ACID AND BASE
Venkata Kumar Sahu
Pharmaceutical Analysis

OBJECTIVE:
• 1. To Understand the Definitions of Acid and
Base
• 2. To learn theories of acid and base

Common Lab acids

Common Lab Bases

Every day example of Neutralization
Indigestion remedies are bases that
neutralise excess stomach acid
Toothpaste is a base that neutralises
acid in the mouth

Lewis Theory
A Lewis acid is an electrophilic species that accepts a pair of electrons
from ions or molecules.
A Lewis base is a nucleophilic species that donates a pair of electrons to
ions and molecules.
Lewis acid and base can react with one another by transferring electrons to
form a covalent bond resulting in a simple or complex molecular structure.
The product formed by a Lewis acid-base reaction is known as adduct

Mechanism

Lewis Acid and Base

Examples of Lewis Acid and Base
Lewis acid can be
• Any cation that can accept
electron pair: H+, Mg2+, Fe3+,
NH4
+, and H3O+
• Any molecule with an incomplete
octet of electrons: BF3 and AlCl3
• Any molecule where the central
atom can have more than 8
electrons in its valence shell:
SiCl4 and SiF4
• Any molecule that can have
multiple bonds between two
atoms of different
electronegativity: CO2 and SO2
•
Lewis base can be
• Any simple anion that can give
up a lone pair of electrons: O2-,
Cl–, F–, and H–
• Any anionic species containing
a lone pair of electrons: OH–,
CN–, CH3
–, and SO4
2-
• Any molecule with an extra
pair or pairs of electrons: H2O,
NH3, and C(CH3)Cl
• Any electron-rich pi system:
benzene (C6H6), ethylene
(C2H4), and ethyne (C2H2)
•

APPLICATION
Applications of Lewis
Acid
• Friedel-Crafts alkylation
reaction
(method of generating alkylbenzenes by using alkyl
halides as reactants)
Applications of Lewis
Base
• Formation of ligands
• Modification of metallic
catalyst
• Form bonds with Lewis
acids to produce
chelating agents

THEORIES OF ACID AND BASE AND
THEIR SCOPE
Illustration of the hierarchy of acid-base theories. Arrhenius acids and bases are a
sub-class of Brønsted acids and bases, which are themselves a subclass of Lewis acids

USANOVICH THEORY
Usanovich (1934) modified the Lewis concept of acid and base by removing
the restriction of either donation or acceptance of the electron pair in a more
generalized fashion.
Acid : It is a chemical species that reacts with a base thereby giving up
cations or accepting anions or electrons.
Base : It is a chemical species that reacts with an acid thereby giving up
anions or electrons or combines with cations
Usanovich’s concept in a much broader sense includes all the oxidizing agents
as acids and the reducing agents as bases, e.g.,

LUX-FLOOD CONCEPT
The concept of acid-base reactions with respect to the oxide ion was first
introduced by Lux (1929) and supported by Flood (1947). According to the
Lux-Flood concept—‘an acid is the oxide-ion acceptor while a base is the
oxide donor’.
Merit:
This approach emphasizes the acid and basic-anhydride aspects. Acidic oxides
are acid anhydride and in the aqueous medium generate protonic acids,
while, basic oxides are basic anhydride and generate hydroxyl ions in the
aqueous medium.
Demerit:
The usefulness of this concept is limited to species that contain oxide ion.
The concept is applicable only in the molten state of an oxide.