Weak bases are good lcaving groups. 6-8 Structure andS2 Reactivity:The Nucleophile ohilicity of the nucleophile depends upon: ote the sequence:1>Br Ch>F i charge increase ydecreaeto the right in the Consider these experiments: +HN: CHCHNH++-a CHOH+:: Cm+5一i+n GH:+hg一CH+:-na CHCH+4一CHCH,,+smtm CHG:+HN:一CHNH+G:e C,Ce◆5：一H,HH+0sww Conclusion:Nucleophilicity correlates with basidty Sce9eie0aetorgtacrosthepenodcable, Should basicity and nucleophilicity be correlated? tion impedes nucleophilicity. Nudeophilicity is a kinetic phenomenon c亦+t 55 Weak bases are good leaving groups. Leaving group ability is inversely related to base strength. Weak bases are best able to accommodate negative charge and are the best leaving groups. (Weak bases are the conjugate bases of strong acids.) Note the sequence: I- > Br- > Cl- > F- 6-8 Structure and SN2 Reactivity: The Nucleophile Nucleophilicity of the nucleophile depends upon: •Charge •Basicity •Solvent •Polarizability •Nature of substituents Increasing negative charge increases nucliophilicity. Consider these experiments: Conclusion: Comparing nucleophiles having the same reactive atom, the species with the negative charge is the more powerful nucleophile. A base is always more nucleophilic than its conjugate acid. Nucleophilicity decreases to the right in the periodic table. Consider these experiments: Conclusion: Nucleophilicity correlates with basicity. As we proceed from left to right across the periodic table, nucleophilicity decreases. (best) H2N- > HO- > NH3 > F- > H2O (worst nucleophile) Should basicity and nucleophilicity be correlated? Basicity is a thermodynamic property: K A + H O AH + HO K = equilibrium constant 2 − − ←⎯⎯⎯⎯→ Nucleophilicity is a kinetic phenomenon: k Nu + R-X Nu-R + X k = rate constant − − ⎯⎯→ Despite this difference in definition, there is a good correlation between nucleophilicy and basicity in the cases of charged versus neutral nucleophiles along a row in the periodic table. Solvation impedes nucleophilicity. Consider these experiments: Conclusion: Nucleophilicity increases in the progression down a column of the periodic table which is opposite the trend predicted by the basicity of the nucleophiles tested