Mesomeric Effect: –M or +M | Organic Chemistry | Stereochemistry | Chemistry

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ORGANIC CHEMISTRY Nature of Bonding and Stereochemistry
Topic
Electronic Effects : Inductive and Mesomeric Effects
Learning Outcomes
After studying this lesson, you shall be able to:
compare the extent of –M or +M in the comparable groups
Delocalization

. In order to classify most of the groups into +M/+R or –M/-R just look at the following parameters required (given in following table) for the nature of first atom
+ M/+R Effect

If the first atom of the group either has a lone pair or negative charge.

• e.g. – NH2, -OH, -OCH3, NHCOCH3 etc.

- M/-R Effect

1. If the first atom of the group has positive charge and after withdrawing a pair of electrons ,it is not violating it’s octet.

• e.g. -PH3+ and NOT –NH3+ (octet of N will be voilated if it withdraws electrons)

2. If the first atom of a neutral group has neither a lone pair nor any charge, and is joined to more electronegative atom(s) than itself by double/triple bond.

• e.g. -CHO, -CN, -SO3H, etc.
How to compare the extent of –M or +M in the comparable groups (having same first atom)?

–M Effect: Here, due to the vacant p orbital on the first atom, the conjugated Π electrons are withdrawn towards it. The further attached atoms or groups shall enhance the –M if they also pull by –I or –M effect, but will decrease the –M effect if they are electron donating by +I or +M effect

For example, the order of –M effect between the following groups is
+M Effect: Here, due to the lone pair or –charge in the p orbital of the first atom, the electrons are donated to the conjugated system away from itself.

The further attached atoms or groups shall enhance the +M if they push by +I or +M effect, but will decrease the +M effect if they are electron withdrawing by –I or -M effect.

For example, the order of +M effect between the following groups is
Delocalization may involve:

(i) Overlap of p orbitals involved in the formation of л bonds (л-л overlap).
Delocalization of л electrons through л-л overlap occurs in conjugated systems (alternate single and double bonds) i.e. systems where л bonds are separated through a sigma bond only, for example, in 1,3-butadiene, each carbon is sp2 hybridized and thus each carbon has a p orbital.

These four p orbitals overlap each other to form л bonds.

The л bonds are not localized between two carbons, C-1 and C-2, but are delocalized or distributed equally over all four carbons.

It is this delocalization that helps in lowering of energy and stabilizes the system.

Similarly, in benzene the л electrons are delocalized over all the six carbon atoms
(ii) Overlap of p orbital of an atom (vacant or filled) with p orbitals involved in л bond formation (p- л overlap)
Delocalization of л electrons through p- л overlap occurs in a system where p orbital of an atom (vacant or filled) is separated from a л bond through one σ bond only.
In other words, the p orbital may contain a lone pair of electron or a single electron or may be vacant, for example, in case of chlorobenzene or vinyl chloride, the lone pair of electrons is present in p orbital of chlorine.
Chapters:
0:00 Organic Chemistry
0:13 +M/+ Effect &\ -M-R\ Effect
1:23 How to compare the extent of -M or +M in the comparable groups
1:36 -M Effect
2:30 +M Effect
3:18 Delocalization May Involve
4:49 These Four p Orbitals
5:50 Overlap of p Orbital
10:44 Questions of Organic Chemistry
#Donor #Vacant #Stabilizes #Mesomeric #Distributed #Stabilizes #Conjugated #Butadiene #Delocalization #Electron #TestPrep #ugcnet #doorsteptutor #examrace

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