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23 thoughts on “Trapped in SN1/SN2/E1/E2 Hell? Start Here”

1. I teach it to my class like we’re prosecutors in court trying to build a case for either SN1, SN2, E1, or E2. How do we build the case? We examine the evidence.

   What evidence do we get from the nucleophile? It’s a good nucleophile, like I-? That’s evidence for SN2. It’s a poor base, like I-? That’s evidence against E2.

   What evidence do we get from the elctrophile? It’s a primary halide? That’s evidence for SN2/E2 and evidence against SN1/E1 (because the resulting carbocation in the SN1/E1 mechanism would be a primary carbocation and we won’t form primary, unstable carbocations).

   What evidence do we get from the solvent? It’s a polar aprotic solvent? That increases the nucleophilicity of the already strong nucleophile. That’s evidence for SN2

   Then put it all together – look at all the evidence as a whole. I’ve got lots of evidence against an SN1/E1, a couple of pieces of evidence pointing toward SN2, and some evidence against E2.

   Sounds like we’re doing an SN2 reaction!

   Problem is, sometimes the evidence points in a couple of different directions – so how do we sort out the conflicting evidence and put together a case for one reaction pathway over another? That’s why this unit is so difficult. Sometimes deciding what the evidence means is really really hard.

   I actually think trying to flow chart this problem is a bad way to approach it. It makes students think this is all ‘rules based,’ when really its so much more nuanced than that. To say we can flow chart this unit makes students think they can apply a set of rules and if I do things the same way every time, I’ll get a nice, neat, one single right answer every time. It’s way more complicated and nuanced than that.

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   1. Just curious, im taking ochem right now…
      Is this site still active/maintained?

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      1. hell yeah!

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         1. do you do short private tutoring sessions? say if i had a few questions on a certain homework problems, and need verfication, would you be available within a certain timeframe to be able to provide aid and guidance?

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            1. right now given my current schedule that is just not possible. I realize there’s a demand for this kind of service however.

2. Good point re the “evidence-based” approach versus a “rules-based approach”.

   In light of the fact that ochem is a premed requirement, another way to look at it might be as a “diagnosis”. What can we rule out, based on the evidence? What’s consistent? What’s inconsistent?

   This thought process is pervasive in medicine. My dad’s a retired GP and he had to use this kind of thinking all the time.

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   1. I realize this is a really old comment but I just have to say that I love this idea. I’m going to try to think about it that way and maybe make a study sheet to help me see it like that. I’m pre-med and love chemistry so it’s perfect :)

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3. HI James!!! Thanks for checking in and seeing that things are better with me and organic chemistry I am enjoying it a lot more than the first time around. I like th exam generator. That’s pretty cool! I’ve realized that it’s a looot of practice and critical thinking skills. It’s not something that you can memorize instead really thing about what is the next next. Over all I like it more than I thought I would in organic chemistry II. Thank you!!! i will use some of the info that you have on your blog for midterm II. Thank you!

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   1. You’re welcome! Glad I could be of help.

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4. Thank you thank you thank you! I love the flow chart. My Org 1 Final exam is in two days, wish me luck! :) Thanks for the great resources.

   ps I’m a McGillian too!

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5. Thanks for the comments, very helpful and easy to understand!

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6. I just came across this blog and it has been very helpful. I am taking Org 1 and 2 this summer… quite a challenge. Very fast paced so it is crucial to stay on top of the material and practice as much as possible. This past week we began learning mechanisms and they are giving me a hard time. I understand them individually but do not know how to determine which one needs to be used in different situations.

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7. This is an extremely common problem – for instance, understanding when something like OH(-) acts as a base and when it acts as a nucleophile can be tricky at first. The more problems/reactions you see, the more you’ll get the hang of it.

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8. THANKS !

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9. thanq very much it is very useful to teach students

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10. Thanks mate.. Really helpful info

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11. This is an absolute lifesaver. Thank you. I was having a really hard time understanding how to choose one over the other. The flow chart was especially useful.

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    1. Awesome, mak!

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12. James,

    Thank you so much. I was so confused with substitution and elimination reactions. I understand them more clearly now all thank to you. Again, thank you. I truly appreciate the time and effort that you put in to make the tutorials available for free. Please keep up your good work.

    Best.

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13. The whole overview for SN1, SN2, E1 and E2 is very clear. The only think I miss, which would make this picture even more complete to my opinion, is the E1cB mechanism. In which in which first a deprotonation of a β-hydrogen occurs followed by the loss of a leaving group from the α-carbon.

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14. Thanks so much for making this website. Big help!

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    1. Glad to hear you find it useful!

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15. What happens when 2-Bromo,2-Chlorobutane is treated with NaI/acetone in Sn2 Condition? And i love ur work

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