Which of the following order(s) is/are correct for Nucleophilic substitution reaction?
Which of the following order(s) is/are correct for Nucleophilic substitution reaction?
The correct options are
B
D
All the reactions have alkyl halides as the substrate - but with different carbon chain structures and different leaving groups.
In the SN1 mechanism, the leaving group departs in a slow, rate-determining step - leaving behind a carbocation. We want the leaving group to be as good as it can be and the resulting carbocation to be as stable as it can be. But the rate expression is given by:
For the SN1 reaction, the rate of reaction ∝[R−X]
On the other hand, the SN2 mechanism is a concerted one. The incoming nucleophile approaches from a direction opposite to that of the leaving group at the tetrahedral carbon centre where the action happens. The better the nucleophile, the lower the activation energy; also, we want the leaving group to be a good one for better yields. But the overall rate expression has the activities or concentrations of both the substrate (alkyl halide) and the nucleophile. Sine the pathway needs the nucleophile and the C−LG bond in a concerted arrangement - where both bond-breaking and bond formation happen simultaneously, it is ideal that the carbon where the attack happens be as unhindered sterically as possible. All other things being equal, the rate expression is:
For the SN2 reaction, the rate of reaction ∝[R−X][:Nu]
B
D
All the reactions have alkyl halides as the substrate - but with different carbon chain structures and different leaving groups.
In the SN1 mechanism, the leaving group departs in a slow, rate-determining step - leaving behind a carbocation. We want the leaving group to be as good as it can be and the resulting carbocation to be as stable as it can be. But the rate expression is given by:
For the SN1 reaction, the rate of reaction ∝[R−X]
On the other hand, the SN2 mechanism is a concerted one. The incoming nucleophile approaches from a direction opposite to that of the leaving group at the tetrahedral carbon centre where the action happens. The better the nucleophile, the lower the activation energy; also, we want the leaving group to be a good one for better yields. But the overall rate expression has the activities or concentrations of both the substrate (alkyl halide) and the nucleophile. Sine the pathway needs the nucleophile and the C−LG bond in a concerted arrangement - where both bond-breaking and bond formation happen simultaneously, it is ideal that the carbon where the attack happens be as unhindered sterically as possible. All other things being equal, the rate expression is:
For the SN2 reaction, the rate of reaction ∝[R−X][:Nu]
