A LITTLE LIGHTBULB MOMENT
Substitution and elimination mechanisms
SN2: one-step backside attack
Sign in to bookmark this concept and add private notes →
Backside shove flips the umbrella in one motion
The idea behind the cue
SN2 is a concerted substitution: nucleophile attack and leaving-group departure happen together. It is fastest at less hindered centers, especially methyl and primary substrates. Because attack occurs from the backside, a stereocenter undergoes inversion of configuration.
Practice this flashcard →Make it memorable
Short & simple
Backside shove flips the umbrella in one motion
Acronym
CBIU — Clumsy Badgers Inspect Umbrellas.
Story
A tiny locksmith slides in from the door’s backside, kicks the occupant out, and locks the door behind them in the same motion. The umbrella on the table flips inside out as the crowd gasps.
Connect the pieces
- C (Clumsy) → Concerted: Attack and leaving-group departure happen in the same step.
- B (Badgers) → Backside: The nucleophile must come in from the backside of the electrophile.
- I (Inspect) → Inversion: A stereocenter flips configuration, giving inversion of configuration.
- U (Umbrellas) → Unhindered: SN2 is fastest at methyl and other unhindered primary substrates; tertiary centers are too crowded.
- Clumsy · Concerted: Attack and leaving-group departure happen in the same step.
- Badgers · Backside: The nucleophile must come in from the backside of the electrophile.
- Inspect · Inversion: A stereocenter flips configuration, giving inversion of configuration.
- Umbrellas · Unhindered: SN2 is fastest at methyl and other unhindered primary substrates; tertiary centers are too crowded.
Know the limits
Works best for methyl and unhindered primary substrates; severely hindered tertiary centers do not undergo SN2 well.
Further reading · OpenStax Organic Chemistry ↗Check your understanding
Which description best matches an SN2 reaction?
A. A two-step process through a carbocation intermediate with racemization
B. A two-step elimination that forms an alkene and a protonated base
C. A radical chain substitution requiring light and peroxides
D. A one-step backside attack that causes inversion of configuration
Reveal explanation
A one-step backside attack that causes inversion of configuration
SN2 is defined by concerted backside attack and inversion. The other choices describe SN1, E1/E2, or radical chemistry.