Acids and bases
HA−−→←−H++A−
The Brønsted-Lowry definiton of acids and bases is one of two definitions still in common
use by modern chemists.
11.3 Lewis Acids and Bases: Electron donors and acceptors.
The second definition in widespread use deals not with a molecule’s propensity for accepting
or donating protons but rather with accepting or donating electrons, thereby demonstrating
a slightly different emphasis and further broadening the explanatory and predictive powers
of acid-base chemistry.
Definition of Lewis Acids and BasesA Lewisacidis anelectron acceptorand a
Lewisbaseis anelectron donor.Probably the most important aspect of Lewis acids and bases is which types of atoms can
donate electrons, and which types of atoms can receive them. Essentially atoms with lone
pairs, i.e. unshared pairs of electrons in an outer shell, have the capability of using those
lone pairs to attract electron-deficient atoms or ions. This is why ammonia can bond a
fourth hydrogen ion to create the ammonium ion; its lone pair of electrons can attract and
bond to a free H+ion in solution and hold on to it. For the same reason, methane cannot
become methanium ion under ordinary circumstances, because the carbon in methane does
not have any unshared pairs of electrons orbiting its nucleus. Generally speaking, Lewis
acid are in the nitrogen, oxygen or halogen groups of the periodic table.
11.4 Nucleophiles and Electrophiles
Whether or not an atom can donate or accept electrons it can be called a nucleophile or
electrophile, respectively. Electrophiles(literally, ”lovers of electrons”) are attracted to
electrons. Electrophiles therefore seek to pair with unshared electrons of other atoms.Nu-
cleophiles, or ”nucleus lovers”, seek positively charged nuclei such as those available in
acidic solutions as hydronium ions. It is important to note that electrophiles and nucle-
ophiles are oftenions, but sometimes they are not.
Understandingelectrophiles andnucleophiles goes beyond simply ideas of acids and
bases. They are, in a majority of cases, the major players in organic reactions. As we will,
over and over again, find reactions that are the result of nucleophilesattackingelectrophiles.
Keep in mind that the idea of nucleophiles and electrophiles is very related to the ideas of
acids and bases in the Lewis context.
But it is also important to understand that, while they are related, they are not exactly
the same thing either. An ion or molecule can be a strong nucleophile and a weak base
(e.g. N 3 - , RS-, I-, Br-and CN-). Another ion can be a poor nucleophile and a strong base
((CH 3 ) 3 CO-, R 2 N-). And yet others are strong nucleophiles and strong bases (R 3 C-, RO-,
HO-) and poor nucleophiles and poor bases (RCO 2 - , ROH, NH 3 ).