SN1 and SN2 Nucleophilic Substitution Mechanisms
ChemistryOrganic ChemistryAges 17–18
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Organic Chemistry
Substitution reactions of halogenoalkanes can follow two pathways: the one-step SN2, where the nucleophile attacks from behind as the leaving group departs, or the two-step SN1, which proceeds through a carbocation. Choose the carbon's degree and the solvent to see which mechanism dominates, alongside an energy diagram showing the transition state and activation energy.
Lesson: Reaction mechanisms in organic chemistry
What it shows
Halogenoalkanes react with nucleophiles such as OH⁻ by two limiting mechanisms. In SN2 the nucleophile attacks the carbon from the side opposite the bromine; bond making and bond breaking happen together in one step through a single transition state, and the configuration inverts. The rate depends on both [RBr] and [OH⁻]. In SN1 the C–Br bond breaks first to give a planar carbocation, which is then attacked from either side, so the rate depends only on [RBr]. Steric hindrance around the carbon, carbocation stability and the way the solvent surrounds ions decide which pathway dominates.
How to use
Start with a primary substrate in an aprotic solvent, press Run or drag the Progress slider, and follow the energy diagram with its single peak. Then set the Degree of the halogen-bearing carbon to tertiary and the Solvent to protic, and compare the two-step profile with its carbocation dip. Use the View SN1 or View SN2 button to switch mechanism, and read the SN2 : SN1 ratio as you change one factor at a time.
Parameters you can change
- Degree of the halogen-bearing carbon Primary (CH₃CH₂Br), Secondary ((CH₃)₂CHBr), Tertiary ((CH₃)₃CBr)
- Solvent Protic polar (water, ethanol), Aprotic polar (acetone, DMSO)
Questions to explore
- Why does (CH₃)₃CBr react almost entirely by SN1, while CH₃CH₂Br reacts by SN2?
- Which step is rate-determining in SN1, and how can you recognise it on the energy diagram?
- How does changing from a protic to an aprotic solvent affect the SN2 : SN1 ratio for a secondary halogenoalkane?
