One feature of the SN1 type of reaction?
a) Racemization
b) Dehydration
c) Planarization
d) Elimination
e) a and c
The correct answer and explanation is :
Correct Answer: (e) a and c (Racemization and Planarization)
The SN1 (Substitution Nucleophilic Unimolecular) reaction is a two-step process involving the formation of a carbocation intermediate. The key features of an SN1 reaction include:
- Formation of a Planar Carbocation (Planarization)
In the first step, the leaving group departs, forming a positively charged carbocation. This intermediate has a trigonal planar geometry due to the sp² hybridization of the central carbon. This planar structure is crucial because it allows the nucleophile to attack from either side of the molecule, leading to racemization if the substrate is chiral. - Racemization
Since the carbocation intermediate is planar, the incoming nucleophile can attack from either the front or the back with equal probability. If the starting material is chiral, this results in a mixture of retention and inversion of configuration. However, in most cases, a slight preference for inversion is observed due to the partial shielding effect of the departing leaving group. Nevertheless, the overall product is a racemic mixture. - Why Not the Other Options?
- (b) Dehydration – This is a characteristic of elimination reactions (E1/E2) rather than SN1.
- (d) Elimination – While SN1 reactions can compete with E1 elimination, elimination is not a defining feature of SN1.
Conclusion
The two key features of the SN1 mechanism are planarization due to carbocation formation and racemization of chiral substrates. Hence, the correct answer is (e) a and c.