Organic Chemistry Help: Resonance and SN1/SN2

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.

Dr. Michael Pa got a bachelors degree in chemistry from Binghamton University, a masters degree in organic chemistry from the University of Arizona and a Ph.D. in organic chemistry from the University of Arizona. His research focus was on novel pain killers which were more potent than morphine but designed to have fewer side effects. There may even be a patent or two that came out of it. Prior to all of this, he was a chemist at Procter and Gamble. After all of that, he (briefly) worked as a post-doctoral assistant at Syracuse University, working on novel organic light-emitting diodes (OLEDs). In between, he did NOT compete at the 1996 Olympics, make the Atlanta Braves opening day roster, or become the head coach of the Indiana Pacers, as he had intended. #fail During this entire time, he always loved helping students, especially if they were struggling with organic chemistry. In 2006, Dr. Pa founded in order to make learning organic chemistry fast and easy. 14 years and about 60,000 students later we are still helping students to learn organic chemistry one reaction at a time at

1 thought on “Organic Chemistry Help: Resonance and SN1/SN2”

  1. Can I add something?

    A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by mass.

    There are some exceptions to the definition above. Certain crystalline compounds may be treated as chemical compounds despite varying in composition according to the presence or otherwise of elements trapped within the crystal structure. Some compounds regarded as chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, which will make the ratio of elements by mass vary slightly. A compound therefore may not be completely homogenous, but for most purposes in chemistry it can be regarded as such.

    Not all molecules are compounds. A diatomic molecule of hydrogen, represented by H2, is homonuclear — made of atoms of only one element, so is not regarded as a compound. Compounds are pure substances that contain two or more elements combined in a definite fixed proportion.

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