Chemistry · Ch 8 — Organic Chemistry – Some Basic Principles and Techniques

Resonance Effect

8.7.7

Resonance Effect

The resonance effect arises when a molecule contains a conjugated system — a chain of alternating single and double bonds — and a nearby atom or group disturbs the electron distribution through that system. The textbook defines it as the polarity produced in a molecule by the interaction of two π-bonds, or between a π-bond and a lone pair of electrons on an adjacent atom. This effect is transmitted through the chain of atoms, not through space.

The resonance effect is also called the mesomeric effect and is denoted by the symbol R or M. It is fundamentally different from the inductive effect, which operates through sigma bonds and dies out after a few atoms. The resonance effect, by contrast, can propagate over the entire conjugated system.

Note

The resonance effect is a permanent effect — it exists in the ground state of the molecule, not just during a reaction. It determines the polarity and reactivity of the molecule even when no reagent is present.

There are two types of resonance effect, depending on the direction of electron displacement.

Positive Resonance Effect (+R Effect)

In the +R effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system. The substituent donates electron density into the conjugated system, making certain positions in the molecule richer in electrons.

The classic example is aniline ( $C_6H_5NH_2$ ). The lone pair on the nitrogen atom is in conjugation with the π-electrons of the benzene ring. The electron displacement can be shown through resonance structures:

\ce{C6H5-\overset{..}{N}H2} \longleftrightarrow \ce{^{-}C6H5=\overset{+}{N}H2} \longleftrightarrow \text{(further ortho/para contributing structures)}

The lone pair on nitrogen moves into the ring, creating negative charge density at the ortho and para positions. This is why aniline undergoes electrophilic substitution preferentially at the ortho and para positions.

Tip

To identify a +R group: look for an atom with a lone pair directly attached to the conjugated system. The lone pair can be delocalised into the π-system.

Substituents that show the +R effect include:

Halogens ( $-F, -Cl, -Br, -I$ )
$-OH$ (hydroxyl)
$-OR$ (alkoxy)
$-OCOR$ (acyloxy)
$-NH_2$ (amino)
$-NHR$ (secondary amino)
$-NR_2$ (tertiary amino)
$-NHCOR$ (amido)

Negative Resonance Effect (–R Effect)

In the –R effect, the transfer of electrons is towards the atom or substituent group attached to the conjugated system. The substituent withdraws electron density from the conjugated system, creating positive charge density at certain positions in the molecule.

The classic example is nitrobenzene ( $C_6H_5NO_2$ ). The nitro group has a π-bond between nitrogen and oxygen, and this group is in conjugation with the benzene ring. The electron displacement can be shown as:

\ce{C6H5-\overset{+}{N}O2^{-}} \longleftrightarrow \ce{^{+}C6H5=\overset{+}{N}(O^{-})2} \longleftrightarrow \text{(further ortho/para contributing structures)}

The nitro group pulls electron density away from the ring, creating positive charge density at the ortho and para positions. This deactivates the ring towards electrophilic substitution and directs incoming electrophiles to the meta position.

Watch out

A common mistake is to think that –R groups always make the molecule completely electron-deficient. They create partial positive charges at certain positions, but the molecule as a whole remains neutral.

Substituents that show the –R effect include:

$-COOH$ (carboxyl)
$-CHO$ (aldehyde)
$>C=O$ (carbonyl, as in ketones)
$-CN$ (cyano)
$-NO_2$ (nitro)

The Conjugated System

The presence of alternate single and double bonds in an open chain or cyclic system is termed a conjugated system. Examples include 1,3-butadiene ( $CH_2=CH-CH=CH_2$ ), aniline, and nitrobenzene. In such systems, the π-electrons are not localised between two atoms but are delocalised over the entire conjugated framework. …

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