What is the #+M# and #-M# effect? What are examples of electron releasing and electron withdrawing groups?

Question

Ellie Adams · Accepted Answer

The mesomeric effect (or resonance effect) is the movement of π electrons toward or away from a substituent group.

#bb "-M effect"#

For example, propenal has a mesomeric contributor in which the π electrons move towards the oxygen atom.

and a #δ^+# charge on #"C-3"#.

Since the electrons have moved away from the rest of the molecule and towards the #"C=O"# group, the effect is called a #bb "-M effect"#.

Other #"–M"# substituents are #"–COR"#, #"-CN"#, and #"-NO"_2#.

#bb "+M effect"#

If the π electrons move away from the group and towards the rest of the molecule, the effect is called a #bb "+M effect"#.

An example is the donation of electrons from an amino group into a benzene ring, putting #δ^-# charges on the ortho and para positions.

Other #"+M"# substituents are #"–OH"#, #"-OR"#, #"-OCOR"#, #"-NR"_2#, and #"–NHCOR"#.

Ellie Adams · Answer

undefined The +M effect, or mesomeric effect, refers to the electron-donating or electron-releasing ability of a substituent group in an organic molecule due to resonance. The -M effect, or mesomeric effect, refers to the electron-withdrawing ability of a substituent group in an organic molecule due to resonance. Examples of electron-releasing groups include alkyl groups (e.g., methyl, ethyl), phenyl groups, and amino groups. Examples of electron-withdrawing groups include nitro groups, carbonyl groups (e.g., ketones, esters), and halogens (e.g., fluorine, chlorine).