Mesomeric Effect

The mesomeric effect, commonly referred to as the resonance effect, describes the electron-releasing or electron-withdrawing influence exerted by substituents or functional groups through delocalisation of π electrons or lone pairs within a conjugated system. It accounts for a form of chemical polarity that arises from interactions between adjacent π bonds or between a π bond and a lone pair of electrons. Through these interactions, a molecule can be represented by several resonance structures, while the true structure corresponds to a resonance hybrid with electrons distributed across the conjugated framework. The effect plays a central role in understanding reactivity, stability, and electronic distribution in organic molecules and is particularly significant in aromatic systems, conjugated chains, and carbonyl compounds.

Background and Fundamental Principles

The mesomeric effect originates from the capacity of substituents to participate in electron delocalisation by virtue of lone pairs, π bonds, vacant orbitals, or formal charges. When such features interact with a conjugated system, the electrons may shift either towards or away from the substituent. This shift is a resonance phenomenon and not a physical movement of electrons from one atom to another; instead, it is a conceptual device used to describe electron distribution across several resonance structures that collectively represent the molecule.
Electron delocalisation lowers the overall energy of the system, contributing to resonance stabilisation. Molecules with lower ionisation potentials tend to exhibit stronger mesomeric effects because their electrons can be redistributed more readily, resulting in resonance structures of comparatively lower energy. The resonance hybrid generally possesses a lower energy than any individual canonical form, and the energy difference between these is known as the resonance or mesomeric stabilisation energy. This stabilisation is a key factor in the unusual stability of aromatic compounds and extended π systems.
The mesomeric effect is symbolised M, with a positive value (+M) indicating electron-donating behaviour and a negative value (–M) indicating electron-withdrawing behaviour. These qualitative descriptors are widely used in organic chemistry to predict reactivity patterns.

Positive Mesomeric (+M) Effect

The +M effect occurs when a substituent donates electron density into a conjugated system. This donation typically requires the presence of a lone pair of electrons or a negative charge on the substituent. As electrons flow from the substituent towards the conjugated framework, the electron density within the system increases, often resulting in the development of a partial negative charge in regions distant from the substituent.
Common electron-donating groups exhibiting a strong +M effect include:

• –OH
• –OR
• –NH₂ and substituted amines
• –NHR and –NR₂
• –O⁻
• –S⁻

The increase in electron density generally enhances reactivity towards electrophiles, which are attracted to electron-rich centres. For example, the –OH group on a phenyl ring activates the aromatic system and directs substitution preferentially to the ortho and para positions. In conjugated polyenes, +M substituents stabilise cationic intermediates, making such systems more reactive in electrophilic addition reactions.

Negative Mesomeric (–M) Effect

The –M effect is observed when a substituent withdraws electron density from the conjugated system. Substituents that display this effect typically possess a positive charge or a vacant orbital capable of accepting electron density. In this case, π electrons shift away from the conjugated region and towards the electron-withdrawing substituent, rendering the system electron-deficient.
Prominent –M groups include:

• –NO₂
• –CN
• –CHO
• –COOH
• –COR
• –SO₃H
• carbonyl groups in general

As the conjugated system becomes electron-poor, it becomes less reactive towards electrophiles, which are repelled by the reduced electron density. Conversely, the system becomes more reactive towards nucleophiles, which are drawn to electron-deficient regions. A typical example is the nitrobenzene ring, which is strongly deactivated towards electrophilic aromatic substitution and directs incoming groups predominantly to the meta position due to the electron-withdrawing action of the nitro group.

Transmission of the Mesomeric Effect in Conjugated Systems

In conjugated frameworks, the mesomeric effect can travel across several carbon atoms through continuous overlap of p orbitals. This transmission gives rise to extended resonance networks, as found in polyenes, aromatic rings, and conjugated carbonyl compounds. The capacity of electrons to delocalise over long distances contributes significantly to molecular properties such as:

• resonance stabilisation
• altered acidity or basicity
• changes in reactivity
• diamagnetic anisotropy in aromatic systems
• shielding and deshielding phenomena observed in NMR spectroscopy

For example, in benzoic acid derivatives, electron-withdrawing substituents increase the acidity of the carboxylic group by stabilising the conjugate base through the –M effect. Conversely, electron-donating groups decrease acidity by destabilising the conjugate base via the +M effect.

Quantitative Evaluation of the Mesomeric Effect

Several substituent constants have been developed to quantify the mesomeric or resonance effect. These parameters are integral to linear free-energy relationships used in physical organic chemistry:

• Swain–Lupton resonance constant (R)
• Taft resonance constant
• Oziminski and Dobrowolski π-electron donor–acceptor parameter

These constants allow chemists to predict reaction rates, equilibrium positions, and substituent effects on diverse organic transformations. They separate the overall substituent influence into field/inductive components and resonance/mesomeric components, enabling a more detailed analysis of reaction mechanisms.

Mesomeric Effect versus Inductive Effect

Whilst both the mesomeric and inductive effects influence electron distribution within molecules, they stem from different origins and operate through distinct mechanisms. The inductive effect arises solely from electronegativity differences between atoms and is transmitted through σ bonds. It depends on the topology of the molecule—specifically, the sequence of atoms connected through single bonds—and affects primarily atoms in the immediate vicinity, diminishing rapidly with distance.
In contrast, the mesomeric effect involves electron delocalisation through π bonds and is governed by orbital overlap rather than electronegativity. As a result, the mesomeric effect can be transmitted across entire conjugated systems and is generally stronger and more influential in determining molecular reactivity. Importantly, the mesomeric effect does not alter the electronegativity-based inductive influence, and the two effects often act simultaneously, sometimes in reinforcing and sometimes in opposing directions.
An example illustrating the difference is found in substituted benzoic acids: a –NO₂ group exerts both a strong –I (inductive withdrawing) and –M (mesomeric withdrawing) effect, both of which enhance acidity. A halogen substituent, however, provides –I withdrawing but +M donating behaviour, resulting in a more complex influence on reactivity.

Historical Context and Terminology

The term mesomeric effect was introduced in 1938 by Christopher Kelk Ingold as part of a broader set of concepts designed to explain substituent influences on organic reactivity. Ingold’s terminology of mesomerism was intended as an alternative to Linus Pauling’s concept of resonance. Although both describe the same underlying phenomenon of delocalised electron systems, the term resonance has become predominant in English-language chemical literature. Nonetheless, mesomerism remains commonly used in German and French texts, reflecting regional preferences in chemical nomenclature.
The historical development of resonance theory contributed to modern understanding of aromaticity, reaction mechanisms, and the electronic structure of organic molecules. The mesomeric effect remains an essential theoretical tool in education and practice.

Applications and Chemical Significance

The mesomeric effect is crucial across a wide spectrum of organic chemistry. It influences:

• Electrophilic and nucleophilic aromatic substitution, determining activation, deactivation, and directing effects.
• Acid–base behaviour, particularly in substituted carboxylic acids, phenols, and amines.
• Stability of intermediates, such as carbocations, carbanions, and radicals.
• Spectroscopic properties, including UV–visible absorption of conjugated systems and NMR chemical shifts.
• Reactivity in conjugated dienes, enones, and polyaromatic hydrocarbons.