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Comparing the SN1 and SN2 Reactions

Hey! You can now download a free 1-page Summary Sheet of SN1 vs SN2 reactions containing all the material on this series of blog posts here: Summary Sheet – SN1 vs SN2


 

Since we’ve gone through the different factors that impact the SN1 [see post] and SN2 [see post] reactions, it’s worthwhile to review and summarize  the different factors behind each of these two reactions. But first – have you ever heard the story of the cats and the comfy chair?

Illustration by the talented Canadian political cartoonist Graeme Mackay. Thanks Graeme!

Cat #1 finds Cat #2 on his comfy chair and wants to sit. He has two options.

  1. He can wait for Cat #2 to leave, and then sit in the comfy chair.
  2. He can kick the Cat #2 out of his comfy chair.
Think about that for a second. In the meantime, let’s compare the SN1 and the SN2.

The Mechanism

The Big Barrier

This is the most important thing to understand about each reaction. What’s the one key  factor that can prevent this reaction from occurring?

The dependence of rate upon the substrate

  • For the SN2, since steric hindrance increases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest).
  • For the SN1, since carbocation stability increases as we go from primary to secondary to tertiary, the rate of reaction for the SN1 goes from primary (slowest) << secondary < tertiary (fastest)
Remember that SN1 and SN2 reactions only occur for alkyl halides (and related compounds like tosylates and mesylates). If the leaving group is directly attached to an alkene or alkyne, SN1 or SN2 will not occur!

The Nucleophile

  • The SN2 tends to proceed with strong nucleophiles; by this, generally means negatively charged nucleophiles such as CH3O(–), CN(–), RS(–), N3(–), HO(–), and others.
  • The SN1 tends to proceed with weak nucleophiles – generally neutral compounds such as solvents like CH3OH, H2O, CH3CH2OH, and so on.

The Solvent

  • The SN2 reaction is favored by polar aprotic solvents – these are solvents such as acetone, DMSO, acetonitrile, or DMF that are polar enough to dissolve the substrate and nucleophile but do not participate in hydrogen bonding with the nucleophile.
  • The SN1 reaction tends to proceed in polar protic solvents such as water, alcohols, and carboxylic acids. These also tend to be the nucleophiles for these reactions as well.

Stereochemistry

  • Since the SN2 proceeds through a backside attack, if a stereocenter is present the SN2 reaction will give inversion of stereochemistry.
  • By contrast, if the SN1 leads to the formation of a stereocenter, there will be a mixture of retention and inversion since the nucleophile can attack from either face of the flat carbocation.

Back To The Cats

So does the story about the cats and the comfy chair make sense now?
  • In the SN2, the nucleophile (Cat #1) forms a bond to the substrate (comfy chair) at the same time the leaving group (Cat #2) leaves.
  • In the SN1, the leaving group (Cat #2) leaves the substrate (comfy chair), and then the nucleophile (Cat #1) forms a bond.
If this makes sense, you might be ready for the Quick N’ Dirty Guide to SN1/SN2/E1/E2 reactions. Otherwise, join us for our next post when we discuss rearrangement reactions. 

Don’t forget – you can download a free 1-page Summary Sheet of SN1 vs SN2 reactions containing all the material on this blog post here: Download SN1 vs SN2 Summary Sheet PDF


Cat Illustration by my talented cousin, political cartoonist Graeme MacKay 

UPDATE . The most perfect cat video ever. Thanks to Alex Roche (Rutgers U.) for sending.

There Can Be Only One

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