Leaving Group Ability (Best → Worst)

$$\boxed{ \text{Sulfonium} > \text{Quaternary ammonium} > \text{Sulfonate (OTs/OMs)} > I^- > Br^- > Cl^- > F^- > H_2O > ROH > OH^- > OR^- > NH_2^- }$$

Sulfonate ester

Sulfonium ion structure

Why HO⁻ and RO⁻ are Poor Leaving Groups

• HO⁻ (hydroxide ion) and RO⁻ (alkoxide ion) are:
  • Strong bases
  • High-energy species
• A good leaving group should be:
  • Stable after leaving
  • Weak base

Since HO⁻ and RO⁻ are strong bases, they are unstable when free, so they do not want to leave.

---

2. Comparison with Alkyl Halides

• In alkyl halides (R–X), the leaving group is X⁻ (Cl⁻, Br⁻, I⁻):
  • These are weak bases
  • Highly stable ions

Consequence in Reactions

• Alcohols (R–OH) and ethers (R–O–R’):
  • Do not undergo SN1/SN2/E1/E2 easily
  • Because –OH or –OR cannot leave directly

So they are less reactive than alkyl halides.

---

4. Activation of Alcohols & Ethers

To make them reactive, we convert the poor leaving group into a good one.

Common activation methods:

(A) Protonation (Acidic Medium)

• In presence of acid (like H₂SO₄ or HCl):

$$R-OH + H^+ \rightarrow R-OH_2^+$$

• Now leaving group = H₂O (water)
• Water is neutral and stable → good leaving group

---

Conversion to Better Leaving Groups

• Convert –OH into:
  • Tosylate (–OTs)
  • Mesylate (–OMs)

These are excellent leaving groups.

---

(C) Reaction with HX

$$R-OH + HCl \rightarrow R-Cl + H_2O$$

---

Summary (Exam Ready)

• HO⁻ and RO⁻ = strong bases → poor leaving groups
• Alkyl halides = weak base leaving groups → high reactivity
• Alcohols/ethers = less reactive
• Need activation (protonation or conversion) before reaction

---