Unlikely Analogy of the Day: Nucleophilic Substitution and Girlfriends

Aliphatic nucleophilic substitution is one of the most important reactions in organic chemistry; it is highly versatile consenting to prepare a wide varity of important chemicals, and has been studied in depth for decades.

The reaction needs a substrate and a nucleophile: the nucleophile bonds to the substrate replacing the part called leaving group - hence the substitution. It proceeds through two distinct mechanisms: S_N1 and S_N2. S_N1 entails initially the loss of the leaving group with formation of a positive ion from the substrate (carbocation), which then reacts with the nucleophile to give the product(s). S_N2 instead is a concerted mechanism: the nucleophile attacks the substrate and pushes the leaving group away yielding the product. Certain substrates manifest ambiguous behaviour, of course.


How does this relate to girlfriends, tho? This way...
A relationship ends for some reason, and the guy remains single, ready to attach himself to another girl. This is analogous (with some fantasy) to the S_N1 mechanism.

In the other case, a second girl gets close to the couple, bonds to the man and pushes the former girlfriend out, in a fashion resembling the S_N2 mechanism.

Threesomes are not stable, much like the transition state for S_N2 with its pentavalent carbon atom isn't.

And now I want my engineer's disease certificate, pronto!