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Mesomeric effect and Resonance


Mesomeric  effect


         It is special type of resonance of in which lone pair of electrons are delocalized over the alternative double bond and lone pair always present on same heteroatom in function group.
Heteroatom=> 
                                       
These types of functional group are called +M Group .same function groups are present on which lone pairs are not present but these functional groups show mesomeric effect and these types of functional groups are called –M group
+M group = O- >NR2 >NHR,>NH2 >HO>NHCOR>-OCOR and alky |& ary |groups.
-M group = -NO2>-CN>-CHO>COR>COR>-COOH>-COOP>CONH2 etc
+M group =ortho-para directing and activating groups.
In this example negative charge observed at ortho and para position to the –NH2 group and
negative charge means extra electron is present in the ring and ring is activated and electrophile   attacks on ortho & para positions.
-M group => meta directing and deactivating groups.
e.g.
In this example positive charge observed at ortho and para position to the –NO2 group and positive charge means electron density decreases in  ring at ortho and para positions results deactivation  of the ring and electrophile attacks on the meta position.
Resonance = It is definel as the delocalisation  of electrons in conjugation electron moves from higher electron density to lower electron conjugation is the alternation of double bond with single bond ,negative charge ,positive to charge delocalization over the bonds.
Resonance effect tells about the stability of most of intermediates.
(A)   Stability of carbocation=
When resonance increases in any organic compound, then the stability of corbocation increases.
e.g
(A)  



(B)
(C)
In above three examples ( C) is most stable carbocation  because positive change delocalized over the three example  (A) is the least stable than  (C) & (D) because the positive charge delocalized over only one double bond .
Thus the stability order is
((B)   Stability of corbanions =>
When resonance increases in any organic compound than the stability of corbanion also increases
(A)


(B)
In above example
(A) is more stable than(B) because in (A) three three double bond are present and in (B) Only one double bond present
                                                

Note= Halogens are exception which are ortho & para directing groups



1 . According to resonance halogens are ortho & para directing groups
X=F,CL,Br,I



In this example lone pair present on halogen resonace with the double bond in benzene ring and give the position on which electropile are attack .(i.e inductive effect tell about orientation)



2 . Acording to inductive effect halogens are deactivators because halogen are higher electronegative atoms and they pull the electrons from electrons from benzene ring (i.e. Inductive effect about nature of halogens)
Thus the halogens are ortho,para directing and deactivating group
Electonagative order/inducative effect =F>CI>Br>I
Resonance effect order=F>CI>Br>I  (Because ‘F’is very small halogen and better overlapped with carbon offering while  I is very bigger halogen
Resonance effect generally observed higher electron density to lower electron density.




(A) Nagative charge to double ()
(B)   Lone pair to  double bond     ()
              
(C) Double bond to positive charge ()


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