Potential energy profile
ReactantsInt.Products
Step 0
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3-bromo-3-methylhexane, a tertiary halogenoalkane. The carbon bonded to Br also carries CH₃, C₂H₅ and C₃H₇, so it is chiral. The C–Br bond is polarised (Cδ+ Brδ-).
Step 1 — slow
C–Br bond breaks
Heterolytic fission of C–Br. Br departs with both bonding electrons as Br⁻. Rate-determining step.
Intermediate
Carbocation
Trigonal planar carbocation (sp² hybridised). Empty p orbital exposed above and below the plane. Simulation paused.
Step 2 — fast
Nucleophilic attack
OH⁻ can attack from either face with equal probability, giving pathways (a) and (b).
Result
Racemic mixture
Equal amounts of the two enantiomers of 3-methylhexan-3-ol, one from inversion and one from retention: a racemic mixture with no net optical activity.
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What is SN1?
Unimolecular nucleophilic substitution. Rate depends only on the substrate — it is a two-step mechanism with a carbocation intermediate.
Rate-determining step
Step 1 (heterolytic C–Br fission) is slow and endothermic. It is the rate-determining step. Step 2 (nucleophilic attack) is fast and does not affect the overall rate.
Carbocation intermediate
The carbocation is trigonal planar (sp² hybridised) with an empty p orbital perpendicular to the plane. Both faces are equally accessible to the nucleophile.
Racemisation
The carbocation carbon carries three different groups (CH₃, C₂H₅ and C₃H₇). OH⁻ attacks from both faces with equal probability, giving equal amounts of the two enantiomers of 3-methylhexan-3-ol: a racemic mixture with no net optical activity.