Elimination E1 Flashcards
1
Q
Describe the reaction kinetics of an E1 reaction
A
- Unimolecular and 1st order
- The reaction rate is only proportional to the concentration of the electrophile (1st order)
- Only the electrophile is involved in the rate-limiting step
2
Q
What is the rate equation for an E1 reaction
A
- Rate= k[electrophile]
3
Q
Describe the basic mechanism of an E1 reaction
A
- 2 steps
- First curly arrow from C-X bond to X- slow step forming a carbocation
- Then curly arrow from base lone pair to H and from C-H to C-C
- Forms C=C bond + Base.HX
4
Q
What affects the reaction rate of an E1 reactiion
A
- Electrophile structure
- Leaving group
- Temperature
- NOT base
5
Q
Where is the geometry of an E1 reaction determined
A
- In the deprotonation step
6
Q
What determines whether it is an E/Z alkene
A
- For the alkene to form a new pi-bond, the empty p-orbital of the cation and the breaking C-H bond must be parallel
- Conformation leading to the E-alkene is less sterically hindered and therefore lower in energy
7
Q
What does the Hammond postulate predict
A
- The transition state leading to the major product will also be lower in energy (kinetically favourable)
8
Q
Which conformation of alkene is most stable
A
- E-alkenes are more stable than Z-alkenes
- As heats of combustion are less negative
- The formation of E-alkenes is therefore both thermodynamically and kinetically favourable
9
Q
How can you explain regioselectivity of E1 reactions
A
- If there are 2 possible Beta hydrogens
- More substituted alkenes are more stable (thermodynamic product)
- Transition state leading to more substituted alkene is also lower in energy (kinetic product)
10
Q
What are alkenes stabilised by
A
- Hyperconjugation of their empty pi* antibonding orbital with adjacent filled orbitals of adjacent C-H or C-C bonds
- More substituents leads to greater stabilisation
- Overlap provides stability
- Same stabilisation is also present in transition states, so more substituted partial double bond has greater stabilisation
- Removal of hydrogen leading to more substituted product therefore has a lower energy transition state.
11
Q
What type of substrate undergoes E1 elimination reactions
A
- Only substrates that can form a relatively stable carbocation intermediate can undergo E1- same as for SN1
- Benzylic and allylic substrates containing suitable beta- hydrogens can also eliminate via an E1 mechanism