Conjugation in chemistry refers to three or more adjacent resonant atoms that overlap with each other. There are five types of atoms that exhibit resonance:
- Double-bonded atoms
- Triple-bonded atoms
- Anions
- Radicals
- Cations
A conjugated system involves compounds that have covalent bonds or a group/chain of atoms that are not in a single-bond formation. The atoms in the group or chain have the capacity to modify the behaviour of the others.
In a conjugated system, molecular energy is decreased and stability is increased.
You’ll find some of the largest conjugated systems in state-of-the-art materials such as graphene and conductive polymers.
In this post:
What is a Conjugated System?
The term ‘conjugated’ as it applies to chemistry was first used by German chemist Johannes Thiele in 1899. Thiele investigated unsaturated systems-structures that have alternate single and double bonds in a chain of atoms.
The partial valences of the atoms cancel each other out, leaving only the ends of the molecule reactive. The model of atomic electron orbitals is based on quantum mechanics, which hadn’t been developed when Thiele published his paper in 1899. 
The modern definition of a conjugated system is based on the concept of overlapping orbitals. This type of system consists of connected p-orbitals with delocalised electrons. A p-orbital overlaps with another adjacent orbital across an adjacent sigma (σ) bond. Sigma bonds are considered the strongest of all covalent bonds.
The Meaning of Conjugation in Chemistry
Widely used in organic chemistry, conjugation is a description of pi bonds (double bonds) that are linked together. Typically, conjugation occurs when there are alternating single and double bonds. Each atom in the chain provides a p-orbital that’s perpendicularly oriented to the plane of the molecule.
Conjugation also occurs when there are available p-orbitals in all the contiguous atoms in a chain.
One example of this is furan (C4H4O). Structurally, this compound is a five-membered ring with two alternating double bonds. The double bonds are flanking an oxygen atom, which has two lone pairs of electrons. One of these lone pairs is occupying a p-orbital that’s perpendicularly oriented to the plane of the ring. This means it maintains the conjugation of the ring by overlapping with the adjacent carbon atoms, as shown in the diagram below. The other lone pair doesn’t participate in the conjugation. 
Organic Chemistry and Conjugation
Conjugation occurs under the following two conditions:
- There are p-orbitals that overlap or are connected
- There are delocalised electrons between them
These conditions are fulfilled in several ways in many unsaturated organic compounds. ‘Unsaturated’ means there are double or triple covalent bonds between atoms.
Conjugated systems are central to organic chemistry and its practical applications. Many synthetic materials are either derived from or are themselves conjugated systems. Synthetic polymers such as polyethene, polypropylene, and polystyrene, for instance, have a wide range of commercial and industrial uses.
Conjugated systems are inherently stable when it comes to thermodynamics and molecular structure. Their stability can be empirically verified by measuring the hydrogenation heat produced by two different dienes.
A diene – also known as alkadiene – is an organic compound with two double covalent bonds. Hydrogenation is a chemical process that adds a hydrogen molecule to a p-bond. A non-conjugated diene has two double bonds that are separated by more than one single bond, such as in the case of 2,5 heptadiene. 
Conjugated dienes have a lower energy yield during the hydrogenation process when compared with non-conjugated dienes.
Pentane, for example, can be derived from two diene compounds through the hydrogenation of either 1,3-pentadiene or 1,4-pentadiene. The first option features conjugated double bonds and releases an energy yield of around 225 kJ per mole of pentane. The second (non-conjugated) method produces a higher energy yield of about 250 kJ per mole.
Examples of Conjugate Pairs in Chemistry
Conjugate pairs in chemistry is a different concept to that of conjugations and conjugated systems.
In chemistry, conjugate pairs always refer to an acid-base pair. This could be a combination of either a weak acid and a strong base, or a strong acid and a weak base. Here are some examples of conjugate pairs:
Conjugate pairs are related to the Bronsted-Lowry theory of acids and bases, wherein an acid acts as a proton donor and the base acts as a proton acceptor. The pairs also differ in their hydrogen ion affinity.
As you can see from the balanced equation above, the conjugate pairs are in dynamic equilibrium. The products are written as ions, differing by one hydrogen ion.
Some other examples of conjugate pairs are outlined below.
Perchloric acid
Perchloric acid has the chemical formula HClO4 and its conjugate base is perchlorate ion (ClO4).
Sulphuric acid
Sulphuric acid has the chemical formula H2SO4 and its conjugate base is hydrogen sulphate ion (HSO4).
Nitric acid
Nitric acid has the chemical formula HNO3 and its conjugate base is nitrate ion (NO3).
Hydronium ion
Hydronium ions have the chemical formula H3O+. The conjugate base is water, otherwise known as H2O.
The examples above are in descending order of acid strength and ascending alkaline strength.
