Hi everybody, I wanted to talk briefly today about resonance and sterics and how it can affect and SN1 or SN2.  For background, I hope everyone knows when it comes to SN1 reactions, tertiary substrates are the fastest and primary substrates are the slowest (because of carbocation stability).  Conversely, when it comes to SN2, it is all about steric hindrance, so primary is the fastest and tertiary is the slowest.  But what happens when there are other factors involved?

As shown here, the benzyl cation was a primary cation, but can undergo resonance stabilization that moves the cation all throughout the ring.  This serves to further stabilize it and makes the benzyl cation have the reactivity of a secondary carbocation when it comes to SN1.

Lesser known is the neopentyl bromide, which is a primary substrate so it should react quickly via SN2, but it does not.  This is because, even though it is primary, it has a very large t-butyl group close, which blocks the reaction site.  This makes neopentyl bromide less reactive than one would expect.  In fact, it has reactivity somewhere between a secondary and tertiary substrate, for SN2 reactions.

For more information on this, please visit Organic Chemistry.


As always, good luck and happy reacting.