Final answer:
The true statement about an SN1 mechanism is that it involves a single-step reaction, which specifically refers to the rate-determining step being unimolecular. SN1 reactions are characterized by the formation of a carbocation intermediate, follow first-order kinetics, and are common with tertiary substrates, not necessarily symmetrical ones.
Step-by-step explanation:
The statement that is true about an SN1 mechanism is (a) SN1 involves a single-step reaction. However, this statement requires clarification because while the rate-determining step is unimolecular and involves only the substrate, the overall SN1 mechanism is not a single step but a two-step process.
The correct statements about SN1 mechanisms are:
- SN1 reactions are characterized by a carbocation intermediate during the reaction process.
- SN1 kinetics is first order, meaning the rate is proportional to the concentration of only one reactant, the substrate.
- SN1 mechanisms are not known for a simultaneous attack by the nucleophile; this description pertains to SN2 mechanisms, which are bimolecular.
- SN1 reactions are commonly seen with tertiary substrates due to their ability to stabilize carbocations, while symmetrical substrates are not a specific characteristic of SN1.
Therefore, option (b) SN1 reactions exhibit second-order kinetics, is incorrect because SN1 is first order. Option (c) SN1 reactions are characterized by a nucleophile attacking a substrate simultaneously, is also incorrect; this is a characteristic of SN2. Lastly, (d) SN1 reactions are common in symmetrical substrates, is not accurate since SN1 favors substrates that can form stable carbocations, like tertiary substrates, not necessarily symmetrical ones.