Effect Of Leaving Group Flashcards
What is the affect of the leaving group dependent on?
Dependent on RX bond strength and the stability of the leaving group
How does bond strength affect the leaving group?
For a strong bond, it is harder for the leaving group to leave
For a weak bond it is easier for the leaving group to leave
What is a good leaving group?
Small, stable and neutral molecule or stable anions that are weakly basic
How does acid strength affect the leaving group?
A strong acid will completely dissociate which means it is happy to release as leaving group
A weak acid will only partially dissociate which means it is reluctant to release the leaving group (poor leaving group)
How does the leaving group affect the rate of SN1 and SN2 reactions?
Have a good leaving group increases the rate of both
This is because the rate determining step in both reactions involves the cleavage of the CX bond
The better the leaving group, the faster the cleavage
Which halogen is the best leaving group?
The iodine ion is the best leaving group
This is because it forms the weakest bond to the carbon and I- is a weak base
This means iodoalkanes react more rapidly than other halogenoalkanes in both reactions
Give an example of nucleophilic catalysis and explain how it works
Iodine ion is a good leaving group and a good nucleophile (large and polarisable)
This means the iodine is used a catalyst to increase the rate of slow SN2 reaction
Two fast SN2 reactions replaces one slow one
In these reactions the Iodine is first used as the nucleophile and then a leaving group
What is a nucleophilic catalysts?
This is a nucleophile that after it has been added to the bond, can also act as a good leaving group
How is OH- made into a better leaving group?
It is protonated to water which is a better leaving group
How can internal nucleophiles assist?
An internal nucleophile can assist, via the formation of a cyclic intermediate, in the overall substitution process
How can you tell how basic something is? Does this make it a better or worse leaving group?
If an molecule has a strong conjugate acid, then the anion is weakly basic and is a better leaving group
If the a molecule has a weak conjugate acid then it is strongly basic and is a worse leaving group
What is neighbouring group participation?
An internal nucleophile is present that can assist, via the formation of a cyclic intermediate, in the overall substitution process
What does NGP lead to?
An increased rate of reaction
Retention of configuration
Need dilute conditions otherwise SN2 dominates with inversion
