SN1 Reactivity

1. Understanding the Core Mechanism of SN1

The rate-determining step of an SN1 (Substitution Nucleophilic Unimolecular) reaction relies entirely on the loss of a leaving group (like Cl¯) to form a stable carbocation intermediate.

• A stable carbocation must adopt a trigonal planar geometry with an unhybridized, empty p-orbital.
• The ideal bond angles for this planar sp2 hybridization are 120°.

2. Why Option C is Inactive (Bredt's Rule)

Option C represents a bridged bicyclic compound where the chlorine atom is attached to a bridgehead carbon. Attempting an SN1 pathway on this molecule fails severely due to geometric constraints:

3. Comparison with Other Options

Let's look at why the other molecules given in the question are reactive:

• Option A (tert-Butyl Chloride): A standard tertiary (3°) alkyl halide. It quickly undergoes ionization because it is free to become flat, producing a highly stable 3° carbocation stabilized by 9 hyperconjugating α-hydrogens.
• Option B (Allyl Chloride variant): This structure allows the forming carbocation to be stabilized through resonance allylic delocalization, significantly accelerating the SN1 rate.

Summary Conclusion

Because forming a carbocation at the bridgehead position in Option C creates immense angle strain that cannot be relieved, the energy barrier to break the C-Cl bond via an SN1 pathway is too high. Thus, it remains completely inert.