Resonance Effect or Mesomeric Effect with Types & Conditions

The resonance effect is a property of functional group that refers to the polarity generated within a molecule through interactivity between either a lone pair of electrons and a pi-bond present next to each other or between two pi bonds.

• The resonance effect is also known as Mesomeric Effect.
• The effect is observable when molecules are conjugated with double bonds or at least one lone pair of electrons and one double bond.
• It is stronger in compounds with a lower ionization potential. 

The resonance effect in organic chemistry is the variation in electron behaviour when elements other than hydroges and carbon take part actively in the formation of molecular bonds. 

• It also explains the delocalized electrons within certain molecules.
• Carbonate ion is the most common example of resonance ion.

Key Terms: Resonance effect, Resonance, Mesomeric Effect, Lewis Structure, Atoms, Pi Bonds, Electrons, Positive Resonance Effect, Negative Resonance Effect, Ionization Potential, Conditions of Resonance Effect, Orbitals

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Resonance Effect or Mesomeric Effect in Chemistry

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Resonance effect is defined as a type of effect when polarity is induced when a lone pair of electrons interact with a pi bond or two pi bonds interacting with nearby atoms.

• On the mesomeric effect is induced when two -bonds or a -bond and a lone pair of electrons interact.
• The resonance effect is represented by M or R symbols.

The only difference between the resonance effect and the mesomeric effect is that the first describes how a lone pair of electrons and a bond pair of electrons are used to determine its chemical structure, whereas the latter describes how the chemical structure of a molecule is stabilized using a functional group.

• An organic compound with double bonds is characterized by the resonance effect, which exists as a chemical phenomenon. 
• They often have double bonds present in the structure.
• The p-orbitals of carbon atoms are typically overlapping on their two adjacent sides.

The resonance effect is most commonly in molecules with double bonds, triple bonds, or areas of high electron density. It refers to how an electron lone pair interacts with a pi bond to form polarity in a molecule. 

• Also, it occurs through the interaction of two pi bonds in adjacent atoms. 
• A complex Lewis structure is referred to as resonance. 
• Understanding resonance in chemistry helps in figuring out the stability and energy states of a compound.

Various compounds with resonance effects are shown in the figure above.

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Types of Resonance Effects

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 There are two types of Resonance effects namely positive resonance effect and negative resonance effect whose detail description are as follows:

Positive Resonance Effect

The positive resonance effect is seen when electrons in a molecule are transferred away from an atom which is bound to the conjugated system. During the process of delocalization, the groups release electrons to the other molecules, thus creating a positive resonance effect. +R and +M are usually used to denote the groups. 

• Molecular electron density increases as a result. 
• Their positions with high electron densities are caused by electron displacement. 
• This effect in aniline is shown as : 

Negative Resonance Effect

Negative Resonance Effect is created when the groups absorb electrons from other molecules. These groups are typically indicated by -R or -M. According to this process, the molecular electron density decreases.

• In this effect electrons are transferred towards the atom or group associated with the conjugated system.
• When an open-chain or cyclic system contains alternate single and double bonds, it is referred to as a conjugated system. 
• This type of system frequently displays abnormal behavior.
• For example in nitrobenzene this electron displacement can be depicted as :

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Conditions for Resonance Effect

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The conditions for the resonance effect are as follows:

• First and foremost, more than one Lewis structure can be represented by the molecule.
• Resonant structures differ only in electron positions and not in atom or nucleus positions.
• All structures should have an equal number of valence and unpaired electrons.
• It is also based on the suitable alignment of atoms.

Circumstances under which Resonance occurs

The conditions under which resonance takes place are as follows:

• It include the conjugation of two pi bonds
• Secondly, the process is initiated by negatively charged pi bonds and positively charged pi bonds.
• An ionic bond and a lone pair or a free radical are conjugated to produce a positive charge
• It depends on pi bond with a free radical or lone pair.

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Stability of Resonance Structure

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All resonance structures have more in common with the actual molecule than the more unstable ones. Following are some points that can help to explain the stability of resonance structures:

• Due to the identical resonance structures, each resonance hybrid contributes equally.
• The most significant contribution is from the structure with the most covalent bonds. 
• The reason is bonds are formed by releasing energy, which means a molecule will be more stable.
• Due to their higher energy, structures with positive and negative charge separation tend to be less stable.

Structures composed of electronegative atoms and positively charged atoms contribute more towards resonance hybridism when atoms of different electronegativity are involved. Accordingly, the resonance structures above follow the following order in terms of stability:

• Positive charge distribution are delocalized in structures, regardless of whether they are electronegative or electropositive.
• It plays a significant role in their accumulation and functionality.
• These two structures contribute to the dispersion of positive charges.
• In general, the first structure appears to be more stable.

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Application of Resonance Effect

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Some important applications of resonance effect are as follows:

Resonance Energy

Energy derived from the difference in energy between a compound's most stable component and its resonance hybrid is called resonance energy or resonance stabilization energy.

Resonance In Electricity

When alternating current is applied to electrical circuits, resonance occurs. When resonance occurs, a maximum current is generated in the circuit. When the reactance of the inductor and capacitor are equal, it occurs in a series RLC circuit.

• As a result, the circuit's current flow is maximum due to the minimum net impedance.

Resonance Theory

Chemistry resonance is a process that occurs in the presence of a nonbonding electron or a changing position of a Pi bond (S). It is the Pi electron positions or nonbonding electron positions that change the position of an atom as a result of this process.

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Things To Remember

• The resonance Effect occurs when a lone electron pair interacts with a pi bond or two pi bonds close to a molecule.
• It results in an increase in polarity in that molecule.
• The mesomeric Effect explains how a molecule can stabilize its chemical structure using a functional group.
• There are two types of resonance effect, namely positive and negative resonance effect
• The resonance structure of Lewis dot molecules, such as ozone, is called Lewis’s dot structure. 
• Atoms are positioned in the same positions in various compounds, but electron positions vary.
• Lewis structures cannot fully describe the bonding between molecules; thus, resonance hybrids are used.

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