Concept 02 / 06Haloalkanes and Haloarenes
Nucleophilic Substitution II: The SN1 Mechanism and Stereochemistry
The unimolecular SN1 mechanism with first-order kinetics and carbocation intermediate, reactivity order of alkyl halides, allylic and benzylic halides, the role of polar protic solvents, and stereochemical consequences: optical activity, chirality, retention, inversion and racemisation.
SN1 (substitution, nucleophilic, unimolecular). Example: , with rate (first order, independent of the nucleophile).
Mechanism (see figure):
Step 1 (slow, rate-determining): the C-Br bond ionises to give a carbocation and bromide. This needs energy, supplied by solvation of both ions by a polar protic solvent (water, alcohols).
Step 2 (fast): the nucleophile attacks the carbocation.
Reactivity follows carbocation stability: (the reverse of SN2). Allylic and benzylic halides are very reactive in SN1 because their carbocations are resonance-stabilised (, ). Carbocations may rearrange (hydride or alkyl shifts) to more stable ones before capture.
Summary of competition: primary halides react mainly by SN2, tertiary by SN1, secondary by either, depending on nucleophile and solvent.
Stereochemistry.
Optical activity: chiral compounds rotate plane-polarised light, measured with a polarimeter; dextrorotatory (d, ) or laevorotatory (l, ).
Chirality: a molecule with a carbon bearing four different groups (asymmetric carbon) is non-superimposable on its mirror image; the two mirror-image forms are enantiomers. An equimolar mixture of enantiomers is a racemic mixture (racemate, ), optically inactive by external compensation.
Retention: the product has the same spatial arrangement as the reactant (no bond to the stereocentre is broken, or a double inversion occurs).
Inversion: the product has the opposite arrangement (SN2).
Racemisation: conversion of one enantiomer into a racemic mixture. In SN1, the planar carbocation can be attacked equally from both faces, giving both enantiomers (in practice with slight excess of inversion, because the departing halide partly shields one face).
Example: optically active 2-bromobutane undergoes SN2 with inversion, but under SN1 conditions it gives largely racemic butan-2-ol.
Key Takeaways:
- SN1: two steps, first order, carbocation intermediate, rate depends only on [RX].
- Reactivity: ; benzylic and allylic fast.
- Polar protic solvents favour SN1; polar aprotic favour SN2.
- SN1 gives racemisation; SN2 gives inversion.
- Pitfall: benzylic and allylic halides are reactive towards both SN1 (stable cation) and SN2 (unhindered, conjugation stabilises the TS).
Brain-Teaser 1: Arrange in increasing SN1 reactivity: , , , .
Solution: < < < . The carbocations are increasingly stabilised: primary alkyl, benzyl, secondary benzylic (extra methyl), diphenylmethyl (two rings).
Brain-Teaser 2: Optically active (S)-2-bromobutane is heated with aqueous solvent (a weak nucleophile, polar protic medium). Is the butan-2-ol formed optically active?
Solution: These are SN1 conditions: the planar secondary carbocation is attacked from both sides, giving nearly equal amounts of (R)- and (S)-butan-2-ol. The product is largely racemic, so optical activity is lost (a small net rotation may remain from slight inversion excess).
Brain-Teaser 3: Why does react faster than with water, but slower with sodium ethoxide in acetone?
Solution: In water (polar protic, weak nucleophile), SN1 operates, and the tertiary cation forms easily while the methyl cation does not. With a strong nucleophile in an aprotic solvent, SN2 operates: methyl bromide is unhindered, whereas tert-butyl bromide is too crowded for backside attack (and tends to eliminate instead).
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