Mesomeric effect

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In chemistry , the mesomeric effect ( M effect for short ) is an effect of functional groups in chemical compounds . The M-effect works through conjugated systems and influences reaction speeds, ion equilibria and the point at which a molecule reacts (directing effect).

Substituents can only exert a mesomeric effect if they are linked to a conjugated system ( unsaturated bonds or atoms with lone pairs of electrons ). This increases the size of the conjugated system (also: mesomeric system). In this case the first substituent has an M effect. If the substituent has a lone pair of electrons that it can make available for the mesomerism, it adds electron density to the conjugate system , this is referred to as a + M effect . If, on the other hand, the substituent has a double or triple bond, electron density is withdrawn from the mesomeric system; one speaks of an −M effect .

The M-effect affects reaction rates and ionic equilibria because the p orbitals (or π bonds) of a substituent overlap with the rest of the molecule. It comes to the delocalization or their extension. There is also the inductive effect (+ I effect and –I effect).

+ M effect

+ M effect of the methoxy group in an ether

The substituent has a lone pair of electrons that it can make available for mesomerism. With + M effects, the electron density of the mesomeric system increases. A possible electrophilic second substitution is thereby favored, the + M effect thus has an activating effect with regard to reactivity. In the case of halogens as primary substituents on aromatic systems, however, the rate of substitution is overall reduced due to the −I effect , but the + M effect still directs in the ortho / para position (see substitution pattern ).

Example: Reaction of bromine and benzene : Benzene contains a mesomeric system and bromine has three free electron pairs. After the substitution, bromine is bonded to a carbon atom of benzene with a single bond. A free electron pair of the bromine can be included in the mesomerism, which results in new mesomeric boundary structures.

Substituents with + M effect

−M effect

–M effect of the carbonyl group in the α, β-unsaturated aldehyde acrolein

In the case of −M effects, electron density is withdrawn from the mesomeric system because the substituent has a double or triple bond. This makes further substitutions more difficult or increases the activation energy required for this. From a reaction kinetic point of view, the −M effect has a deactivating effect. In addition, the following groups have a meta- directing effect on an electrophilic attack in aromatic systems (see substitution pattern ). This is where the greatest electron density is located, since the ortho and para positions carry a positive charge due to the withdrawal of electrons from the aromatic system .

Substituents with the −M effect

Acid starch

Substituents with an -M effect increase the acid strength of aromatic, acidic compounds, since the substituent removes electron density from the conjugated system. For example, nitrophenols compared to phenol ( pK s  = 9.99) stronger acids. Trinitrophenol reached a pK s value of 0.29. This is because all three nitro groups (-NO 2 ) as listed above have an -M effect. Mesomerism removes electrons from the benzene ring so that it is positively charged in the ortho / para position. Since the hydroxyl group can compensate for this electron deficiency through its unbound electron pair, this positive charge is transferred to the oxygen. This unstable situation tends to split off the hydrogen ion so that the oxygen can retain the binding electrons and is no longer positively charged. Furthermore, the double bonded oxygen to the benzene ring also has an -M effect, so that the resulting anion is mesomeric stabilized. This means that it is not clear which oxygen atoms have the negative charge.

Base strength

In the case of an aromatic ring with an amino group on it, electrons are pushed into the ring due to the + M effect. The functional group becomes more positive and the pK b value increases overall for this compound. That is, the " base strength " decreases.

Conducting effect

Mesomeric effects also influence the position - i.e. at which atom - a further reaction is likely to take place. In the electrophilic second substitution on the benzene ring , the first substituents “direct” (= “determine”) the position of the others. In general, one can say that primary substituents that have a + M effect direct in the ortho / para position, those with a −M effect in the meta position. This can be explained by the stability of the transition states occurring during the substitution.

Individual evidence

  1. ↑ Entry on mesomeric effect . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.M03844 Version: 2.1.5.