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When atoms other than carbon and hydrogen readily participate in the creation of molecular bonds, it alters their electron behaviour in organic chemistry. Organic molecules have different relationships with these molecules. The majority of biological compounds are made by these six elements: carbon, nitrogen, hydrogen, oxygen, sulphur and phosphorus. But these elements don’t stop organic compounds from gaining a wide range of chemical reactivity and physical attributes.
In this article, we will study the meaning and types of resonance effects as well as their importance. These notes will help you in your preparation for the IIT JEE examinations.
Resonance Effect or Mesomeric Effect
A resonance effect or mesomeric effect can be defined as the withdrawal or releasing of electrons related to a certain substituent through the process of the delocalisation of pi-electrons, which can be demonstrated by sketching various canonical structures. Resonance, also known as mesomerism, is shown by organic molecules. The resonance effect is a chemical phenomenon that is observed in organic molecules that contain double bonds. Organic compounds feature these double bonds in their structures, and the p-orbitals on the two opposite sides of carbon atoms are frequently overlapping.
In chemistry, resonance aids in the study of a compound’s energy states. Resonance in organic chemistry can also be termed as the delocalisation of molecules that have more than a single Lewis structure. The delocalised electrons in an ion or molecule can be represented by providing numerous structures known as resonance structures.
The polarity obtained in a molecule by the reaction of a single pair of electrons and a pi bond is defined as the resonance effect. This can also take place when two pi bonds in nearby atoms combine. As a result, resonance can also use molecules that have numerous Lewis structures.
Types of Resonance Effects
There are two types of resonance effects:
Positive resonance effect
Negative resonance effect
Positive Resonance Effect
The positive resonance effect occurs when electrons or pi electrons are transferred from a specific group to a conjugate system, which boosts the system’s electron density. This occurs when the groups are delocalised and release electrons to the other molecules. It is essential that the group must have either a single pair of electrons or a negative charge to produce the positive resonance effect. It is represented by +R or +M in organic chemistry.
The +M effect causes the conjugate system to have a negative charge or the electron density to increase on the conjugate system. These conjugate complexes have a higher electrophile reactivity and a lower nucleophile reactivity. Thus, the molecule’s electron density increases from this action.
The positive mesomeric effect can be seen in the following group in this particular order:
–O− > –NH2 > –OR > –NHCOR > –OCOR > –Ph > –CH3 > –I > –Br > –Cl > –F
Example: In aniline, the -NH2 group likewise has a +R impact. Through delocalisation, it releases electrons towards the benzene ring. The electron density on the benzene ring increases as a result, especially at the ortho and para locations. Hence, aniline activates the ring, allowing it to undergo electrophilic substitution.
Negative Resonance Effect
The negative resonance effect occurs when pi-bond electrons are moved from the conjugate system to a specific group, resulting in a drop in the conjugate system’s electron density. When groups delocalise, they withdraw electrons from other molecules, resulting in a negative resonance effect. It is important that the group must have either a positive charge or a vacant orbital for the negative resonance effect to occur. The groups are commonly represented by -R or -M.
The –R effect makes a molecule more reactive to a nucleophile by lowering the electron density in the conjugate system but also makes it less reactive to an electrophile for the same reasons. Thus, the molecule’s electron density drops during this process.
The negative mesomeric effect can be seen in the following group in this particular order:
–NO2 > –CN > –SO3H > –CHO > –COR > –COOCOR > –COOR > –COOH > –CONH2 > –COO−
Example 1: The carboxyl group’s negative resonance effect (-R or -M) is depicted below. It delocalised electrons, thus, removing them and lowering electron density, especially on the third carbon.
Example 2: Due to delocalisation of conjugated electrons, the nitro group, -NO2, in nitrobenzene, exhibits the -R effect, as seen below. The electron density on the benzene ring is reduced, especially in the ortho and para locations.
Importance of Resonance Effects
As we have studied in the types of resonance effects notes, the importance of resonance effects lies in the fact that it is a chemical phenomenon that is observed in organic molecules that contain double bonds. It plays an important role in the understanding of organic chemistry.
The resonance effect specifies the charge distribution in the compound and aids in determining where electrophiles and nucleophiles attack. It is used to explain the electron-withdrawing or releasing properties of substituents based on relevant resonance structures. Furthermore, it aids in the description of physical properties such as dipole moment and bond length.
Conclusion
Thus, in the types of resonance effects notes, we have learnt the meaning of resonance effects and their importance. The withdrawal or release of electrons associated with a particular substituent via the delocalisation of pi-electrons is known as a resonance effect or mesomeric effect, which can be proven by drawing several canonical configurations. We also detailed the two types of such effects: the positive and negative resonance effects. R+ takes place when electrons or pi electrons are transferred from a specific group to a conjugate system, thus, boosting the conjugated system’s electron density. The –R effect occurs when pi-bond electrons are moved from the conjugate system to a specific group, resulting in a drop in the conjugate system’s electron density.
