A LITTLE LIGHTBULB MOMENT
Substitution and elimination mechanisms
SN1: carbocation first
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The guest leaves; the room waits; then the next guest enters
The idea behind the cue
SN1 is a two-step substitution: leaving group departure forms a carbocation, then the nucleophile attacks. Because the carbocation is planar, attack can occur from either face, giving racemization rather than pure inversion. Rearrangements can occur during the carbocation stage.
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Short & simple
The guest leaves; the room waits; then the next guest enters
Acronym
ICPRR — Impatient Cupcakes Polish Rubbery Rainboots.
Story
At a crowded theater, one person steps out and the aisle briefly opens up. A second person can now enter from either side. The final seating arrangement is mixed, and sometimes a neighbor rushes over before the next person sits down.
Connect the pieces
- I (Impatient) → Ionization: The leaving group departs first to make a carbocation.
- C (Cupcakes) → Carbocation: SN1 proceeds through a carbocation intermediate before nucleophilic attack.
- P (Polish) → Planar: The carbocation is flat, so attack can happen from either face.
- R (Rubbery) → Racemization: Attack from either face gives racemization rather than pure inversion.
- R (Rainboots) → Rearrangement: Hydride or alkyl shifts can occur while the carbocation exists.
- Impatient · Ionization: The leaving group departs first to make a carbocation.
- Cupcakes · Carbocation: SN1 proceeds through a carbocation intermediate before nucleophilic attack.
- Polish · Planar: The carbocation is flat, so attack can happen from either face.
- Rubbery · Racemization: Attack from either face gives racemization rather than pure inversion.
- Rainboots · Rearrangement: Hydride or alkyl shifts can occur while the carbocation exists.
Know the limits
Most favorable for substrates that can form relatively stable carbocations; strongly hindered methyl and many primary substrates are poor SN1 candidates.
Further reading · OpenStax Organic Chemistry ↗Check your understanding
What is the key intermediate in an SN1 reaction?
A. A carbocation
B. A carbanion
C. A radical pair
D. A four-membered ring
Reveal explanation
A carbocation
SN1 proceeds through a carbocation intermediate. The other options are not the hallmark intermediate of SN1.