Substitution — bimolecular
SN2: Hydroxide displaces bromide from bromomethane
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: NaOH (source of HO⁻) or any strong, unhindered Nu:. Conditions: Polar aprotic solvent — DMF, DMSO, MeCN; 25–80 °C. Bonds formed: C–O σ bond; bonds broken: C–Br σ bond. Stereochemistry: Inversion of configuration (Walden). Rate and selectivity: Rate = k[RX][Nu⁻]; Me > 1° > 2° ≫ 3°.
At a glance
Step 1: Backside attack
Hydroxide (HO⁻) approaches methyl bromide from the side opposite the bromine — the backside — because that's where the C–Br antibonding orbital points.
Step 2: Transition state
One curved arrow shows the lone pair on oxygen attacking carbon. A second arrow shows the C–Br bonding pair leaving with bromine as it departs. Both happen in a single, concerted step.
Step 3: Products
Carbon inverts like an umbrella flipping in the wind — this is the mechanistic signature of SN2. The product is methanol; bromide leaves as a free ion.
Abbreviations and terms
- Nu
- Nucleophile. An electron-rich species that donates an electron pair to form a new bond.
- Me
- Methyl. –CH₃.
- DMF
- N,N-Dimethylformamide. Polar aprotic solvent – dissolves ionic nucleophiles but does not H-bond to them, speeding SN2 and SNAr.
- DMSO
- Dimethyl sulfoxide. Polar aprotic solvent; also the oxidant in Swern/Kornblum-type reactions.
- MeCN
- Acetonitrile. Polar aprotic solvent.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- 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].
How it compares with related reactions
| SN2 | SN1 | SN2′ | |
|---|---|---|---|
| Reagents | NaOH (source of HO⁻) or any strong, unhindered Nu: | tert-butyl bromide; H₂O as solvent and nucleophile | Nu⁻ (e.g. Me₂CuLi, amines, alkoxides) + allylic bromide |
| Conditions | Polar aprotic solvent — DMF, DMSO, MeCN; 25–80 °C | Polar protic solvent (H₂O, ROH), 25–60 °C | THF or Et₂O, –78 to 25 °C |
| Stereochemistry | Inversion of configuration (Walden) | Planar cation: attack from both faces → racemisation at a stereocentre | Usually syn to the leaving group (cuprates anti) |
| Rate / selectivity | Rate = k[RX][Nu⁻]; Me > 1° > 2° ≫ 3° | Rate = k[RBr] (first order); 3° > 2° ≫ 1° | Favoured when the α-carbon is hindered |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the SN2?
NaOH (source of HO⁻) or any strong, unhindered Nu:. Conditions: Polar aprotic solvent — DMF, DMSO, MeCN; 25–80 °C.
Which bonds are formed and which are broken?
Formed: C–O σ bond. Broken: C–Br σ bond.
What is the stereochemical outcome?
Inversion of configuration (Walden)
What controls the rate or the selectivity?
Rate = k[RX][Nu⁻]; Me > 1° > 2° ≫ 3°
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go