i) A negatively charged nucleophile is always a more reactive nucleophile than its
conjugate acid ⇒ HO– is a better nucleophile than H 2 O; RO– is a better
nucleophile than ROH.
ii) In a group of nucleophiles in which the nucleophilic atom is the same,
nucleophilicities parallel basicities:
RO– > HO– >> RCO 2 – > ROH > H 2 O
2) Correlation between electrophilicity-nucleophilicity and Lewis acidity-basicity:
i) “Nucleophilicity” measures the affinity (or how rapidly) of a Lewis base for a
carbon atom in the SN2 reaction (relative rates of the reaction).
ii) “Basicity”, as expressed by pKa, measures the affinity of a base for a proton (or
the position of an acid-base equilibrium).
Correlation between Basicity and Nucleophilicity
Nucleophile CH 3 O– HO– CH 3 CO 2 – H 2 O
Rates of SN2 reaction with CH 3 Br 25 16 0.3 0.001
pKa of conjugate acid 15.5 15.7 4.7 –1.7
iii) A HO– (pKa of H 2 O is 15.7) is a stronger base than a CN– (pKa of HCN is ~10)
but CN– is a stronger nucleophile than HO–.
3) Nucleophilicity usually increases in going down a column of the periodic table.
i) HS– is more nucleophilic than HO–.
ii) The halide reactivity order is: I– > Br– > Cl–
iii) Larger atoms are more polarizable (their electrons are more easily distorted)
⇒ a larger nucleophilic atom can donate a greater degree of electron density to
the substrate than a smaller nucleophile whose electrons are more tightly held.
6.14C SOLVENT EFFECTS ON SN 2 REACTIONS:
PROTIC AND APROTIC SOLVENTS
- Protic Solvents: hydroxylic solvents such as alcohols and water
