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In Organic chemistry, the behavior of electrons differs when the elements other than that of the carbon atoms and hydrogen actively take part in the formation of molecular bonds.

The electronic factors influencing the organic reactions includes the electromeric effect, the inductive effect, resonance effects, hyperconjugation, etc. All these factors relate to the organic molecules, in a diverse manner.  Most of the biological molecules consist of a combination of these six elements: carbon, nitrogen, hydrogen, oxygen, sulfur, phosphorus. Yet, they do not prevent the organic compounds from taking on the diverse properties of their chemical reactivity and physical characteristics.

The organic molecules also exhibit the property of resonance or mesomerism. In Organic chemistry, the factor called resonance or Mesomerism describes the delocalized electrons within the certain molecules where one single Lewis structure does not express the bonds. An ion or molecule with these delocalized electrons can be represented by contributing several structures which are known as resonance structures.

Resonance Effect or Mesomeric effect In Chemistry

The resonance effect describes the polarity produced in a molecule by the interaction between a lone electron pair and a pi bond or the interaction of two pi bonds in adjacent atoms. In other words, resonance means that a molecule must be drawn with multiple Lewis structures, but actually exists in a hybrid state between the various configurations. It is usually found in molecules with conjugated double bonds or in molecules having at least one lone pair and one double bond. Understanding resonance is important in understanding the stability of the compound and its energy state.

The withdrawal effect or releasing effect of electrons attributed to a particular substituent through the delocalization of π or pi-electrons that can be seen by drawing various canonical structures is known as resonance effect or mesomeric effect. M or R symbols are used to represent resonance effect.

Resonance Effect or Mesomeric effect In Chemistry
The Above figure shows the different resonance structures of different compounds with resonance effects.

Definition- The resonance effect is a chemical phenomenon observed in compounds characteristic of double bonds of organic compounds. Organic compounds that contain double bonds in their structure are usually made of the overlap of p-orbitals on two adjacent carbon atoms (referred to as pi bonds).

  • A single bond is often called a sigma bond and is present in compounds which have only one bond between the adjacent carbon atoms. Sigma bonds are usually lower in energy than pi bonds and also have higher symmetry than pi bonds.

Delocalization effect- The delocalization effect has been experimentally determined by measuring the heat of formation of the double bond-containing compound alone and comparing it with the heat of formation of the sum of all the double bonds in the compound individually. The result of these measurements shows that the heat of formation of the whole molecule is lower than that of the sum of the heat of formation of its constituent double bonds measured singly.

  • This indicates that the molecules exist in a hybrid resonance state with a lower energy than that of a single individual resonance structure. In other words, they are more stable.
  • Aromatic compounds are especially stable due to this bond delocalization and resonance effect.

Principles of resonance- Not all resonant structures are equally significant to the compound. There are a few principles involved that can help you determine how significant a resonance structure is.

  • The rules of least charges: The resonant form with the lowest overall charge is the most significant.
  • The octet principle: The resonance forms with a full octet are more significant than those lacking a full outer shell.
  • Stabilization of positive charges: Forms, where positive charges are acting on the least electronegative atom are the most significant.
  • Stabilization of negative charges: Forms, where negative charges are acting on the most electronegative atom are the most significant.
  • Covalent bonds: The most significant resonance structure has the most covalent bonds.

Types of Resonance Effects

There are two types of Resonance effects namely positive resonance effect and negative resonance effect.

  1. Positive Resonance Effect- Positive resonance effect occurs when the groups release electrons to the other molecules by the process of delocalization. The groups are usually denoted by +R or +M. In this process, the molecular electron density increases. For example- -OH, -SH, -OR,-SR.
  1. Negative Resonance Effect- Negative resonance effect occurs when the groups withdraw the electrons from other molecules by the process of delocalization. The groups are usually denoted by -R or -M. In this process, the molecular electron density is said to decrease. For example- -NO2, C=O, -COOH, -C≡N.

Drawing Resonance Structures:

1) Draw a Lewis structure of the molecule- A Lewis structure is a simplified representation of a molecule. It shows how atoms are bound together and their valence electron states.

  • Start by writing the chemical symbol of each element.
  • Single bonds are represented with a line connecting the two atoms bound.
  • Double bonds are represented by two lines and triple bonds by three.
  • Valence electrons (electrons in the outer shell of the atom) are represented by dots next to the atom.
  • Remember to indicate the overall charge of the molecule with a “+” or “-“ at the top right of the structure.
  • For example, O3 has three oxygens all bound together. The oxygen in the middle is bound to the other two oxygens by one single bond and one double bond.

2) Identify the bonds that can alternate to form resonant structures- Molecules that have resonant structures actually exist in a hybrid state between the different structures formed by the variations in bonds. While you may draw the various Lewis structures as separate molecules, that is just a way to pictorially represent them. Electrons that form double bonds can switch between atoms, slightly changing the way the structure is drawn.

  • Bonds of this nature are said to be “delocalized”, as they are distributed evenly across all the atoms in the compound.
  • For example, O3 has two resonant structures. The double bond can be between the first and second oxygen or between the second and third oxygen.

3) Diagram all possible resonant Lewis structures – Once you have identified the bonds that can alternate in the compound, you can draw the various Lewis structures for each version. It is also possible to draw a representative hybrid structure by using a dashed line where the bond could be either a single bond or a double bond.

  • For example, You can draw two O3 structures with the two possible bond configurations or one O3 structure with dashed lines representing the bonds.
  • Draw a double-sided arrow between each structure to indicate that they are resonance structures.

Image result for ozone molecule

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