Which of the following potential energy (PE) diagrams represents the SN1 reaction?

Step 1: Understand $S_N1$ mechanism

$S_N1$ is a two-step mechanism:

1. Ionization (slow): $\ce{R-X -> R+ + X-}$

   • Rate-determining step
   • Forms carbocation intermediate
   • High activation energy

2. Nucleophilic attack (fast): $\ce{R+ + Nu -> R-Nu}$

   • Fast step
   • Lower activation energy

Step 2: PE diagram features

For a two-step mechanism:

• Two transition states (peaks)
• One intermediate (valley between peaks)
• First peak is higher (RDS)
• Valley represents carbocation intermediate

Step 3: Identify correct diagram

Option (c): Shows two peaks with valley

• First peak (higher) = Ionization TS
• Valley = Carbocation intermediate
• Second peak (lower) = Nucleophilic attack TS
• Correct for $S_N1$ ✓

Other options:

• (a) One peak = $S_N2$ (one-step)
• (b) Smooth curve = Not typical reaction
• (d) Multiple peaks = Multi-step (>2 steps)

Answer: (c)

Key Concepts:

PE diagram interpretation:

| Feature | Meaning | |---------|----------| | Peaks | Transition states (activated complexes) | | Valleys | Intermediates (stable species) | | Highest peak | Rate-determining step | | ΔH | Overall energy change (products - reactants) | | Ea | Activation energy (height of peak from reactants) |

$S_N1$ vs $S_N2$ diagrams:

$S_N1$:

• Two peaks (two steps)
• One valley (carbocation)
• First peak higher (ionization is RDS)

$S_N2$:

• One peak (one step)
• No intermediate
• Smooth transition from reactants to products