Question

Consider the following reactions I, II and III. The correct order of reactivity of X, Y and Z towards $S_{N}1$ reaction is:

• A. $Z > Y > X$
• B. $X > Z > Y$
• C. $X > Y > Z$
• D. $Z > X > Y$

Hint:

Stable carbocation = Faster $S_{N}1$ reaction.

Answer 1

The Correct Option is A

Step 1: Recall what controls $S_N1$.
In an $S_N1$ reaction, the slow step makes a carbocation. So the reaction is faster when the carbocation formed is more stable.

Step 2: Recall carbocation stability.
Carbocation stability goes tertiary greater than secondary greater than primary. More alkyl groups or resonance make the carbocation more stable.

Step 3: Link stability to rate.
A substrate that makes a more stable carbocation reacts faster by $S_N1$. So we rank the substrates by the stability of their carbocations.

Step 4: Compare the substrates.
Looking at the three substrates X, Y and Z, Z forms the most stable carbocation, then Y, then X forms the least stable one.

Step 5: Write the rate order.
Since faster $S_N1$ follows more stable carbocation, the reactivity order is Z first, then Y, then X.

Step 6: State the answer.
So the $S_N1$ reactivity order is Z greater than Y greater than X. \[ \boxed{Z > Y > X} \]