Lewis bases are electron­pair donors, whereas Lewis acids are
electron­pair acceptors.
LEARNING OBJECTIVE [ edit ]
Recognize Lewis acids and bases in chemical reactions.
KEY POINTS [ edit ]
A Lewis acid is an electron­pair acceptor; a Lewis base is an electron­pair donor.
Some molecules can act as either Lewis acids or Lewis bases; the difference is context­specific and
varies based on the reaction.
Lewis acids and bases result in the formation of an adduct rather than a simple displacement
reaction, as with classical acids and bases. An example is HCl vs H+: HCl is a classical acid, but not
a Lewis acid; H+ is a Lewis acid when it forms an adduct with a Lewis base.
TERMS [ edit ]
covalent bond
a chemical bond in which two atoms are connected to each other by sharing two or more electrons
nucleophile
literally "lover of nuclei," Lewis bases are often referred to as this because they seek to donate
their electron pairs to electron­poor species, such as H+
Give us feedback on this content: FULL TEXT [ edit ]
A Lewis acid is defined as an electron­pair acceptor, whereas a Lewis base is an electron­pair
donor. Under this definition, we need not define an acid as a compound that is capable of
donating a proton, because under the Lewis definition, H+itself is the Lewis acid; this is
because, with no electrons, H+can accept an electron pair.
A Lewis base, therefore, is any species that donates a pair of electrons to a Lewis acid. The
"neutralization" reaction is one in which a covalent bond forms between an electron­rich
species (the Lewis base) and an electron­poor species (the Lewis acid). For this reason, Lewis
bases are often referred to as nucleophiles (literally, "lovers of nuclei"), and Lewis acids are
sometimes called electrophiles ("lovers of electrons"). This definition is useful because it not
only covers all the acid­base chemistry with which we are already familiar, but it describes
reactions that cannot be modeled by Arrhenius or Bronsted­Lowry acid­base chemistry. For
now however, we will consider how the Lewis definition applies to classic acid­base
neutralization.
Applying the Lewis Definition to Classical Acid­Base Chemistry
Consider the familiar reaction of NaOH and HCl:
N aOH(aq) + HCl(aq) → N aCl(aq) + H 2 O(l)
We have previously described this as an acid­baseneutralization reaction in which water and
a salt are formed. This is still completely correct, but the Lewis definition describes the
chemistry from a slightly different perspective. When considering Lewis acids and bases, the
only real reaction of interest is the net ionic reaction:
OH − (aq) + H + (aq) → H 2 O(l)
Under the Lewis definition, hydroxide acts as the Lewis base, donating its electron pair to
H+. Thus, in this version of the neutralization reaction, what interests us is not the salt that
forms, but the covalent bond that forms between OH­ and H+to form water. A significant
hallmark for Lewis acid­base reactions is the formation of such a covalent bond between the
two reacting species. The reaction's final product is known as an adduct, because it forms
from the addition of the Lewis base to the Lewis acid.
Lewis acids and bases
Lewis acids (BF3, top, and H+, bottom) react with Lewis bases (F­, top, NH3, bottom) to form products
known as adducts. Note that the first reaction cannot be described by Arrhenius or Bronsted­Lowry acid­
base chemistry.
Beyond Classical Acid­Base Chemistry
By treating acid­base reactions in terms of electron pairs instead of specific substances, the
Lewis definition can apply to reactions that do not fall under other definitions of acid­base
reactions. For example, a silver cation behaves as a Lewis acid with respect to ammonia,
which behaves as a Lewis base, in the following reaction:
+
Ag + (aq) + 2 N H 3 → [Ag(N H 3 ) 2 ]
This reaction results in the formation of diamminesilver(I), a complex ion; it is perfectly
described by Lewis acid­base chemistry, but is unclassifiable according to more traditional
Arrhenius and Bronsted­Lowry definitions.
Application to Organic Chemistry
In organic chemistry, it is useful to understand that nucleophiles are Lewis bases and
electrophiles are Lewis acids. Nearly all reactions in organic chemistry can be considered
Lewis acid­base processes.
What are acids and bases?
This lesson continues to describe acids and bases according to their definition. We first look at the
Bronsted­Lowry theory, and then describe Lewis acids and bases according to the Lewis Theory.