Final answer:
The tertiary carbocation is the most stable and, therefore, undergoes SN1 reactions the easiest. It benefits from the stabilizing inductive effects of surrounding alkyl groups, which make the formation of the carbocation intermediate more favorable in SN1 reactions. Option 3 is correct.
Step-by-step explanation:
The easiest type of carbocation to undergo SN1 reactions is the tertiary carbocation. An SN1 reaction involves a carbocation intermediate that is stabilized by adjacent alkyl groups through the inductive effect.
Tertiary carbocations are highly stabilized due to the presence of three alkyl groups, which makes them better able to handle the positive charge of the carbocation. This increased stability leads to a faster formation of the carbocation intermediate, which is the rate-determining step in SN1 reactions.
Primary carbocations are the least stable and thus the least likely to undergo SN1 reactions. Secondary carbocations are more stable than primary ones but less stable than tertiary carbocations. Quaternary carbocations do not undergo SN1 reactions, as they would require the formation of a five-coordinated carbon, which is highly unstable and generally not observed.
The formation of a carbocation leads to a trigonal planar intermediate, in which the nucleophile can attack from either side, creating a racemic mixture if the center is chiral. The stability of the carbocation intermediate determines the likelihood of the reaction following the SN1 pathway, so tertiary alkyl halides are typical substrates for SN1 reactions.