reaction mechanism matches the empirical results, it is likely (but you can never be sure!) that the mechanism is correct. If not, a new mechanism should be postulated and tested.

That being said, in many examples it may be possible to make an educated guess of a plausible reaction mechanism - based on your own experience or the experience of others (i.e. from articles etc.) - and then investigate whether the proposed mechanism matches the empirical facts of relevant experiments. Often, the proposed mechanism is more or less correct.

In addition, modern advances in computational chemistry may give useful information regarding likely reaction pathways based on the structure and interaction of the involved molecules. To my knowledge, it is not yet possible to use this in a standalone fashion to figure out the detailed mechanism, but it may help out in terms of making a better educated guess.

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  • $\begingroup$ What about the second question ? Can we know at least if the reaction is a branched one or not ? $\endgroup$
    – Ndrina Limani
    May 7, 2015 at 13:39
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    $\begingroup$ Again, as far as I know, the general answer is still no - unfortunately. It might be easier to predict whether or not branching will occur than predicting the reaction mechanism as a whole - but I'm still quite sure that this must be based on experience and (educated) guessing. If anyone has any counter-example I'll be very interested to know :) $\endgroup$
    – Kjetil Sonerud
    May 7, 2015 at 13:55

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