$word_{count}$: 220

Nucleophilicity (kinetic — how fast Nu attacks carbon) differs from basicity (thermodynamic — how strongly base binds $H^+$).

In polar protic solvents: nucleophilicity = $I^-$ > $Br^-$ > $Cl^-$ > $F^-$ (reverse of basicity — smaller ions more heavily solvated by H-bonding). In polar aprotic solvents: nucleophilicity = $F^-$ > $Cl^-$ > $Br^-$ > $I^-$ (matches basicity — no solvation penalty).

Leaving group ability = inverse of basicity. Better LG = weaker base = more stable anion: $I^-$ > $Br^-$ > $Cl^-$ >> $F^-$. Exception: in SNAr, F is the best LG because addition (not elimination) is RDS.

Strong nucleophile, weak base → SN2 (e.g., $CN^-$, $I^-$, $RS^-$, $N3^-$). Strong base, moderate nucleophile → E2 (e.g., KOtBu, LDA, DBU). Strong nucleophile AND strong base → SN2 or E2 depending on substrate.

An ambident nucleophile has two nucleophilic sites: $CN^-$ attacks via C (→ nitrile, SN2 kinetic product) or N (→ isocyanide). With KCN → RCN (C-attack). With AgCN → RNC (N-attack, thermodynamic).