Substitution — allylic rearrangement
SN2′: nucleophile attacks an allylic system at the far carbon
How the SN2′ works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Nu⁻ (e.g. Me₂CuLi, amines, alkoxides) + allylic bromide. Conditions: THF or Et₂O, –78 to 25 °C. Bonds formed: C–Nu σ bond; new C=C π; bonds broken: C–Br σ; old C=C π. Stereochemistry: Usually syn to the leaving group (cuprates anti). Rate and selectivity: Favoured when the α-carbon is hindered.
At a glance
Step 1: Allylic system, hindered α-carbon
In an allylic halide, the adjacent π bond conjugates with the C–Br σ* orbital, letting a nucleophile attack the far end of the allyl system instead of the carbon bearing the leaving group.
Step 2: Three arrows, one concerted step
Three arrows move together: the nucleophile attacks the terminal alkene carbon, the π bond shifts to become the new C=C, and the C–Br bond breaks as bromide leaves — one concerted step.
Step 3: Rearranged product
The double bond has shifted position, and the nucleophile ends up one carbon away from where the leaving group was — the hallmark of allylic rearrangement.
Abbreviations and terms
- Nu
- Nucleophile. An electron-rich species that donates an electron pair to form a new bond.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- Et2O
- Diethyl ether. Low-boiling ether solvent, classic for Grignard reagents.
- σ*
- Sigma antibonding orbital. The empty orbital an incoming nucleophile attacks in SN2 (the back lobe of C–LG).
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
- SN2′
- SN2-prime (allylic). Nucleophile attacks the alkene terminus with allylic transposition.
How it compares with related reactions
| SN2′ | SN1 | SN2 | |
|---|---|---|---|
| Reagents | Nu⁻ (e.g. Me₂CuLi, amines, alkoxides) + allylic bromide | tert-butyl bromide; H₂O as solvent and nucleophile | NaOH (source of HO⁻) or any strong, unhindered Nu: |
| Conditions | THF or Et₂O, –78 to 25 °C | Polar protic solvent (H₂O, ROH), 25–60 °C | Polar aprotic solvent — DMF, DMSO, MeCN; 25–80 °C |
| Stereochemistry | Usually syn to the leaving group (cuprates anti) | Planar cation: attack from both faces → racemisation at a stereocentre | Inversion of configuration (Walden) |
| Rate / selectivity | Favoured when the α-carbon is hindered | Rate = k[RBr] (first order); 3° > 2° ≫ 1° | Rate = k[RX][Nu⁻]; Me > 1° > 2° ≫ 3° |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the SN2′?
Nu⁻ (e.g. Me₂CuLi, amines, alkoxides) + allylic bromide. Conditions: THF or Et₂O, –78 to 25 °C.
Which bonds are formed and which are broken?
Formed: C–Nu σ bond; new C=C π. Broken: C–Br σ; old C=C π.
What is the stereochemical outcome?
Usually syn to the leaving group (cuprates anti)
What controls the rate or the selectivity?
Favoured when the α-carbon is hindered
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti