SN1 Reactions Flashcards
(10 cards)
Features of SN1 reactions
- Unimolecular = only the substrate is involved in the transition state of the rate determining step
- The starting material is enantiopure
- Products are a racemic mixture —> due to an empty p orbital meaning there is equal probability of attack on each side of the molecule
Equation of SN1 reactions
Rate = k [R-X]
Mechanism of SN1 reactions
Rate determining step is the slowest step as molecule needs to be ionised to trigger a reaction = first step
Slow cleavage of the electrophile
There are 3 transition states
Intermediate cation 1 is when there is a Nucleophilic attack by water
Intermediate cation 2 is attack by the electrophile on the now attached water molecule
Forms alcohol and HX
Reaction profile of SN1 mechanism
Stability of the transition states
Last one is a tertiary cation and has the lowest energy = most stable
First one is a primary cation and has the highest energy = least stable
Activation energy is lower for the tertiary
What is hyperconjugation?
The spread (delocalisation) of charge in orbitals
Hyperconjugation in tertiary cations
Hyperconjugation of neighbouring alkyl group stabilises the positive charge
SN1 vs SN2 in sterics and stability
SN2 is faster when there’s low steric hinderance whereas SN1 is slow at low hinderance as the molecule is unstable
At high steric hinderance such as tertiary alkylchlorides, SN1 reactions are fast as the molecule is stable
When will SN1 reactions occur?
At secondary or tertiary cations
There is a loss of enantiopurity for secondary electrophiles
Describe the general mechanism for SN1 reactions
- Bond is broken between carbon and electrophile
- Nucleophile attacks and a tertiary carbocation is formed
- Proton of the nucleophile is abstracted by the electrophile
- Neutral products are formed
