Substitution Nucleophilic Reactions Flashcards
Write features of Nucleophile substitution reactions.
- Substrate undergoes 2 changes. -:
a. C-X Heterolysis
b. New bond between C and nucleophile - Changes may occur in one or 2 steps
- Mechanism of reactions is deduced from kinetics.
SN2
Write salient features of SN2 mechanism
- Single step
- Simultaneous bond breaking and forming
- Second order
- Rate of reaction is influenced by two reactants
- Backside attack of nucleophile due to steric hindrance of halogen grp
- Transition state contains pentacoordinate carbon having 3 sigma bonds making angle of 120 degrees with each other.
- Inversion occurs
SN1
Write salient features of SN1 mechanism
- 2 Step mechanism
- Heterolysis of C-X Bond is slow and reversible
- Attack of nucleophile is fast.
- Racemic product is formed.
- 1st order
- Rate of reaction depends on concentration of only one reactant.
- 50% Retention 50% inversion
Factors influencing SN1 and SN2
Why is SN2 least favoured in tertiary halides?
Transition state is penta-coordinate and thus offers great steric hindrance.
Factors influencing SN1 and SN2
Why is SN1 most preferred in tertiary halides?
- In SN1 mechanism, planar carbo-cation intermediate is formed.
- No steric crowding observed as bulky grps are accomodated easily
- Secondly, it is stabilised by +I effect of alkyl groups attatched and by hyperconjugation of alpha carbon
- Thus, it is most preferred in tert. halides
Factors influencing SN1 and SN2
Will the electron releasing tendency of reagent play a role in SN2 mechanism?
Nucleophilicity
- Powerful nucleophile favors SN2
- SN1 mechanism is independent on the nature of nucleophile as carbocation intermediate is formed.
Solvation
Which solvents are capable of forming hydrogen bonds?
Polar protic solvents.
e.x. Water, ethanol, methanol, ammonia etc.
Solvation
Give examples of aprotic solvent
Acetone, Dimethylsulfoxide
Solvation
Why does SN1 proceed more rapidly in polar protic solvent
- SN1 proceeds via carbocation intermediate
- Polar solvent stabilises ions by solvation.
- Solvation of anion is important and solvation of carbocation is relatively poor.
- Polar protic solvent facilitates solvation of anions by hydrogen bond.
- Thus, polar protic solvents prefer SN1
Solvation
Why do SN2 reactions occur in aprotic solvent
Protic solvent stabilizes the nucleophile in RDS preventing further reaction.
Solvation
What is solvation?
It is the proces of attraction and association of molecules of solvent with melecules or ions of solute.
