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SN2 - Second-order Nucleophilic Substitution

Leaving Group

What are the characteristics of a good leaving group (nucleofuge), and what is the difference between a leaving group and a nucleophile? Frequently, the leaving group in a nucleophilic substitution is, like the nucleophile, negatively charged. So, why does the nucleophile enter the substrate, while the leaving group leaves it? The ability of the nucleofuge to leave the substrate is directly correlated to its ability in stabilizing the negative charge.

Fig.1
Order of ability to leave the substrate of different halogens.

If the leaving group is a weak base, it is a good nucleofuge. This can be illustrated by the halide anions. The leaving ability decreases from that of iodide to that of fluoride. This tendency is equivalent to the acidity's tendency in the corresponding hydrohalic acids. Iodide is a good leaving group, while SN2 reactions of fluoroalkanes hardly occur, as the fluoride anion is a poor leaving group.

Aside from halide ions, some derivatives of sulfonic acid are also very good leaving groups. These sulfonic acid derivatives are particularly good leaving groups, because they possess substituents that considerably stabilize the negative charge by resonance stabilization.

Fig.2
Resonance stabilization of the trifluoromethanesulfonate anion (triflate anion).

Due to the extraordinary leaving ability of the triflate anion and other similar anions, such as mesylate and tosylate, these leaving groups are frequently applied to organic syntheses.

Fig.3
Mesylate anion (methanesulfonate anion).
Fig.4
Tosylate anion (p-toluenesulfonate anion).
Fig.5
Triflate anion (trifluoromethanesulfonate anion).

Hydroxide as leaving group

The hydroxide anion OH is a particularly poor leaving group, as its negative charge cannot be stabilized. However, a hydroxy group may be substituted if it has been previously chemically altered by one of the following methods:

  • Esterification by one of the sulfonic acid derivatives mentioned above.
  • Conversion to the more efficient leaving group water by protonation.

Exercises

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