Elimination reactions Flashcards
When is there more delocalisation, with a stronger or weaker nucleophile?
A weaker nucleophile has more delocalisation
is a weaker nucleophile, smaller or bigger than a stronger ?
smaller
whats more electronegative, a strong or weak nucleophile?
A weaker nucleophile is more electronegative
Whats a better leaving group, an anion from a strong or weak acid?
A better leaving group is anions from strong acids e.g. Br
In the following reaction, How do the products arise?
The major product is formed from an SN2 nucleophilic substitution reaction
The minor product is formed from an elimination reaction
What does the product ratio depend upon?
The relative rates of the substitution vs. elimination reactions
NB: won’t be expected to predict which reaction is dominant in a given example
Whats a Brønsted base?
A species that accepts a proton
What is the order of processes in an E2 mechanism?
The proton is removed whilst the leaving group departs
What is the order of processes in an E1 mechanism?
The leaving group departs first, then the proton is removed
What is the order of processes in an E1cb mechansim?
The proton is removed first (creating a negative charge), then the leaving group departs
Tell me about E2 reactions?
What is the rate dependent on?
- 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
In E2 reactions, where must the proton be removed from?
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
Example E2 reactions…
Tell me what is required for stereoselectivity of an E2 elimination reaction?
- Anti-periplanar
- cis of A and C
- cis of B and D
What does Anti-periplanar mean?
Anti-periplanar: Dihedral angle between H and X of 180˚
Why does the E2 elimination reaction take place for a specific conformation of the alkyl hydride?
- The conformation is staggered= stable
- The brønsted base and leaving group (both big) will be as far apart as possible
Tell me about E1 reactions?
What is the rate dependent on?
- 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 SN1 nuc. sub. (so in each the rate depends on how stable the leaving group is)
Tell me about E1cb reactions?
What does the rate depend on?
- 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)
What stabilises the carbanion in an E1cb reaction?
The delocalisation of the lone pair (sharing by the electronegative atoms)
In E1cb reactions, what does the loss of the leaving group show?
from either resonance forms
Do E1, E2 and E1cb reactions all occur in biological pathways?
yes
What reaction is most common in enzyme-catalysed dehydration?
E1cb
Give an example of a biochemical elimination reaction using the E1cb mechanism?
Bacterial dehydrosquinase-catalysed dehydration 3-dehydroquinate
