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The Wikipedia article about the Grignard Reaction says that

The addition of the Grignard reagent to the carbonyl typically proceeds through a six-membered ring transition state. enter image description here

This got me thinking: How was it determined that the Grignard reaction has the ring as a transition state? In fact, how are unstable transition states in general determined?

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    $\begingroup$ It is clear from the text that you are curious about transition states. It should be noted that transition states and reaction intermediates are two different things. I would advise you to correct your question. $\endgroup$ – urquiza Feb 25 at 12:44
  • $\begingroup$ It´s called educated guessing. ;) $\endgroup$ – Karl Feb 25 at 20:21
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Experimentally, and surprisingly, there is only circumstantial evidence for most short lived species in any type of reaction, i.e. no one has spectral or structural evidence for a transition state, such as often portrayed in textbooks, other than for example NaI dissociation or atom-diatomic molecule reactions e.g. H+OH etc. done in ultra-low vacuum.

For longer lived intermediates (picoseconds to longer) short pulse laser pump-probe (flash photolysis) methods have to be used to obtain spectral signatures but give no structure, and this method only works for light triggered reactions which are rare. An alternative is stopped flow, but the time scale is slow, many microseconds so usually too slow for most intermediates, and again does not give structure. NMR is immensely slow also. Time resolved x-ray only works on crystals. So far light driven reactions in proteins have been studied down to a few tens of femtoseconds. There is a huge and exciting field here to be explored.

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