What is mesomeric effect



After a substitution reaction on molecules that contain a mesomerism, it can happen that the substituent can participate in the mesomerism. This enlarges the mesomeric system. In this case the substituent has one Mesomeric effect (short: M-Effect). Mesomeric effects influence every further reaction to the molecule in question. They determine at which position - i.e. at which atom - a further reaction is likely to take place. In addition, there are more mesomeric limit formulas, which stabilizes the molecule.

In the electrophilic second substitution on the benzene ring, the first substituents direct (= determine) the position of the other. 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.

+ M effect

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 second substitution is thereby favored, the + M effect thus has an activating effect with regard to reactivity (except halogens).

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

Substituents with + M effect

  • -O
  • –NH2
  • -NO2
  • -OH
  • –OR
  • –NHCOR
  • –OCOR
  • - (aryl) (e.g. -phenyl group)
  • -CH = CH-COOH (b)
  • -Br, -Cl, -I

−M effect

The substituent has a double or triple bond, so that it withdraws electron pairs from the mesomeric system. This makes further substitutions more difficult and increases the activation energy required for this. From a reaction kinetic point of view, the −M effect has a deactivating effect. With −M effects, the electron density of the mesomeric system decreases.

Substituents with the −M effect

  • –COOR
  • -COOH
  • –CHO
  • –COR
  • -CN
  • –NO2
  • -SO3H
  • –NH3+
  • -NO3+

Acid Strength:

Since −M effects cause electrons to be pulled out of the benzene ring by the substituent (e.g. benzoic acid), these reduce the acidity of the substituent.

+ M effects, on the other hand, push electrons into the benzene ring. Due to the mesomeric stabilization of the negative charge in the ring, the proton can be more easily split off from e.g. the OH group as a substituent (phenol). The acid strength increases, phenol is 10 times with a pKa of ~ 106 times more acidic than alkyl alcohols such as. Ethanol (pKa 16).

See also

Category: Chemical reaction