Elimination E1 vs E2

Question 1

What is required from the leaving group in an elimination reaction

Answer 1

1. A good leaving group is required

Question 2

What is a good leaving group

Answer 2

1. The conjugate base of a strong acid

Question 3

Can hydroxides be leaving groups

Answer 3

1. No

2. Must be protonated first

Question 4

What is another example of a good leaving group in elimination reactions

Answer 4

1. Quaternary ammonium ions

Question 5

What happens to entropy in elimination reactions

Answer 5

1. Two molecules of reactants return 3 molecules of products

2. Products are more disordered than the reactants, therefore elimination reactions have positive entropy change

Question 6

What does the entropy of the reaction mean about the temperature

Answer 6

1. Higher temperatures are more favourable
2. DG would be more negative
3. Eliminations normally performed at room temperature or above

Question 7

Substitution versus elimination

Answer 7

1. Table p110

Question 8

What is another type of elimination

Answer 8

1. E1cB

Question 9

What happens in an E1cB elimination

Answer 9

1. Occurs for substrates bearing an electron-withdrawing substituent adjacent to the proton being removed
2. The reversible deprotonated to form an anion which then undergoes elimination of the beta-leaving group in a second step
3. Elimination occurs from the conjugate base of the substrate in the rate-limiting step

Question 10

Describe the kinetics of an E1cB elimination

Answer 10

1. Rate dependent on concentration of conjugate base

2. Overall second order, as concentration of base determines the concentration of the anionic intermediate

Question 11

What can act as a leaving group in E1cB but no other reactions

Answer 11

1. Alcohols can be the leaving group without prior activation
2. Conjugate base of substrate is less stable so preferes to lose hydroxide to form a conjugated alkene product

Question 12

What must substrates that undergo E1cB elimination form

Answer 12

1. Must form a stabilised anionic intermediate

Question 13

Compare the main steps in each elimination mechanism

Answer 13

1. E1- leaving group first, deprotonation follows
2. E2- simultaneous deprotonation and leaving group departure
3. E1cB- deprotonation first, leaving group follows