SN2 Mechanisms Flashcards
(12 cards)
What are the leaving groups in Nucleophilic substitution mechanisms?
The Nucleophile will be C, O, N, P or S based
Leavings groups are Cl- or Br- or I-
Stability of leaving groups
The higher the stability, the better the leaving group
I- is the most stable and Cl- is the least
A carboxylate will be a moderate leaving group whereas thiolates and alkoxides are poor leaving groups (a hydride would be impossible as a leaving group)
What is an SN2 reaction?
Biomolecular Nucleophilic substitutions that have a second order reaction kinetics
Typically 2 step process
What is an SN1 reaction?
A unimolecular Nucleophilic substitution that has fist order reaction kinetics
Equation for SN2 reactions
Rate of reaction depends on the concentration of the 2 components
Rate = k [Nu] [R-X]
Where Nu is the nucleophile and R-X is the electrophile
Inversion of configuration in SN2 reactions
Known as the Walden inversion
Nucleophile attacks the back (rear) of the molecule
Chiral centre of the molecule inverts in the product = product has inverted chirality
There is a transition state where the molecule becomes a zwitter ion with partial charges before the product is formed
Energy profile of SN2 reactions
Reaction coordinate is the energy of the reacting system against the progress of the reaction
Activation energy is what it takes to get to the transition state
End product has lower energy as it is an exothermic reaction
Rates of SN2 reactions
Affected by groups attached to the reacting carbon of electrophile
E.g.
- Primary alkylchloride has a easy SN2 and a relative rate of 1
- Secondary alkylchloride has a relative rate of 0.02
- Tertiary alkylchloride has an impossible SN2 as the nucleophile has to attack the rear end of the molecule which is blocked so has a relative rate of 0
Choline biosynthesis
Starts from ATP
- Triphosphate is a good leaving group
- Nucleophilic sulfur has a methionine group attached
- This forms Triphosphate and SAM which has a methylsulfonium group attached (S has positive charge)
- S is a reactive electrophile which can then be attacked by a nucleophile amine
- This forms Choline and SAH
DNA alkylation
Mustard gas has a sulfonium ion that is a highly reactive electrophile which is attacked by a guanine residue (nucleophile) and forms alkylated DNA
If further reactions occur, this results in cross linked DNA and cell death
Can be used for cancer treatment
Features of transition states
Has partial bonds in the molecule
Sign to represent it is an = with a diagonal line crossing through
When do SN2 reactions occur?
Never at tertiary but at the rest
For secondary electrophiles, there is an inversion of the centre
