Elimination reactions

Question 1

When is there more delocalisation, with a stronger or weaker nucleophile?

Answer 1

A weaker nucleophile has more delocalisation

Question 2

is a weaker nucleophile, smaller or bigger than a stronger ?

Answer 2

smaller

Question 3

whats more electronegative, a strong or weak nucleophile?

Answer 3

A weaker nucleophile is more electronegative

Question 4

Whats a better leaving group, an anion from a strong or weak acid?

Answer 4

A better leaving group is anions from strong acids e.g. Br

Question 5

In the following reaction, How do the products arise?

Answer 5

The major product is formed from an S_N2 nucleophilic substitution reaction

The minor product is formed from an elimination reaction

Question 6

What does the product ratio depend upon?

Answer 6

The relative rates of the substitution vs. elimination reactions

NB: won’t be expected to predict which reaction is dominant in a given example

Question 7

Whats a Brønsted base?

Answer 7

A species that accepts a proton

Question 8

What is the order of processes in an E2 mechanism?

Answer 8

The proton is removed whilst the leaving group departs

Question 9

What is the order of processes in an E1 mechanism?

Answer 9

The leaving group departs first, then the proton is removed

Question 10

What is the order of processes in an E1cb mechansim?

Answer 10

The proton is removed first (creating a negative charge), then the leaving group departs

Question 11

Tell me about E2 reactions?

What is the rate dependent on?

Answer 11

• One step mechanism
• The rate is dependent on both the [Brønsted base] and the [alkyl halide]
• The rate determining (only) step of the elimination is bimolecular (base and alkyl halide) is the mechanism is E2

Question 12

In E2 reactions, where must the proton be removed from?

Answer 12

The carbon (beta-carbon) either side of the carbon with the leaving group attached (alpha carbon)

This means that more than one product may be formed

Question 13

Example E2 reactions…

Answer 13

Question 14

Tell me what is required for stereoselectivity of an E2 elimination reaction?

Answer 14

• Anti-periplanar
• cis of A and C
• cis of B and D

Question 15

What does Anti-periplanar mean?

Answer 15

Anti-periplanar: Dihedral angle between H and X of 180˚

Question 16

Why does the E2 elimination reaction take place for a specific conformation of the alkyl hydride?

Answer 16

• The conformation is staggered= stable
• The brønsted base and leaving group (both big) will be as far apart as possible

Question 17

Answer 17

Question 18

Tell me about E1 reactions?

What is the rate dependent on?

Answer 18

• two-step mechanism

Step 1: the leaving group departs (slow)

A proton is removed (fast)

• The rate depends on only the RDS
• the RDS is unimolecular (alkyl halide only) so this mechanism is called E1

NB: E1 has same RDS as S_N1 nuc. sub. (so in each the rate depends on how stable the leaving group is)

Question 19

Tell me about E1cb reactions?

What does the rate depend on?

Answer 19

• Two-step mechanism

Step 1: the proton is removed

step 2: the leaving group departs

• The rate is dependent on [carbanion] so this is the RDS (STEP 2)
• when a molecule loses a proton, the anion is called the conjugate base
• The RDS is unimolecular
• reaction is favoured if the intermediate carbanion is stable
• only reaction where a wedge can be removed without protonating (a double bond can be formed)

Question 20

What stabilises the carbanion in an E1cb reaction?

Answer 20

The delocalisation of the lone pair (sharing by the electronegative atoms)

Question 21

In E1cb reactions, what does the loss of the leaving group show?

Answer 21

from either resonance forms

Question 22

Do E1, E2 and E1cb reactions all occur in biological pathways?

Answer 22

yes

Question 23

What reaction is most common in enzyme-catalysed dehydration?

Answer 23

E1cb

Question 24

Give an example of a biochemical elimination reaction using the E1cb mechanism?

Answer 24

Bacterial dehydrosquinase-catalysed dehydration 3-dehydroquinate