Nucleophilicity is the ability of a species (usually an ion or molecule with a lone pair or π electrons) to attack an electrophilic center, such as a carbon atom in an alkyl halide.

While several factors affect nucleophilicity (like charge, solvent, and electronegativity), one major factor is steric hindrance — the crowding around the reactive site.

---

What Are Steric Effects?

Steric effects occur when bulky groups around the nucleophile block or hinder its approach toward the electrophilic atom.

Even if the nucleophile is strongly basic and has high electron density, its reactivity (nucleophilicity) decreases if it is physically hindered.

---

General Trend

Smaller, less hindered nucleophiles are more nucleophilic because they can easily reach and attack the electrophile.

Less steric hindrance → higher nucleophilicity

---

Example 1: Alkoxides

Let’s compare methoxide ion (CH₃O⁻) and tert-butoxide ion [(CH₃)₃CO⁻].

• CH₃O⁻: small, unhindered → can easily attack → strong nucleophile
• (CH₃)₃CO⁻: bulky tert-butyl group shields oxygen → difficult to attack → weak nucleophile

Result:
CH₃O⁻ > (CH₃)₃CO⁻ in nucleophilicity
But the reverse can be true for basicity, since tert-butoxide is a strong base.

---

Example 2: Amines

Compare NH₃, CH₃NH₂, and (CH₃)₃N:

Compound	Steric hindrance	Nucleophilicity
NH₃	None	Highest
CH₃NH₂	Mild	High
(CH₃)₃N	Very bulky	Low