Three And A Half Steps To Any Substitution or Elimination Reaction, Step 2: The Nature of The Electrophile by @azmanam
Step 2: What is the nature of the electrophile?
The nature of the electrophile is a bit simpler to assess than the nucleophile. We need to know what the degree of substitution is for the electrophilic carbon atom. Recall that the degree of substitution of a carbon atom is equal to the number of other carbon atoms to which it is attached. The degree of substitution for several carbon atoms is listed below.
To rationalize the evidence we gain from the electrophile, we need to remember how the various mechanisms work. For the SN2 reaction, the nucleophile has to be able to get all the way to the interior of the molecule and get close enough to the electrophilic carbon atom to directly attack and form a new bond. For the SN1 and E1, the leaving group has to leave first to form an unstable carbocation. And for the E2, the base deprotonates the β-carbon atom adjacent to the electrophilic carbon atom.
Different degrees of substitution for electrophiles will facilitate our substitution and elimination mechanisms to a different extent. Tertiary electrophiles are too sterically hindered to allow the nucleophile to get close enough for direct substitution attack, but methyl, primary, and secondary are fine. Methyl and primary electrophiles are too un-substituted to allow a carbocation to form, but secondary and tertiary are ok. Methyl electrophiles don’t even have a β-carbon atom for elimination, but the rest do. So these are the things we think about to help us figure out the evidence we gain from the electrophiles.
|Electrophile Category||Evidence For||Evidence Against|
|Methyl||SN2||SN1, E1, E2|
|Primary||SN2, E2||SN1, E1|
|Secondary||SN1, SN2, E1, E2|
|Tertiary||SN1, E1, E2||SN2|
Note that the secondary electrophile is evidence for all possible mechanisms… it doesn’t help us narrow down our decision. We will need to rely on our other pieces of evidence more in this circumstance.
A common question is: why is a tertiary electrophile evidence for E2? I thought it was sterically hindered! It is… but we need to remember how the mechanism works. The E2 reaction works by the strong base attacking the proton on the β-carbon atom. The β-proton is two whole bonds away from the hindered electrophilic carbon atom and is on the periphery of the molecule. It is not nearly as difficult for a strong base to attack a peripheral proton versus making it all the way to the interior of the molecule to act as a nucleophile and attack the electrophilic carbon atom.
We need to make one more point about carbocations before we break for the day. Carbocations that can be stabilized by resonance are more stable than their degree of substitution would suggest. To a first approximation, the ability to stabilize a primary carbocation by resonance will make the carbocation about as stable as a secondary carbocation. In general, resonance stabilization bumps up the carbocation stability by one level. Thus a resonance stabilized primary carbocation is stable enough to form and engage in SN1 and E1 reactions. Always be on the lookout for resonance!