Conjugation is a key concept in chemistry that explains why some molecules are more stable, reactive, or even colourful than others. It happens when alternating single and multiple bonds allow electron clouds to spread out across several atoms.
This electron sharing makes molecules more stable and influences their behaviour in chemical reactions, as well as their interactions with light.
You’ll see conjugation in everything from organic chemistry to industrial acids and biological molecules. It helps explain why benzene is so stable, why some dyes absorb certain colours, and why strong acids behave the way they do.
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Key Takeaways
Conjugation happens when alternating single and multiple bonds let p-orbitals overlap, spreading out electron density
It stabilises molecules by lowering their overall energy
Conjugated systems influence properties like reactivity, colour, and acidity
Conjugation and resonance are related, but conjugation requires actual orbital overlap
Many strong acids, including sulphuric and nitric acid, rely on conjugation to stabilise their conjugate bases
What is a Conjugated System?
A conjugated system is a structure where alternating single and multiple bonds allow electrons free movement across multiple atoms. The modern definition 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.
Conjugation can often be seen as a ring-only structure, but this isn’t the case. It can also happen in straight-chain molecules like polyenes, which have alternating double and single 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.
What’s The Difference Between Conjugation And Resonance?
Conjugation and resonance both involve electron delocalisation, but they’re not the same thing. Conjugation is a physical phenomenon that requires p-orbital overlap, while resonance is a way to describe it using multiple structures.
In a conjugated system, the orbitals physically overlap, allowing electrons to move freely across the structure. This happens in molecules like benzene and polyenes, where alternating bonds create a continuous electron cloud.
Resonance is a method of representing this delocalisation on paper. A molecule doesn’t actually switch between different resonance forms, but instead exists as a hybrid of all of them.
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 (HClO₄) is one of the strongest acids out there. When it donates a proton, it forms the perchlorate ion (ClO₄⁻).
Following resonance stabilisation, the negative charge spreads across multiple oxygen atoms, making the base very weak and the acid extremely strong.
Sulphuric acid
Sulphuric acid (H₂SO₄) is another strong acid that benefits from conjugation.
When it loses a proton, it forms the bisulphate ion (HSO₄⁻), and a second proton loss creates the sulphate ion (SO₄²⁻). In both cases, the negative charge is spread across multiple oxygen atoms, making the base more stable.
Sulphuric acid is widely used in industry, from fertiliser production to petroleum refining. It’s also key in battery acid and chemical manufacturing.
Nitric acid
Nitric acid (HNO₃) forms the nitrate ion (NO₃⁻) when it donates a proton. This ion is highly stabilised because the negative charge is delocalised across three oxygen atoms, making the conjugate base very weak.
Nitric acid is a major ingredient in fertilisers, explosives, and dyes. It’s also used in metal processing and cleaning applications.
Hydronium ion
Hydronium ion (chemical formula H₃O⁺) is the conjugate acid of water. When an acid donates a proton in water, it forms hydronium.
This is an important chemical in biological and industrial reactions, affecting enzyme function, chemical equilibrium, and how acids behave in solutions.
Conclusion
Conjugation is an essential part of chemistry, influencing stability, reactivity, and even colour. It helps explain why some molecules are more acidic, why certain compounds are brightly coloured, and why aromatic rings behave the way they do.
