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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.
However, another effect that plays a role is the +M effect which adds electron density back into the benzene ring (thus having the opposite effect of the -I effect but by a different mechanism). This is called the mesomeric effect (hence +M) and the result for fluorine is that the +M effect approximately cancels out the -I effect.
The pEDA parameter (pi electron donor-acceptor) is a pi-electron substituent effect scale, described also as mesomeric or resonance effect. There is also a complementary scale - sEDA. The more positive is the value of pEDA the more pi-electron donating is a substituent.
The reason for this is that while the inductive effect is still negative, the mesomeric effect is positive, causing partial cancellation. The data also show that for these substituents, the meta effect is much larger than the para effect, due to the fact that the mesomeric effect is greatly reduced in a meta substituent.
Founded on a few general principles that govern how orbitals interact, the stereoelectronic effect, along with the steric effect, inductive effect, solvent effect, mesomeric effect, and aromaticity, is an important type of explanation for observed patterns of selectivity, reactivity, and stability in organic chemistry. In spite of the ...
3,5-Dinitrobenzoic acid is an odorless, yellowish solid. Due to the mesomeric effect of the two nitro groups, it is more acidic (pK a = 2.82) than benzoic acid (pK a = 4.20) and 3-nitrobenzoic acid (pK a = 3.47).
Distinguishing between both mechanisms requires consideration of inductive and mesomeric effects that could stabilize or destabilize a potential carbocation in the S N 1 pathway. Usage of hydrohalic acids takes advantage of the fact that these agents are able to protonate the ether oxygen atom and also provide a halide anion as a suitable ...
Memory effect (electric batteries) Mesomeric effect (chemical bonding) Microwave auditory effect (cognitive neuroscience) (espionage) (hearing) (human psychology) (less-lethal weapons) (mind control) (sound) Mid-domain effect (macroecology) (biogeography) (biodiversity) Mikheyev–Smirnov–Wolfenstein effect (particle physics)