SN2 vs SN1 rxns – Stories of Eternal Love & Conflict

Guest post by:  Rami Ibo

On the occasion of the 2011 International Year of Chemistry, the Physical and Biophysical Chemistry Division (I) of IUPAC announced a Student Chemistry Cartoon Competition. The goal of the competition was to illustrate a chemistry principle in a manner that can enrich the teaching of chemistry (http://www.chemistry2011.org/participate/activities/show?id=361). So I decided to participate in this event and I drew by hand a chemistry cartoon which I called: SN2 vs SN1 rxns – Stories of Eternal Love & Conflict. 🙂

I sent in the cartoon along with the description that they asked for.  Unfortunately, I did not win 🙁 (tears in my eyes and tissue box next to me :P)… However, I would still like to share it with you here on Sandpaw since I think the idea of teaching chemistry (or any science for that matter) in an accessible/humorous way is relevant to the discipline of Science Communication.

Here is the description of the cartoon. I hope you find it as informative and enjoyable as I did!

How does the cartoon relate to a chemistry principle?

It teaches in a simple and humorous way, the difference between S_N2 (Substitution, Nucleophilic, Bimolecular) and S_N1 (Substitution, Nucleophlic, Unimolecular) reactions in organic chemistry. I hope students will be able to relate to the cartoon and be able to notice the key features that distinguish between an S_N2 reaction and an S_N1 reaction.

Summary of the cartoon:

 The cartoon is drawn on an A4 page and is split into two halves: the first half deals with S_N2 reaction while the second half deals with the S_N1 reaction. In the S_N2 story, I introduce the main character of that story: CH₃Cl in which the carbon is annoyed by the electronegative chlorine and how it withdraws all the electron density towards it [the chlorine]. Then I introduced the second character, OH^–, which comes into the scene lonely and floating by. As soon as the carbon in CH₃Cl and OH^– see one another, they fall in love (much to chlorine’s dislike). As the bond between the OH^– and the carbon forms, the bond between the carbon and chlorine starts to break (which is the key feature of S_N2 reactions – it is a concerted mechanism where bond making and bond breaking happens at the same time). Chlorine leaves the molecule and becomes a chloride ion (which happens to be a very good leaving group as it can easily accommodate the extra negative charge).

 In the S_N1 story, OH^–‘s twin brother (who unsurprisingly happens to be another OH^– ion) meets CH₃Cl’s older sister CR₃Cl. Again, the OH^– and the carbon in CR₃Cl fall in love. However, it is not possible to form a bond between the OH^– and the carbon of CR₃Cl due to the bulkiness of the R (alkyl) groups (in other words, steric hindrance about the carbon prevents and S_N2 reaction from taking place). This makes the couple sad. But in no time, the carbon in CR₃Cl gets an idea! It figures out that in order to bond with OH^–, it would need to break a bond first to create a vacancy (an S_N1 reaction!). And in a very menacing move, the carbon “kicks out” the much disliked chlorine (in the form of a good leaving) from the molecule and becomes [the carbon] formally positively charged (a carbocation). And now, the OH^– and the carbocation are able to form a bond and live happily ever after!

Rami Ibo

Note: For those who would like to use the cartoon, feel free to do so but please credit my name. Thank you.