NS8. Nucleophilicity

The nucleophile can sometimes play a pronounced role in nucleophilic substitutions.  The following relative rates have been observed when these nucleophiles reacted with methyl bromide in methanol:

Figure NS8.1. Trends in nucleophicity in alcohol solvent. Note: Ph = phenyl, C₆H₅; Ac = acetyl, CH₃C=O; Et = ethyl, CH₃CH₂.

Presumably, some of the species react much more quickly with methyl bromide because they are better nucleophiles than others.

Problem NS8.1.

Sometimes we can draw general conclusions about kinetic factors by looking at sub-groups among the data.  Determine how the following factors influence nucleophilicity (the ability of a species to act as a nucleophile).  Support your ideas with groups of examples from the data (preferably more than just a pair of entries).

a)  charge on the nuclophile

b)  size of the atom bearing the lone pair

c)  electronegativity of the atom bearing the lone pair

d) delocalization of charge

Problem NS8.2.

Nucleophilicity plays a strong role in the rate of one type of substitution mechanism, but not the other. 

a) In which mechanism is it important?  Support your idea.

b)  Is the reaction of methyl bromide likely to proceed via this mechanism?  Why or why not?

Problem NS8.3.

A trend very similar to the data above is found in substitution reactions of py₂PtCl₂ (py = pyridine) in methanol.  Draw a mechanism for this substitution and explain why nucleophilicity plays an important role.

Problem NS8.4.

Very fast nucleophiles are sometimes more likely to undergo S_N2 reactions than S_N1 reactions.  Explain why.

This site was written by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (retired) with other authors as noted).  It is freely available for educational use.

Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. 

Send corrections to cschaller@csbsju.edu

This material is based upon work supported by the National Science Foundation under Grant No. 1043566.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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