4.2 Nucleophiles, Electrophiles, and Leaving

Groups

LEARNING GOALS

After   Chapter 4.2,    you will    be  able    to:

Distinguish nucleophiles    and electrophiles   from    Lewis   acids   and bases

Recall  the four    main    trends  to  determine   nucleophilicity of  a   molecule

Describe    the relationship    between electrophile    and leaving group   in  a   substitution

reaction

Identify    the traits  that    increase    electrophilicity

Recall  the traits  of  a   good    leaving group

Almost all reactions in organic chemistry can be divided into one of two groups: oxidation–
reduction reactions or nucleophile–electrophile reactions. Nucleophiles, electrophiles, and leaving
groups are particularly important to the reactions of alcohols and carbonyl-containing compounds,
which we will look at in depth in later chapters. Let’s take a look at how each of these terms is
defined.

NUCLEOPHILES

Nucleophiles are defined as “nucleus-loving” species with either lone pairs or π bonds that can form
new bonds to electrophiles. You may have noted that nucleophilicity and basicity appear to have
similar definitions—and this is true! Good nucleophiles tend to be good bases. There is, however, a
distinction between the two. Nucleophile strength is based on relative rates of reaction with a
common electrophile—and is therefore a kinetic property. Base strength is related to the
equilibrium position of a reaction—and is therefore a thermodynamic property. Some common
examples of nucleophiles are shown in Figure 4.2.