A chemist intends to run the following reaction on the three substrates shown below: H₂O R-CI CI room temp. CI product (1) (2) (3) They find one will react quickly, one slowly, and one will not react at all. Which is which, and why? HINT: What is the reaction they're trying to do? Does that mechanism tell you anything about why something would be favored?
The SN1 reaction is a substitution reaction that takes place in two steps. In this reaction, a nucleophile replaces a leaving group in two steps.
- In the first step, the leaving group leaves and forms a carbocation. This is the slowest step in this mechanism and hence the rate-determining step.
- In the second step, the nucleophile attacks the carbocation and forms the substituted product.
Whether a reaction proceeds by the SN1 mechanism is determined by the stability of the carbocation intermediate formed. If the carbocation intermediate formed can be stabilized, then the reaction will follow the SN1 pathway. If it is not stabilized, the reaction will not proceed by the SN1 pathway (In this case, the reaction may proceed through the SN2 pathway, if the substrate is suitable for an SN2 reaction).
An SN2 reaction is also a substitution reaction similar to an SN1 reaction. But the difference is in the SN2 reaction, the attack of nucleophiles (nucleophile attack from the backside) and the leaving of the leaving group take place in a single step. So, sterically less hindered C centers will undergo SN2 reaction.
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