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Resonance Effect

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Last updated date: 13th Jul 2024
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The resonance effect in Organic Chemistry is the electron behaviour differs when the elements other than the hydrogen and carbon atoms take part in the formation of molecular bonds actively.


The electronic factors influencing the organic reactions include the inductive effect, the electromeric effect, resonance effects, hyperconjugation, and more. Diversely, all these factors relate to the organic molecules. Many biological molecules consist of a combination of these six elements: nitrogen, carbon, hydrogen, sulphur, oxygen, and phosphorus. However, they do not prevent the organic compounds from taking on the various properties of their physical characteristics and chemical reactivity.


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The organic molecules also exhibit resonance or mesomerism properties. The factor called Mesomerism or resonance in Organic Chemistry explains the delocalized electrons within certain molecules where a single Lewis structure does not express the bonds. A molecule or ion with these delocalized electrons can be described by contributing various structures known as resonance structures.


Resonance Effect Or Mesomeric Effect In Chemistry

The withdrawal or releasing effect of electrons attributed to a specific substituent through the delocalization of π or pi-electrons, which can be seen by drawing different canonical structures, is known as a resonance or mesomeric effect. 


The symbols M or R are used to represent the resonance effect.


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The above representation shows various resonance structures of different compounds with the respective resonance effects.


The definition of the resonance effect explains the polarity caused by the interaction between the electron lone pair and the pi bond in a molecule. It also occurs by the interaction of 2 pi bonds present in the adjacent atoms. 


In simple words, resonance is the molecule with multiple Lewis structures. Resonance in Chemistry helps to understand the stability of a compound along with the energy states.


Definition Of Resonance Effect In Chemistry

The resonance effect definition can be given as a chemical phenomenon, observed in the characteristic compounds containing double bonds in the organic compounds. The organic compounds have these double bonds in the structures and have the overlapping of the p-orbitals, usually on the two adjacent sides of carbon atoms.


Types Of Resonance Effects

Two types of Resonance effects exist, namely:

  • Positive resonance effect

  • Negative resonance effect


Positive Resonance Effect

The positive resonance effect happens when the groups release electrons to the other molecules by the delocalization process. Usually, the groups are denoted by +R or +M - the molecular electron density increases in this process. The positive resonance effect examples are -OH, -OR,-SH, and -SR.


Negative Resonance Effect

The Negative resonance effect happens when the groups withdraw the electrons from the other molecules by the delocalization process. Usually, the groups are denoted either by -R or -M. The molecular electron density is said to decrease in this process. The negative resonance effect examples are, C=O, -COOH, -C≡N, and -NO2.


Resonance Structure

Several organic compounds cannot be represented by one structure accurately. For suppose, benzene is ordinarily represented as follows.


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This structure contains three C-C bonds and three C=C bonds.


Single bond length of Carbon-carbon = 1.54A


Double bond length of Carbon-carbon = 1.34 A


However, experimentally, it has been determined that all carbon-carbon bonds present in benzene are identical and have the same bond length (1.39A).


Therefore, the structure of benzene cannot be represented by a single structure. It can be represented equally well by the energetically similar structures of I and II. These two structures are known as resonance structures.


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The actual structure of benzene is a resonance hybrid of both structures I and II. Another example of resonance is given by nitromethane (CH3N02), which can be represented by the two Lewis structures, as given below.


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The actual structure of nitromethane is the resonance hybrid of two canonical forms (I and II).


Resonance Energy

The difference in the energy between the most stable contributing structure for a compound and its resonance hybrid is resonance energy or resonance stabilisation energy.


Resonance In Electricity

Resonance in the electrical circuits takes place in the cases of alternating current. The current in the circuit at resonance attains its maximum value. For example, in the case of a series R-L-C circuit, resonance occurs when the reactance of the inductor and capacitor is equal. This results in the minimum net impedance, which results in the maximum current flow happening in the circuit.


Resonance Theory

Resonance in Chemistry is an intramolecular electronic process that involves the change in position of Pi bond (s) or nonbonding electron (also called sigma bond). However, the position of an atom occurs in this process by changing the Pi electrons' position or the non-bonding electrons.


Condition for Resonance include:

  • Every participating atom must be coplanar

  • Every participating atom must have the parallel p- atomic orbital (d atomic orbital, sometimes)


Resonance occurs in the Following Cases

  • A Pi bond conjugated with the other pi bond

  • A Pi bond conjugated with a negative charge

  • A Pi bond conjugated with a positive charge

  • A positive charge conjugated with a lone pair or a negative charge

  • A Pi bond conjugated with lone pair or free radical


Inductive Effect

The inductive effect is the result of an electrical charge being transmitted via a chain of atoms. This charge transmission will eventually lead to atoms having a fixed electrical charge. Differences in the electronegative values of atoms in a molecule cause the inductive effect.


Higher electronegativity atoms are more likely to attract electrons to themselves than lower electronegative atoms. In a covalent bond between a highly electronegative atom and a low electronegative atom, the bond electrons are drawn to the highly electronegative atom. The low electronegative atom gains a partially positive charge as a result of this. A partial negative charge will be applied to the highly electronegative atom. Bond polarisation is the term for this.

