Substitution — unimolecular
SN1: Water substitutes on a tertiary carbocation
How the SN1 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: tert-butyl bromide; H₂O as solvent and nucleophile. Conditions: Polar protic solvent (H₂O, ROH), 25–60 °C. Bonds formed: C–O σ bond; O–H (H₃O⁺); bonds broken: C–Br σ bond; O–H. Stereochemistry: Planar cation: attack from both faces → racemisation at a stereocentre. Rate and selectivity: Rate = k[RBr] (first order); 3° > 2° ≫ 1°.
At a glance
Step 1: Ionisation (slow, rate-limiting)
tert-Butyl bromide ionizes on its own: the C–Br bonding pair leaves entirely with bromine, forming a planar, positively charged carbocation. This step doesn't need the nucleophile yet.
Step 2: Water attacks the planar cation
A lone pair on water's oxygen attacks the empty p-orbital of the flat carbocation. Because the cation is planar, water can attack from either face.
Step 3: Deprotonation
Loss of a proton from the resulting oxonium ion gives tert-butanol. Two steps, one slow (ionization) and rate-limiting — that's the '1' in SN1.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- SN1
- Unimolecular nucleophilic substitution. Two steps: leaving group departs (rate-determining) to give a carbocation, then nucleophile attacks.
How it compares with related reactions
| SN1 | SN2 | SN2′ | |
|---|---|---|---|
| Reagents | tert-butyl bromide; H₂O as solvent and nucleophile | NaOH (source of HO⁻) or any strong, unhindered Nu: | Nu⁻ (e.g. Me₂CuLi, amines, alkoxides) + allylic bromide |
| Conditions | Polar protic solvent (H₂O, ROH), 25–60 °C | Polar aprotic solvent — DMF, DMSO, MeCN; 25–80 °C | THF or Et₂O, –78 to 25 °C |
| Stereochemistry | Planar cation: attack from both faces → racemisation at a stereocentre | Inversion of configuration (Walden) | Usually syn to the leaving group (cuprates anti) |
| Rate / selectivity | Rate = k[RBr] (first order); 3° > 2° ≫ 1° | Rate = k[RX][Nu⁻]; Me > 1° > 2° ≫ 3° | 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 SN1?
tert-butyl bromide; H₂O as solvent and nucleophile. Conditions: Polar protic solvent (H₂O, ROH), 25–60 °C.
Which bonds are formed and which are broken?
Formed: C–O σ bond; O–H (H₃O⁺). Broken: C–Br σ bond; O–H.
What is the stereochemical outcome?
Planar cation: attack from both faces → racemisation at a stereocentre
What controls the rate or the selectivity?
Rate = k[RBr] (first order); 3° > 2° ≫ 1°
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