Consider a student giving an exam. He is well prepared and writes the exam comfortably. However, when the invigilator approaches him and glances at the answers written on his answer sheet, a temporary uneasiness is observed in the student while writing further on the answer sheet. As soon as the invigilator goes away, he starts answering the questions again. Similarly, the electromeric effect results in polarisation, which is temporary and occurs only in the presence of a reagent.
Table Of Content
There are two main types of Electromeric effects based on the direction in which the electron transfer occurs.
1. +E effect
2. -E effect
The positive electromeric effect occurs when the shared electron pair is transferred towards the attacking reagent.
The attacking reagent forms a bond with the atom that has received the shared pair of electrons. In this case, the reagent is positively charged and is seeking a negatively charged site.
This +E effect can be observed in the reaction involving the addition of acid to alkenes. A classic example portraying the positive electromeric effect is the protonation of ethene.
Here, π electrons of the multiple bonds are transferred to the atom to which the reagent gets attached.
A negative electromeric effect is observed when the shared pair of electrons is transferred away from the attacking reagent.
In simple terms, the nucleophilic reagent attacks the atom that has lost its shared pair of electrons and forms a bond with it.
This phenomenon is greatly observed in reactions involving carbonyl carbon atoms like aldehydes and ketones. For example, the reaction of an aldehyde with a cyanide ion.
Here, π electrons of the multiple bonds are transferred to the atom to which the reagent does not get attached.
There are commonly observed electronic effects in an organic reaction caused by the attacking reagent: electromeric effect, and resonance effect. All three effects induce polarity in the organic substrate.
The electromeric effect can be defined as a temporary effect that induces polarity in the pi bonded atoms of an organic molecule.
It is only observed in pi bonds and not in sigma bonds, unlike the inductive effect.
Electromeric effects are classified as +E and -E effects based on which atom the attacking reagent (electrophile/ nucleophile) interacts with.
An electrophilic reagent causes a +E effect, whereas a nucleophilic reagent causes a -E effect.
An example of the +E effect is the protonation of ethene, and an example of the -E effect is the reaction of carbonyls with cyanides.
If the Electrons in a conjugated system completely shift towards a specific direction, which is known as the mesomeric effect.
It does not depend on distance like the inductive effect.
In the case of 1,3-butadiene, the delocalization of electrons can take place in either of the directions leading to structures(II) and (III) (both are equivalent). Therefore, 1,3-butadiene does not show a mesomeric effect.
In The case of propenal, the delocalization of electrons takes place predominantly in a particular direction (towards oxygen). Since oxygen is more electronegative than carbon, it is more likely to hold a negative charge. Therefore, propenal shows a mesomeric effect.
There are two types of Mesomeric effects.
1. +M effect (Positive Mesomeric Effect)
2. -M effect (Negative Mesomeric Effect)
If a group donates electrons to a conjugated system then the group is said to show a Positive mesomeric effect.
a. Consider the given molecule showing a mesomeric effect. In this case, the group, -X, donates its electron pair, showing a +M effect. In general, the group showing the +M effect either possesses a negative charge or a lone pair.
b. In the given molecule, the O of -OCH3 the group possesses a lone pair, showing the +M effect.
c. In the phenoxide ion, the oxygen atom possesses a negative charge, showing the +M effect with the benzene ring.
Relative order of Positive Mesomeric Effect (+M Effect)
As electronegativity increases across the period, the electron-donating tendency decreases. Hence, the +M effect decreases along the period. On moving down the group, the size increases. Therefore, the tendency to form pi bonds also decreases, decreasing the +M effect.
The donors from -O- to -OR are powerful donors. Amines and substituted amines are also good donors. Amides (-NHCOR) to phenyl(-Ph) are moderate donors. Halogens and NO are poor donors.
In general substituents attached to the conjugated system possesses the following substituents that show the -M effect.
a. Empty orbital
Example: Boron has a vacant orbital and hence, it can take electrons towards itself from a conjugated system.
(Where the electronegativity of Y is greater than that of X)
−M effect is observed when a conjugated system has an EWG (electron-withdrawing group) like −COOH, −CHO, −COOR, −CN, −NO2 attached to it.
Relative order of Negative Mesomeric Effect (-M Effect)
They are atoms/substituents attached to a conjugate system that can show both +M/ −M effects.
Examples: -N=O, -SH, -Cl, -N=N, −Phenyl, -CH=CH2, and more.
Q 1. What is the correct order of the +M effect of the N-containing functional group on the benzene ring of the given compounds?
Answer: The Correct Order of the +M effect of the N-containing functional group on the benzene ring of the given compounds is II > I > III > IV.
The strong +I effect of the methyl group increases electron density on nitrogen so the II molecule has more +M effect than I.
In III and IV molecules, the IV molecule consists of two amide groups which decrease electron density at nitrogen as compared to the III molecule.
Q 2. Which of the following shows −M effect?
D. All of these
Answer: When atoms or substituents that are directly attached to the conjugated system have −X=Y, where the electronegativity of Y is greater than that of X, they show the −M effect. Here, -CHO, -COOCH3, and -CONH2 have a −C=O group and the electronegativity of O is greater than that of C. Hence, all of these groups show the −M effect.
Hence, option (D) is the correct answer.
Q 3. Which of the following groups has the +M effect?
D. All of these
Answer: When an atom or a group that is directly attached to the conjugated system contains a lone pair or an electron pair, it shows the +M effect. Here, the groups given in options (A), (B), and (C) have lone pairs on the oxygen atom. Hence, they show the +M effect.
Hence, option (D) is the correct answer.
Q 4. Which of the following groups has the +M effect?
C. Both A and B
D. None of these
Answer: Molecule in option (A) has lone pair on the nitrogen atom. Hence, it shows the +M effect when attached to a conjugated system.
The group in option (B) has a −C=O entity that is directly attached to the conjugated system, and oxygen is more electronegative than carbon. Hence, this group shows the −M effect when attached to a conjugated system.
Hence, option (A) is the correct answer.
Q 1. Explain the types of mesomeric effects.
Answer: There are two types of mesomeric effects. Positive mesomeric effect and negative mesomeric effect.
Q 2. Give examples of some molecules which show the + M effect.
Answer: -OCH3, -NH2, -OHare the groups which show positive mesomeric effect.
Q 3. Give examples of some molecules which show the - M effect.
Answer: −COOH, −CHO, −COOR, −CN are the groups that show the -M effect.
Q 4. Is the electromeric effect a permanent or temporary effect?
Answer: Electromeric effect is a temporary effect, it can be applied only in the presence of reagents.