The inductive effect can be found in two ways:


Inductive Effect of Electron Withdrawal

When a molecule is attached to a strongly electronegative atom or group, this occurs. The electrons from the remainder of the molecule will be drawn to this atom or group.


Inductive Effect of Electron Releasing

When alkyl groups are attached to a molecule, this effect is seen. These groups tend to supply electrons to the remainder of the molecule because they are less electron-withdrawing.


Differences between Resonance Effect and Inductive Effect

Definition: 

Inductive Effect: The inductive effect is the result of the passage of an electrical charge across a chain of atoms.


Resonance Effect: The effect of the interaction between pi bond electrons on the stability of a molecule is known as the resonance effect.


The Cause of the Effect:

Inductive Effect:  The inductive effect develops as a result of the polarisation of bonds.

Resonance Effect: When single and double bonds are present together, a resonance effect arises.


Factors that Influence These Effects:

Inductive Effect:  The degree of inductive impact is affected by the electronegativity values of atoms.


Resonance Effect: The resonance effect is influenced by the number of double bonds and their order.


The induced electrical charges in atoms of a molecule generate the inductive effect. The difference in the electronegativity values of atoms causes charge induction. Atoms with a high electronegativity have a tendency to attract bond electrons. The resonance effect, on the other hand, is distinct from the inductive effect. The resonance effect of a molecule occurs when the molecule has double bonds. The inductive effect describes the transmission of electrical charges between atoms in a molecule, whereas the resonance effect describes the transmission of electron pairs between atoms in a molecule.


FAQs on Resonance Effect

1. What is the Current Resonance?

A phenomenon where the current starts oscillating between the two impeding components is referred to as current resonance. Consider an inductor(L), capacitor(C) circuit, as given below.

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Each component provides resistance (impedance) in its own way. Let us assume that there is an applied AC voltage (where we use L and C). The capacitor offers a high impedance for the lower frequencies and the inductor offers the least impedance. Hence, all the current flows through the inductor now, as the frequency increases, the impedance switches. It means the capacitor starts providing the least resistance, and the inductor gives high resistance. Hence, all the current flows through the capacitor now.


There exists an absolute frequency value, where the impedance (resistance) offered by the inductor and capacitor is the same. This condition is known as resonance. The frequency value at which this happens is known as the resonant frequency. In this case, the current starts oscillating between the inductor and capacitor. The current at the resonant frequency is known as the resonant current.

2. What is the Resonant Circuit?

The resonant circuit is just an LC circuit. It is the electric circuit. It is a combination of an inductor and a capacitor. The resonant circuit is used mainly in frequency mixers, oscillators, tuners, and more.

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Basic Operation

As per the above representation given, the LC circuit oscillating at its natural resonant frequency can store electrical energy. The capacitor can store the energy in an electric field between its plates, depending on the voltage across it. Inductor store energy in its magnetic field is based on the current passing through the inductor coils.

3. What is the resonance effect?

The resonance effect, also known as the mesomeric effect, is a concept in chemistry that explains the delocalization of electrons in certain molecules. Delocalization means that electrons are not confined to a single bond or lone pair, but rather spread out over multiple atoms. This delocalization gives the molecule more stability than any single Lewis structure could represent.

4. What causes the resonance effect?

The resonance effect occurs in molecules that have:


  • Double bonds: The pi electrons in double bonds can delocalize into a π-orbital cloud that extends over both atoms involved in the bond.

  • Lone pairs of electrons: These electrons can interact with a nearby π-bond, delocalizing the electron density and affecting the distribution of charges in the molecule.

5. What are the effects of the resonance effect?

  • Increased stability: Resonance structures contribute to the overall stability of a molecule. The more resonance structures a molecule has, the more stable it is.

  • Polarity changes: The delocalization of electrons can create partial charges on certain atoms, even if the individual Lewis structures show no formal charges.

  • Bond length changes: Resonance can affect the lengths of bonds in a molecule. For example, in the benzene molecule, all C-C bonds have the same length due to the delocalization of electrons throughout the ring.

6. How are resonance structures used?

Resonance structures are not actual structures of the molecule, but rather represent different ways to depict the electron distribution. They are used to:


  • Explain the observed properties of a molecule, such as its stability, polarity, and bond lengths.

  • Predict the reactivity of a molecule in chemical reactions.

  • Draw conclusions about the relative acidity or basicity of different molecules.

7. What are some examples of the resonance effect?

  1. The nitrate ion (NO3-): The negative charge is not localized on a single oxygen atom, but is delocalized over all three oxygen atoms due to resonance.

  2. The benzene molecule: The π-electrons in the ring are delocalized over all six carbon atoms, making the molecule unusually stable.

  3. The amide group (NH2-CO-): The lone pair of electrons on the nitrogen atom can delocalize into the π-orbital system of the carbonyl group, affecting the reactivity of the molecule.