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I noticed (lim) is sometimes added to the SN1 or SN2 notation. I suppose it means limiting, but still do not understand it completely. Can someone please explain how is SN1 and SN2 different to SN1 (lim) and SN2 (lim)? first example second example Text from the first row:

Existence of an intermediate with a lower coordination number

No intermediate

Existence of an intermediate with a higher coordination number

Text from the first collumn:

Stoichiometric mechanism

intimate-mechanism

Ingold-Definition

Dependance of the speed on the nature of the entering group

Text from the last row:

The speed does not depend on the entering group.

The speed does depend on the entering group.

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  • $\begingroup$ Do you have an example which you could add in text (preferably) or as picture? $\endgroup$ – Jori Aug 13 '15 at 12:58
  • $\begingroup$ For those of us who cannot speak German, it will be useful if you can put the text from the image (above) into your question, which will enable translation to users' native languages upon viewing it. $\endgroup$ – Todd Minehardt Aug 13 '15 at 14:36
  • $\begingroup$ I think, but this is just an educated guess, that the lim notation refers to limit, meaning an ideal nucleophilic substitution, while the other notations refer to mechanisms that behave more like the parent mechanism. There is a point where SN1 and SN2 cannot be determined anymore, which is somewhere in the middle. $\endgroup$ – Martin - マーチン Aug 13 '15 at 15:13
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    $\begingroup$ This probably has to do with the rate-limiting step of the reaction mechanism, i.e. the slower step. $\endgroup$ – khaverim Aug 13 '15 at 15:36
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I think long story short: the difference between (lim) and not is that the dominant step drives the limiting case more strongly than the non-limiting case.

There are two aspects to a substitution reaction:

  • Stoichiometric mechanism: the sequence of elementary steps that take us from reactants to product
    • Associative (A): the bond formation between the metal and the entering ligand i
    • Dissociative (D): the bond breaking between the metal and the exiting ligand is the important step, i.e. the intermediate has a decreased coordination number
    • Interchange (I): there is no evidence of an intermediate
  • Intimate mechanism: what affects the rate constants of particular steps?
    • The reaction rate is affected by the entering group (a)
    • The reaction rate is affected by the leaving group (d)

http://nptel.ac.in/courses/104106063/Module%205/Lectures%208-10/Lectures%208-10.pdf

You can see that SN1 and SN1 (lim) both have dissociative intimate mechanism, which means that the breaking of the bond between the metal and the leaving group is the rate determining step. In SN1 (lim), the leaving ligand bond breaks first forming an intermediate with a lower coordination number; the entering ligand then forms a bond with the metal. However, SN1 is an interchange stiochiometric mechanism which means that there isn't a true intermediate where the leaving group has totally left. In this case you could imagine that the leaving metal-ligand bond merely weakens or 'stretches' before the incoming ligand bonds to the metal.

Similar principle applies to SN2 (lim) and SN2. In SN2 (lim), an intermediate with higher coordination number is formed when the entering ligand bonds with the metal. For SN2, no such intermediate step is observed. You could say that the incoming ligand simply begins to form a bond to the metal before the leaving group bond starts breaking.

(http://nptel.ac.in/courses/104106063/Module%205/Lectures%208-10/Lectures%208-10.pdf for image above)

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