What does mesomeric effect mean?

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Area of ​​Expertise - Organic chemistry, quantum physics

The mesomeric effect or M-effect describes the ability to change the electron density in a π-electron system. The molecular geometry plays a major role in this.

The increase or decrease in the electron density takes place through mesomerism, i.e. through the formation of π bonds through the participation of free electron pairs of the substituent or the acceptance of π electrons by the substituent. In the case of π-electron donors, one speaks of an electron-donating mesomeric effect (+ M effect), in the case of π-electron acceptors of an electron-withdrawing mesomeric effect (-M effect).

Electron-donating mesomeric effect (+ M effect): Substituents with conjugatable free electron pairs (π electron donors) such as halogens (), the amino () or the hydroxyl group () increase the electron density of the π system and facilitate electrophilic reactions inortho- or para-Position on the aromatic.

The + M effect is greater, the smaller the electronegativity, the smaller the atomic radius and the greater the negative charge of the substituent.

Electron-withdrawing mesomeric effect (-M effect): If the substituent on the aromatic contains a polar multiple bond, e.g. a carbonyl (), nitrile () or nitro group () (π-electron acceptors), this remainder takes on the charge density of the π system and thus favors a nucleophilic attack the ortho- and para-Position of the ring.

The -M effect is greater, the more polar the multiple bond of the substituent and the smaller the radius of the substituent atom.

In addition to mesomeric effects, there are also electron-donating and electron-withdrawing inductive effects (+ I and -I effects) in substituted aromatic systems. M and I effects of a substituent can be in the same direction, but they can also compete. The overall activating or deactivating influence of a first substituent in the case of a second substitution therefore results from the sum of its inductive and mesomeric effects.

See also: mesomerism

Learning units in which the term is dealt with

Aromatic reactions (total)90 min.

chemistryOrganic chemistryAromatic chemistry

This learning unit begins with a description of the mechanism of electrophilic aromatic substitution, including energetic considerations of the intermediates and transition states. Furthermore, the dependencies between inductive and mesomeric effects and the reactivity and the secondary substitution of substituted aromatics are dealt with. The most important reactions are also presented. Finally, the less important nucleophilic aromatic substitution is explained, including the mechanism, intermediate stages, transition states, etc., etc. The conclusion is a description of the "production" and structure of arynes.

Aromatic reactions - Ar-SE - basics and theory30 min.

chemistryOrganic chemistryAromatic chemistry

This learning unit begins with a description of the mechanism of electrophilic aromatic substitution, including energetic considerations of the intermediates and transition states. Furthermore, the dependencies between inductive and mesomeric effects and the reactivity of the second substitution of substituted aromatics are treated.