F– > Cl– > Br– > I–This is because there are no protons to get in the way of the attacking nucleophile. In aprotic
solvents, nucleophilicity relates directly to basicity.
We won’t use nonpolar solvents with this type of reaction because we need our nucleophile to
dissolve. Because charged molecules are polar by nature, a polar solvent is required to dissolve the
nucleophile as well because like dissolves like . Examples of strong nucleophiles include HO–, RO–,
CN–, and . NH 3 and are fair nucleophiles, and H 2 O, ROH, and RCOOH are weak or very
weak nucleophiles. As far as functional groups go, amine groups tend to make good nucleophiles.
KEY CONCEPT
We can’t use nonpolar solvents in these nucleophile–electrophile reactions because our
reactants are polar—they wouldn’t dissolve!ELECTROPHILES
Electrophiles are defined as “electron-loving” species with a positive charge or positively polarized
atom that accepts an electron pair when forming new bonds with a nucleophile. Again, this
definition brings to mind Lewis acids. The distinction, as with nucleophiles and bases above, is that
electrophilicity is a kinetic property, whereas acidity is a thermodynamic property. Practically,
however, electrophiles will almost always act as Lewis acids in reactions. A greater degree of positive
charge increases electrophilicity, so a carbocation is more electrophilic than a carbonyl carbon.
Some comparisons between electrophiles are drawn in Figure 4.4. Additionally, the nature of the
leaving group influences electrophilicity in species without empty orbitals; better leaving groups
make it more likely that a reaction will happen. If empty orbitals are present, an incoming
nucleophile can make a bond with the electrophile without displacing the leaving group.
Electrophilicity and acidity are effectively identical properties when it comes to reactivity. Just as
alcohols, aldehydes and ketones, carboxylic acids, and their derivatives act as acids, they also act as
electrophiles, and can make good targets for nucleophilic attack.