|Class 11 Chemistry Organic Chemistry||Electron Displacement in Covalent Bonds|
ELECTRON DISPLACEMENT IN COVALENT BONDS
Whenever any reaction occurs, old bonds break and new bonds are formed. This formation is due to shifting of electrons. There are four types of electron displacements:
It is a permanent effect that arises when some electron releasing or withdrawing group is attached to chain of alkane.
It is of two types
-I effect –It is shown when electron withdrawing group is attached to the carbon chain.
NO2 > CN> COOH > COO> F> Cl> Br> I
+I effect - It is shown when electron releasing group is attached to carbon chain .
Example – Alkyl groups.
2. Electrometric effect
+E effect -
It is that effect when electron of π bond is transferred to atoms to which attacking agent attaches itself.
-E effect –
“It is that effect when π electrons are transferred to atom other
than the atom, to which attacking agent finally attaches itself.”
It occurs due to delocalisation of π electron cloud so as to get best structure as compared to other structure.
Example: Structure of CO2
But, it was seen that Bond Length of that CO bond is not same as that of C=O. It lies in between C-C and C O. So, as to explain the Resonating structures were proposed.
Resonance is best seen in Benzene: It is also called as Mesomeric effect .It arises due to polarity developed in Benzene ring due to delocalisation of π electron.
+ R effect -It is shown when electron donating group is attached to benzene ring. Like
If any positively charged group approaches it attacks at Ortho and Para positions.
-R effect – Shown by electron withdrawing group when attached to benzene ring.Example:
It is like resonance but the difference is that there is no bond present from where the electrons are shifted.
The Condition required: It arises when = and – bond is at adjacent position.