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Chapter 7 · Substitution & Elimination212

7.4Competing Mechanisms: SN1 and SN2

Both pathways replace a leaving group with a nucleophile, yet differ sharply in mechanism and stereochemistry (Figure 7.9).

‡Nuδ–CHHHLGδ–backside attack— 180° —inversion
Figure 7.9 In the SN2 transition state the nucleophile approaches 180° from the leaving group; the three remaining groups invert like an umbrella in the wind.
Explain this figure · p. 212

In the SN2 mechanism, the nucleophile attacks the electrophilic carbon from the side opposite the leaving group, in a single concerted step. This backside attack inverts the configuration of the stereocenter — Walden inversion.

Because bond-making and bond-breaking occur together, the rate depends on the concentration of both the substrate and the nucleophile, giving second-order kinetics. Bulky groups around the carbon slow the reaction dramatically.

The SN1 mechanism, by contrast, proceeds in two steps through a planar carbocation intermediate, so the nucleophile may attack either face and the product is typically racemic.

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Chapter 7 · p. 212
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7.4 · SN1 and SN2 · p. 212

Both substitution pathways replace a leaving group with a nucleophile.

In the SN2 mechanism, the nucleophile attacks from the side opposite the leaving group, in a single concerted step.

This backside attack inverts the stereocenter — Walden inversion.

The rate depends on both the substrate and the nucleophile — second-order kinetics.

Notes 0
"…attacks from the side opposite the leaving group"p. 212
"backside attack inverts the stereocenter"p. 212
"depends on both the substrate and the nucleophile"p. 213
7.4 — SN1 and SN2 · p. 212

In the SN2 mechanism, the nucleophile attacks the carbon from the side opposite the leaving group, in a single concerted step.

This backside attack inverts the configuration of the stereocenter.

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Chapter 7 · p. 212
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3 highlights
"The nucleophile attacks the electrophilic carbon from the side opposite the leaving group, in a single concerted step."p. 212
"This backside attack inverts the configuration of the stereocenter, an outcome known as Walden inversion."p. 212
"The rate depends on the concentration of both the substrate and the nucleophile, giving second-order kinetics."p. 213
PodcastMind map
0:14
5:12
Alexintro0:00
So — what actually separates SN1 from SN2?
Mayaexplain0:14
Timing. SN2 is one concerted step — the nucleophile attacks as the leaving group leaves.
Alexquestion0:36
And that backside attack is why the stereocenter flips?
Substitution
SN2
SN1
Leaving groups
Nucleophiles
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card 3 of 12
SN2 mechanism
What happens to the stereocenter in an SN2 reaction?
Why does SN2 invert the configuration?
AA planar carbocation forms
BThe nucleophile attacks the opposite side
CIt proceeds in two discrete steps
Substitution mastery76%
Review next: Nucleophilicity — your weakest concept
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An SN2 reaction proceeds in a single concerted step: as the nucleophile bonds to the carbon, the leaving group departs, and the molecule passes through a five-coordinate transition state.

‡ transition statereactantsproductsenergyreaction coordinate →
Figure 7.11 — Reaction energy profile
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"The nucleophile attacks the electrophilic carbon from the side opposite the leaving group, in a single concerted step."p. 212
"This backside attack inverts the configuration of the stereocenter — Walden inversion."p. 212
"The rate depends on both the substrate and the nucleophile, giving second-order kinetics."p. 213
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FrontSN2 mechanismWhat happens to the stereocenter in an SN2 reaction?
AnswerIts configuration inverts— the nucleophile's backside attack flips the stereochemistry (Walden inversion).▤ p. 212 · ch. 7
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Why does an SN2 reaction proceed with inversion of configuration?
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Nucleophilic
Substitution
SN2 mechanism
SN1 mechanism
Influencing Factors
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Second-order rate
Carbocation
Racemization
Leaving groups
Nucleophilicity
Processp. 212

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0:000:32
Alexintro0:00
So — what really separates an SN2 reaction from an SN1?
Mayaexplain0:04
It comes down to timing. SN2 happens in one step — the nucleophile attacks just as the leaving group departs.
Alexquestion0:13
So there's no intermediate?
Mayapayoff0:15
None. It passes through a single transition state and flips the stereocenter inside-out — that’s Walden inversion.
Alexquestion0:23
And SN1?
Mayapayoff0:25
Two steps, through a flat carbocation — so you get a racemic mixture instead of clean inversion.

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