chemistry

Mesomeric effect

Mesomeric effect is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mesomeric effect rather than just read about it. In short: In chemistry, the mesomeric effect (or resonance effect) is a property of substituents or functional groups in a chemical compound. It is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond and lone pair of electrons present on an adjacent atom.

Key takeaways

Mesomeric effect belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mesomeric effect to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mesomeric effect from memory before moving on to harder problems.

Reference excerpt

In chemistry, the mesomeric effect (or resonance effect) is a property of substituents or functional groups in a chemical compound. It is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond and lone pair of electrons present on an adjacent atom. This change in electron arrangement results in the formation of resonance structures that hybridize into the molecule's true structure. The pi electrons then move away from or toward a particular substituent group. The mesomeric effect is stronger in compounds with a lower ionization potential. This is because the electron transfer states will have lower energies.

Representations of the mesomeric effect The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant resonance structures and is symbolized by the letter M. The mesomeric effect is negative (−M) when the substituent is an electron-withdrawing group, and the effect is positive (+M) when the substituent is an electron donating group. Below are two examples of the +M and −M effect. Additionally, the functional groups that contribute to each type of resonance are given below.

+M effect The +M effect, also known as the positive mesomeric effect, occurs when the substituent is an electron donating group. The group must have one of two things: a lone pair of electrons, or a negative charge. In the +M effect, the pi electrons are transferred from the group towards the conjugate system, increasing the density of the system. Due to the increase in electron density, the conjugate system will develop a more negative charge. As a result, the system under the +M effect will be more reactive towards electrophiles, which can take away the negative charge, than a nucleophile.

+M effect order: −O− > −NH2 > −NHR > −NR2 > −OH > −OR > −NHCOR > −OCOR > −Ph > −F > −Cl > −Br > −I > −NO

−M effect The −M effect, also known as the negative mesomeric effect, occurs when the substituent is an electron-withdrawing group. In order for a negative mesomeric (−M) effect to occur, the group must have a positive charge or an empty orbital in order to draw the electrons towards it. In the −M effect, the pi electrons move away from the conjugate system and towards the electron drawing group. In the conjugate system, the density of electrons decreases and the overall charge becomes more positive. With the −M effect the groups and compounds become less reactive towards electrophiles, and more reactive toward nucleophiles, which can give up electrons and balance out the positive charge.

−M effect order:

−NO2 > −CN > −SO3H > −CHO > −COR > −COOCOR > −COOR > −COOH > −CONH2 > −COO−

Mesomeric effect vs. inductive effect The net electron flow from or to the substituent is determined also by the inductive effect. The mesomeric effect as a result of p-orbital overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect has purely to do with the electronegativity of the atoms and their topology in the molecule (which atoms are connected to which). Specifically the inductive effect is the tendency for the substituents to repel or attract electrons purely based on electronegativity and not dealing with restructuring. The mesomeric effect however, deals with restructuring and occurs when the electron pair of the substituents shift around. The inductive effect only acts on alpha carbons, while the mesomeric utilizes pi bonds between atoms. While these two paths often lead to the similar molecules and resonance structures, the mechanism is different. As such, the mesomeric effect is stronger than the inductive effect. The concepts of mesomeric effect, mesomerism and mesomer were introduced by Ingold in 1938 as an alternative to Pauling's synonymous concept of resonance. "Mesomerism" in this context is often encountered in German and French literature, but in English literature the term "resonance" dominates.

Mesomerism in conjugated systems Mesomeric effect can be transmitted along any number of carbon atoms in a conjugated system. This accounts for the resonance stabilization of the molecule due to delocalization of charge. The energy of the actual structure of the molecule, i.e. the resonance hybrid, may be lower than that of any of the contributing canonical structures. The difference in energy between the actual inductive structure and the (most stable contributing structures) worst kinetic structure is called the resonance energy or resonance stabilization energy. For the quantitative estimation of the mesomeric/resonance effect strength, various substituent constants are used, i.e. the Swain-Lupton resonance constant, Taft resonance constant or Oziminski and Dobrowolski pEDA parameter. Additionally, the resulting resonance structures can give the molecule properties that are not inherently evident from looking at one structure. Some of these properties include different reactivities, local diamagnetic shielding in aromatics, deshielding, and acid and base strengths.

References

Illustrations

Worked examples

Example 1 — a first encounter with Mesomeric effect

Start with the simplest possible case. Write down what Mesomeric effect claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mesomeric effect before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mesomeric effect ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mesomeric effect

In research: Mesomeric effect appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mesomeric effect in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mesomeric effect is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical bonding, so understanding it makes those chapters shorter.
In everyday life: Look for Mesomeric effect outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Mesomeric effect in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mesomeric effect means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mesomeric effect out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mesomeric effect in simple terms?

Why does Mesomeric effect matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mesomeric effect?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mesomeric effect.