Nucleophilic Substitution Reactions III

SN reactions (continued III)

Effect of Nucleophile:

Nucleophiles and bases are structurally similar: both have a lone pair or a π bond. They differ in what they attack

Although nucleophilicity and basicity are interrelated, they are fundamentally different.

Basicity is a measure of how readily an atom donates its electron pair to a proton. It is characterized by an equilibrium constant, Ka in an acid-base reaction, making it a thermodynamic property.

Nucleophilicity is a measure of how readily an atom donates its electron pair to other atoms. It is characterized by a rate constant, k, making it a kinetic property.

Nucleophilicity parallels basicity in three instances:

1. For two nucleophiles with the same nucleophilic atom, the stronger base is the stronger nucleophile. The relative nucleophilicity of HO¯ and CH3COO¯, two oxygen nucleophiles, is determined by comparing the pKa values of their conjugate acids (H2O = 15.7, and CH3COOH = 4.8). HO¯ is a stronger base and stronger nucleophile than CH3COO¯.

2. A negatively charged nucleophile is always a stronger nucleophile than its conjugate acid. HO¯ is a stronger base and stronger nucleophile than H2O.

3. Right-to-left-across a row of the periodic table, nucleophilicity increases as basicity increases:

Nucleophilicity does not parallel basicity when steric hindrance becomes important.

• Steric hindrance is a decrease in reactivity resulting from the presence of bulky groups at the site of a reaction.

• Steric hindrance decreases nucleophilicity but not basicity.

• Sterically hindered bases that are poor nucleophiles are called non nucleophilic bases.

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