5- E1

Question 1

what is the general reaction process for E1 reactions?

Answer 1

• Competitive processes to the substitution reaction SN1.
• E1 is carried out with weak bases.
• E1 occurs with substrates that would generate stable carbocations.

Question 2

how many steps are involved in an E1 reaction.

Answer 2

two-step mechanism

Question 3

what is the rate-determining step in E1 reactions?

Answer 3

Loss of the Leaving Group and formation of a stable carbocation (slow; Rate-determining step)

Question 4

what is the fast step in an E1 reaction?

Answer 4

Removal of the proton from the β-carbon to form the π bond (fast).

Question 5

how many molecules are at the RDS in an E1 reaction. what does this mean for the order of the reaction?

Answer 5

one molecule at RDS = unimolecular, first-order reaction

Question 6

describe the regioselectivity of E1 reactions with respect to Zaitsev’s Rule

Answer 6

for E1 reactions, only consider the Zaitsev product. the most substituted product will always be favoured

Question 7

describe the stereochemistry of E1 reactions in terms of E and Z

Answer 7

• in Z form, there is repulsion because the groups are closer together (higher in energy)
• in E form, there is less repulsion because the groups are further away (lower in energy)
• products in E form tend to be the majority

Question 8

what is the order of leaving group stability?

Answer 8

secondary alkyl, tertiary alkyl, primary allylic, primary benzyllic

Question 9

what is the differentiation between E1 and SN1

Answer 9

• Weak Nucleophiles/Bases such as polar protic solvents, tend to produce a mixture of SN1 and E1 products.
• Heating tend to promote elimination reactions: E1 predominates with the addition of heat (∆).

Question 10

which reaction will predominate in the presence of a weak base and no heating?

Answer 10

SN1

Question 11

what is the differentiation between E1 and E2

Answer 11

• E1 requires a weak base

- E2 requires a strong base

Question 12

what is an E1cb reaction and what are the three conditions you need for the reaction to occur?

Answer 12

Elimination Unimolecular Conjugate Base.

Conditions: acidic proton present, OH is a bad leaving group, basic solution

Question 13

describe the two step mechanism for E1cb reaction

Answer 13

• Abstraction of the acidic proton (α to C=O) to form a stabilized carbanion (not carbocation)
• Formation of the π bond with expulsion of the poor LG (by the move of the lone pair)

Question 14

how many transition states does the energy diagram for E1cb reaction have

Answer 14

2 ( 1 for acid-base reaction, 1 for expulsion of the LG)