If nucleophilic attack occurs simultaneously at two seperate electrophilic sites of the molecule following SN2 at each site, will the moleculularity still be 2? Does molecularity depend on the total number of reacting entities, or the number of reacting entities at a particular site?

I think molecularity will be defined wrt a specific site, in a molecule.

Please elaborate.

  • $\begingroup$ It's the molecularity not some "sitality" or whatever. It's not like simultaneous attack is likely - and truly trimolecular reactions common. $\endgroup$
    – Mithoron
    Mar 23, 2022 at 18:29
  • $\begingroup$ @Mithoron But if we have a very large molecule, like some really complex organic molecule, having different active sites, then can't multiple attacks take place in different parts of the molecule simultaneously? $\endgroup$ Mar 24, 2022 at 3:07
  • $\begingroup$ Being very large hardly changes anything. Important issue is that kinetics deal with practical result - even if your contrived scenario happened once in a billion times, all others would be simply two step and effect on reaction would be none. $\endgroup$
    – Mithoron
    Mar 24, 2022 at 14:17

1 Answer 1


If the reaction requires simultaneous nucleophilic attack of both sites to proceed through an elementary reaction step, then we would describe that step as having molecularity of three.

If it just so happened that a particular molecule in a reaction had two simultaneous nucleophilic attacks at different reaction sites that could each have proceeded at different times, then the two would be considered different elementary steps each with molecularity of two.


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