SN2 and SN2'

Is there a way of writing a reaction so that Nu^- reacts with CH₃CH=CHCH₂Br to give *both* CH₃CH=CHCH₂Nu *and* CH₃CH(Nu)CH=CH₂?





In the more general case, if I have R¹CH=CHCHR²Lg, can I write a reaction that will give me either R¹CH=CHCHR²Nu or R¹CH(Nu)CH=CHR², depending on the relative sizes of R¹ and R²?

The first reaction seems to have SN2 mechanism, but the other one is rather SN1 (I think, that it is typical for allyl halides). The two mechanisms are different such as their stereo behaviour. Since our reactions are classified mechanistically, I do not think that it is possible to merge them into one reaction scheme. I propose to make two reactions instead and apply both to your reactant.

The two mechanisms can be SN2 and SN2', or they can be SN1 and SN1'. Regardless, if I put the reactions in separate files, is there a mechanism to determine, based on the relative sizes of R¹ and R², which would be the major product? In other words, can I apply a selectivity rule across reactions? Or do I just have to program it independently?

Yes, there is. We have two functions at the moment which might be applied. Both calculate the hindrance of a given atom using the covalent radii of the neighbouring atoms weighted by their distances.





The stericEffectIndex function of the TopologyAnalyser plugin calculates with the topological distances. It is not too precise, but fast. The other is the hindrance method of the Geometry plugin (renamed to steric hindrance in the new version). It first calculates the 3D geometry of the molecule when necessary and calculates with geometrical distances. Though it can provide 3D hindrance values, apart from its slower speed (owing to the geometry optimization), this latter has the disadvantage of conformation dependence.





If none of the above possibilities fulfill your requirements you can use your own calculation method by making a custom plugin.





At the moment, selectivity rules cannot be applied across reactions, each reaction has its own selectivity rule. However. You can prohibit each reaction by adding the calculation to the reactivity rule. For example, your SN2 reaction will run only if the steric conditions around the reaction center are OK for that reaction. Otherwise it will not provide products.