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Electron displacement Effects in Covalent Bonds

The electron displacement in an organic molecule may take place either in the ground state under the influence of an atom or asubstituent group or in the presence of an appropriate attacking reagent. The electron displacements due to the influence of an atom or a substituent group present in the molecule cause permanent polarisation of the bond. Inductive effect, hyperconjugation and resonance effects are examples of this type of electron displacements.

Inductive Effect

In the C - X and C - Y type organic molecules, the symmetry of organic molecular orbitals will be destructed due to polarity.

The symmetry of molecular orbitals, of C - X will be disturbed in favour of X if 'X' is more electronegative substituent, while in case of C - Y, the molecular orbital will be disturbed in favour of the C - atom if C - atom is more electronegative than Y.

Polarity is induced on either the carbon atom or the substituent, attached to it due to displacement of bonding electron pair, caused by their different electronegativities, is known as Inductive effect (I - effect)

If the substituent (X) electron attracting it develops a -ve charge. In this case the inductive effect is said to be -ve or(-I effect).

-I Effect Groups (Electron Attracting Groups)
NO2 > F > COOH > Cl > Br > I > OH > C6H5

If the substituent (Y) is electron releasing it acquires a +ve charge. In this case the inductive effect is said to be +ve or +I effect

+I - Effect Groups (Electron Pumping)
(CH3)3 C -      > (CH3)2 CH - > CH2CH2 > CH3
tertiary butyl      Isopropyl        Ethyl      Methyl
Inductive effect in carbon chain

If the electron attractive substituent X is at the terminal C - atom in a carbon chain, the flow of electron pair starts gradually from the γ - Carbon atom. This is because first the electron pair is pulled from α - Carbon atom by 'X' as result it becomes δ + which in turn pulls electrons pair from β - Carbon atom which becomes δ δ +. This in turn pulls electron pair from γ - Carbon atoms which in turn becomes δ δ δ +, δ +, δ δ + and δ δ δ + denotes decrease is in magnitude of +ve charge. Thus the inductive effect falls rapidly as the distance from the functional group increases.
In a nut shell, the Inductive effect is
  1. Shown by molecules containing - bond
  2. Permanent
  3. Take place under the influence of substituent, linked to the terminal C - atom.
  4. The polarity, caused by the C -atom linked to the substituent is a small fraction δ+ or δ−.

Hyper Conjugation

The mechanism of electron release by an alkyl group when it is attached to unsaturated system is called Hyper conjugation.

Hyper conjugation effect takes place through the interaction of σ - electrons of C - H bond with π - electrons of = bond.

Let us consider CH2 - CH2(Ethyl cation) in which the positively charged carbon atom has an empty p-orbital. One of the C - H bonds of methyl group can align in the plane of this empty p-orbital and the electron constituting the C - H bond in plane with this p-orbital can then be delocalised into empty p-orbital as follows

It is interesting to note that CH3+ lacks hyper conjugative stability as the C - H bonds lie in nodal plane of the vacant 2p orbital and hence cannot overlap it. (CH3)2 C+ has greater conjugation interaction than CH3CH2+ as the former has no C -H bonds. 

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