Difference between electromeric effect and mesomeric effect
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The interaction between lone elctron pair on atoms and the pi electron bond pairs of adjacent chemical bonds results in resonances. A molecule can have several resonance forms depending on the number of lone electron pairs and pi bonds. But the actual structure of the molecule is a hybrid of all possible resonance structures.
The above image shows resonance structures of nitrate ion. Here, the lone electron pairs on the oxygen atoms interact with the pi bond electrons. This results in the delocalization of electrons.
mesomeric effect occurs due to the presence of substituent groups or functional groups.
if functional group donated electron pair in benzen ring and delocalize with conjugated system. the effect is -ve mesomeric effect
if functional group is electron withdrawing group the positive charge in benzene is in congugation with pi system.. this effect is known as +ve mesomeric effect.
e.g
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OTHER ANSWERS
Preetham Gampa
Preetham Gampa, Co-Founder Green Wave
Answered Jun 2, 2015
Originally Answered: What is the difference between resonance and mesomeric effect?
Both resonance and mesomerism represent the same phenomenon, they differ in the following respect : Resonance involves all types of electron displacements while mesomerism is noticeable only in those cases where a multiple bond is in conjugation with a multiple bond or lone pair of electron.
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Arunavo Chakraborty
Arunavo Chakraborty, Undergraduate, Indian Institute of Science, Bengaluru
Answered Nov 22, 2015
Resonance refers to delocalization of electrons in a given system. Mesomeric effect is the electron donating or withdrawing nature of a substitutent due to resonance.
Hence, resonance is the mechanism by which mesomeric effect occurs. Resonance leads to several other "effects" as well that are non-mesomeric.
Difference between electromeric and mesomeric effect is as follow:--
1)Electromeric effect is temporary effect which involves complete transfer of pie electrons to one of the bonded atoms whereas mesomeric is resonance effect which involves permanent transfer of pie electrons or lone pair of electrons.
2)Electromeric operates in unsaturated compounds which contain one multiple bond whereas mesomeric operates only in unsaturated conjugated system with more than one multiple bond.
3)Electromeric effect operates n presence of outside reagent whereas mesomeric effect can occur both in presence or absence of reagent.
4)Electromeric is temporary effect whereas mesomeric is permanent effect.