That is the rate determining step in a SN1 reaction?
a) Back side attack
b) Formation of the carbocation
c) Dehydration
d) Formation of the double bond
e) Formation of a transition state
The correct answer and explanation is :
Correct Answer:
b) Formation of the carbocation
Explanation:
The rate-determining step (RDS) in an SN1 (Substitution Nucleophilic Unimolecular) reaction is the formation of the carbocation. This step is crucial because it involves the breaking of the bond between the leaving group and the substrate, creating a positively charged carbocation intermediate.
Why is carbocation formation the slowest step?
- Bond Breaking Requires Energy:
The departure of the leaving group requires energy to break the bond between the carbon and the leaving group. This process is endothermic and slower compared to other steps in the reaction. - Unimolecular Rate Law:
The rate of an SN1 reaction depends only on the concentration of the substrate (the alkyl halide or alcohol derivative), not the nucleophile. The rate law is:
[
\text{Rate} = k[\text{R-X}]
]
This suggests that the slowest step involves only the substrate—i.e., the formation of the carbocation. - Stability of the Carbocation:
The rate of SN1 reactions is highly influenced by the stability of the carbocation. Tertiary carbocations are the most stable due to hyperconjugation and inductive effects, making SN1 reactions more favorable for tertiary substrates compared to primary or secondary ones. - Subsequent Steps Are Fast:
After the carbocation forms, the nucleophile quickly attacks, leading to the final product. Since this attack occurs almost instantaneously compared to carbocation formation, it does not determine the reaction rate.
Why are the other options incorrect?
- (a) Backside attack – This is characteristic of SN2 reactions, not SN1.
- (c) Dehydration – This occurs in elimination reactions, not SN1.
- (d) Formation of a double bond – This is part of E1 (elimination) reactions, not SN1.
- (e) Formation of a transition state – The carbocation is an intermediate, not a transition state.
Thus, the formation of the carbocation is the rate-determining step in SN1 reactions.