Elimination E1

Question 1

Describe the reaction kinetics of an E1 reaction

Answer 1

1. Unimolecular and 1st order
2. The reaction rate is only proportional to the concentration of the electrophile (1st order)
3. Only the electrophile is involved in the rate-limiting step

Question 2

What is the rate equation for an E1 reaction

Answer 2

1. Rate= k[electrophile]

Question 3

Describe the basic mechanism of an E1 reaction

Answer 3

1. 2 steps
2. First curly arrow from C-X bond to X- slow step forming a carbocation
3. Then curly arrow from base lone pair to H and from C-H to C-C
4. Forms C=C bond + Base.HX

Question 4

What affects the reaction rate of an E1 reactiion

Answer 4

1. Electrophile structure
2. Leaving group
3. Temperature
4. NOT base

Question 5

Where is the geometry of an E1 reaction determined

Answer 5

1. In the deprotonation step

Question 6

What determines whether it is an E/Z alkene

Answer 6

1. 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
2. Conformation leading to the E-alkene is less sterically hindered and therefore lower in energy

Question 7

What does the Hammond postulate predict

Answer 7

1. The transition state leading to the major product will also be lower in energy (kinetically favourable)

Question 8

Which conformation of alkene is most stable

Answer 8

1. E-alkenes are more stable than Z-alkenes
2. As heats of combustion are less negative
3. The formation of E-alkenes is therefore both thermodynamically and kinetically favourable

Question 9

How can you explain regioselectivity of E1 reactions

Answer 9

1. If there are 2 possible Beta hydrogens
2. More substituted alkenes are more stable (thermodynamic product)
3. Transition state leading to more substituted alkene is also lower in energy (kinetic product)

Question 10

What are alkenes stabilised by

Answer 10

1. Hyperconjugation of their empty pi* antibonding orbital with adjacent filled orbitals of adjacent C-H or C-C bonds
2. More substituents leads to greater stabilisation
3. Overlap provides stability
4. Same stabilisation is also present in transition states, so more substituted partial double bond has greater stabilisation
5. Removal of hydrogen leading to more substituted product therefore has a lower energy transition state.

Question 11

What type of substrate undergoes E1 elimination reactions

Answer 11

1. Only substrates that can form a relatively stable carbocation intermediate can undergo E1- same as for SN1
2. Benzylic and allylic substrates containing suitable beta- hydrogens can also eliminate via an E1 mechanism