Chemistry

The key to $S_N1$ is recognizing that this mechanism happens in two distinct steps with very different activation energies. The first step—ionization of the C–X bond to form a carbocation intermediate—is slow and difficult because you're breaking a strong bond without help from an incoming nucleophile. The second step—the nucleophile attacking the carbocation—is fast because the carbocation is highly reactive and electron-deficient. Since the rate-determining step is always the slowest step, breaking the C–X bond controls the overall reaction rate, not the nucleophile attack that follows.

The key to $S_N1$ is recognizing that this mechanism happens in two distinct steps with very different activation energies. The first step—ionization of the C–X bond to form a carbocation intermediate—is slow and difficult because you're breaking a strong bond without help from an incoming nucleophile. The second step—the nucleophile attacking the carbocation—is fast because the carbocation is highly reactive and electron-deficient. Since the rate-determining step is always the slowest step, breaking the C–X bond controls the overall reaction rate, not the nucleophile attack that follows.