Conjugate Definition in Chemistry

The Different Meanings of Conjugate in Chemistry

In chemistry, conjugate may refer to acid-base pairs in the Bronsted-Lowry theory.
In chemistry, conjugate may refer to acid-base pairs in the Bronsted-Lowry theory. Cultura Asia/Rafe Swan / Getty Images

Conjugate Definition

In chemistry, there are three possible definitions of the term "conjugate".

(1) A conjugate refers to a compound formed by the joining of two or more chemical compounds.

(2) In the Bronsted-Lowry theory of acids and bases, the term conjugate refers to an acid and base that differ from each other by a proton. When an acid and base react, the acid forms its conjugate base while the base forms it conjugate acid:

acid + base ⇆ conjugate base + conjugate acid

For an acid HA, the equation is written:

HA + B ⇆ A- + HB+

The reaction arrow points both left and right because the reaction at equilibrium occurs in both the forward direction to form products and the reverse direction to convert products back into reactants. The acid loses a proton to become its conjugate base A- as the base B accepts a proton to become its conjugate acid HB+.

(3) Conjugation is the overlap of p-orbitals across a σ bond (sigma bond). In transition metals, d-orbitals may overlap. The orbitals have delocalized electrons when there are alternating single and multiple bonds in a molecule. Bonds alternate in a chain so long as each atom has an available p-orbital. Conjugation tends to lower the energy of the molecule and increase its stability. 

Conjugation is common in conducting polymers, carbon nanotubules, graphene, and graphite.

It's seen in many organic molecules. Among other applications, conjugated systems can form chromophores. Chromophores are molecules that can absorb certain wavelengths of light, leading them to be colored. Chromophores are found in dyes, the photoreceptors of the eye, and glow in the dark pigments.