Conjugation:

The word "conjugation" is derived from a Latin word that means "to link together".  In organic chemistry terms, it is used to describe the situation that occurs when π systems (e.g. double bonds) are "linked together".

What is a conjugate system in organic chemistry? 

Conjugation in organic chemistry deals with organic compounds that contain extended pi systems. In conjugation chemistry, these systems have a p orbital on an atom adjacent to a multiple bond. This p orbital may be part of another multiple bond, radical, empty p orbital, or a lone pair of electrons. The extended pi systems can be acyclic or cyclic compounds, but the extended pi system must be flat to allow for p orbital overlap.

What are conjugated double bonds in organic chemistry?

Conjugated carbon–carbon double bonds are two carbon–carbon double bonds that are separated by one carbon–carbon single bond and are called dienes. These conjugated double bonds are not limited to just two double bonds but can contain multiple carbon–carbon doubles. Carbon is not the only atom that can make up a double bond. Conjugated double bonds can consist of only carbon atoms or contain other atoms like oxygen and nitrogen.

Conjugated systems are not limited to just double bonds. Triple bonds can be a part of conjugated systems. If a carbon–carbon triple bond is separated by one single bond from a carbon–carbon double bond, then this organic compound is a conjugated system.

Example of a conjugated system with an alkene, alkyne, and carbonyl bond

Mechanism of the Conjugated System

A student may ask themselves what a conjugated pi system is and how to spot one in an organic compound. A conjugated pi system is when an organic compound has a p orbital on an atom adjacent to a multiple bond (double or triple bond). The electrons in the p orbitals that make up the pi bond are called delocalized pi electrons if the pi bond is delocalized in a conjugated compound. Multiple bonds which are double and triple bonds contain pi bonds. Double bonds contain one pi bond while triple bonds contain two pi bonds. Overlapping p orbitals on adjacent atoms can form an extended pi bond system.

Allylic carbocation, carbonion and carbon radical are all conjugation systems as shown by their resonance structure.

 

Examples of the Conjugated System:

There are many examples of conjugated systems in organic chemistry. The high level of stability in the conjugated system allows for the usefulness of the compound. These conjugated compounds see use in unique reactions and even use as dyes.

1,3-butadiene:

1,3-butadiene

1,3-butadiene is a conjugated diene, where the two carbon–carbon double bonds are separated by one single bond. This diene can undergo a special reaction called electrophilic addition. In this reaction reagents like hydrogen bromide (HBr) or bromine (Br₂) are used to form the 1,2-adduct and 1,4-adduct products. A famous pericyclic reaction that 1,3-butadiene can undergo is the Diels-Alder reaction. In this reaction, 1,3-butadiene will react with a dienophile (alkene or alkyne) in a [4+2] cycloaddition. The product, when the dienophile is an alkene, is a cycloalkene. If an alkyne is used as the dienophile, then an isolated diene contained in a ring is formed.

 

Benzene:

Benzene

 

·        An "isolated" π (pi) system exists only between a single pair of adjacent atoms (e.g. C=C)

·        An "extended" π (pi) system exists over a longer series of atoms (e.g. C=C-C=C or C=C-C=O etc.).

·        An extended π (pi) system results in a extension of the chemical reactitvity.

The fundamental requirement for the existence of a conjugated system is revealed if one considers the p orbitals involved in the bonding within the π system.

·        A conjugated system requires that there is a continuous array of "p" orbitals that can align to produce a π bonding overlap along the whole system.

·        If a position in the chain does not provide a "p" orbital or if geometry prevents the correct alignment, then the conjugation is interupted (broken) and therefore lost at that point.

You can investigate these differences by studying the following examples, paying particular attention to the "p" orbitals in the π system. Use the JSMOL models to look at the hybridisations of the atoms in the systems.

The result of conjugation is that there are extra π bonding interactions between the adjacent π systems. This extra bonding results in an overall stabilisation of the system. This increased stability due to conjugation is refered to as the delocalisation energy or the resonance energy or conjugation energy.

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