What are Conjugated Compounds in Organic Chemistry?

The delocalization of electrons in a molecule is called conjugation in organic chemistry. This delocalisation process of electrons leads to the shortenings or elongations of chemical bonds, but at the same time it causes changes in the chemical properties in conjugated molecules as compared to the non-conjugated ones. For example, conjugated molecules absorb light at longer wavelengths. 

Conjugated compounds are those compounds at which pi- bonds are present in alternating positions of compounds; it can also be defined as compounds having pi-bonds and positive charge or negative charge at alternative positions. Alternative position is always called a conjugation position.

Chemical Bonding in a Conjugated System

• Formation of the conjugated system is only possible when there are alternating single and double bonds in which each atom has the ability to supply a p-orbital perpendicular to the plane of the molecule. 
• Along with that, conjugated system can also be formed if an atom has p-orbital in a continuous atom chain. Example of such conditioning is Furan (it is a five membered ring having alternative double bond
• One of the most common models which is used for treatment of conjugated molecules is composite valence bond or Huckel Molecular Orbital Theory (VB/ HMOT) treatment. 
• Under the treatment procedure, the σ framework of the molecule is separated from the π system of the molecule. Along with that, we can also treat pi- bonding by using the approach of delocalization of electrons. 

Formation of a Conjugated Compound

Compounds which are known to be a conjugated compound should have any one of the below conditions:

• Compound in which pi-bond is at alternating position. 

Example: CH₂=CH-CH=CH₂

• Compound in which pi bond and positive charge is at alternative position. 

Example: CH₂=CH-CH₃⁺

• Compound in which pi bond and negative charge is at alternative position. 

Example: CH₂=CH-CH^-

• Compounds in which pi bond and lone pair is present at alternative position. 

Example: CH₃=CH-NH₂

Conditions Under which Conjugation Occurs

There are certain mandatory conditions which are needed for conjugation to take place or for the formation of conjugated compounds:

• In a conjugated system there is a requirement of p-orbitals. As p-orbital is involved in the bonding within the π system.
• In case if the position in a chain does not have continuous p-orbital or if geometry stops the correct alignment then there is interruption in the conjugation process which causes loss of conjugation at that point.
• If more than one pi-bond is present in a conjugated system then we consider only one pi-bond in the conjugated system. 
• If more than one lone pair is present at conjugation position then only one lone pair is considered. 
• If pi bond and positive charge is present in position then only pi-bond will be considered. 
• If pi-bond and odd electron are present in an atom then pi -bond will be considered. 

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for exams like NEET, JEE, PAT and other competitive exams and also for Bachelors and Masters in Chemistry.

Related concepts

• Organic chemistry.
• Bonding. 
• Effect of electron density.
• Resonance.
• Resonating structure.
• Resonance effect.
• Hyperconjugation.
• Molecular orbital.
• MO theory.

Practice Problems

1. What is the difference between conjugation and resonance?

Ans. Conjugation of a system occurs when three or more p-orbital join together for the formation of a larger pi-system while resonance is different arrangements of electrons within that pi system.

2. What is a conjugated double bond?

Ans. The term conjugated double bond means that there is an alternative single and a double bond. This alternative nature of bond allows the delocalization of electrons over the whole system and thus, the electrons can be shared by many atoms present in the whole system.

 3. What is the Huckel rule?

Ans. The Huckel rule was proposed by Erich Huckel in 1931. In this rule he determined if a planar ring molecule would have aromatic properties. In this rule it is stated that if a cyclic, planar molecule has (4n+2)π electrons in the system then it is an aromatic system.

5. What is the conjugative effect?

Ans. The effect in which molecular orbitals (MOs) are conjugated to form new molecular orbitals that are more delocalized and therefore have lower energy is termed as conjugative effect. These delocalized electrons have a tendency to move freely in these new extended orbitals.

6. Why do conjugated systems absorb light?

Ans. The molecules having conjugated systems of electrons have both the ground and the excited state electrons which are closer in energy in comparison to the non-conjugated compounds, in which both the states are farther from each other. Because of this, any radiation having lower energy cab possibly excite the electrons from the lower state to the higher state as it is achievable easily. This proves that conjugated compounds absorb light. 

7. Why do conjugated systems show color?

Ans. Conjugated systems show color because of absorption of light in the visible region. As they are those compounds in which the gap between the ground as well as the excited state becomes less due to conjugation (due to stability), light in the visible region has enough energy to excite these electrons from the ground state to the excited state and when these electrons return to their ground states, they emit light which falls in the visible range of the spectrum, thus can be seen as the compounds are showing colors.