Chapter 1:- Structure and Reactivity 1st year Book
Organic Chemistry
(Page 6)

TYPES OF ELECTRONIC DISPLACEMENTS – Inductive Effect & Electromeric Effect

Inductive Effect :

The electron cloud in a σ-bond between two unlike atoms is not uniform. It is denser towards the more electronegative of the two atoms e.g. the electron pair forming the σ – bond is slightly displaced towards X when substituent X is either a more electronegative atom or an electron-attracting group. Atom X thus acquires a slight negative charge (δ−) and the carbon atom has a slightly positive charge (δ+). This effect is denoted by the symbol (→−), of which the arrowhead is pointed towards the highly electronegative atom. i.e. the bond is polarised. If the electronegative atom is joined to a chain of carbon atoms, the positive charge on the carbon atom is relayed to the other carbon atom as shown below :
Since C1 is slightly positively charged it exerts a pull on the electrons forming a covalent bond between C1 and  C2 but less strongly than X on C1. The effect thus diminishes rapidly with distance. Hence, the effect is not significant beyond the second carbon atom. Thus, shifting of σ-bond pair electron through a covalent bond either from the substrate to the substituent or vice-versa is known as an “Inductive effect“. It is a permanent state of polarization represented as ‘ I ‘ which may be either −I or +I. The electron-withdrawing character is indicated by the −1 effect and the electron-releasing character by the +1 effect. The effect is additive, the greater the number of electron-withdrawing groups the stronger the effect. Relative inductive effects have been measured with reference to hydrogen. The order of the electron-withdrawing effect is :
 Electron withdrawing groups ⟷—————-⟷ Electron releasing groups  
  ( −I effect)                                             (+ I-effect) 
The alkyl groups are less electron withdrawing than hydrogen and are therefore considered as electron releasing.

Significance: Inductive effect is useful in correlating structure with reactivity.

(a) Acid strength of aliphatic carboxylic acids :

The strength of an acid depends on the extent of its ionization, the greater the extent of ionization the stronger the acid. The strength of an acid is denoted by the numerical value of pKa (pKa is equal to −log10⁡ Ka, where Ka is the acidity constant) Smaller the numerical value of pKa stronger the acid. In acetic acid the electron -releasing. inductive effect of the methyl group hinders the breaking of the O−H bond. Consequently, reduces the ionization However, this effect is absent in formic acid.
Greater ionization in formic acid over acetic acid makes formic acid (pKa=3.77) stronger than acetic acid (pKa=4.76). Monochloroacetic acid (pKa=2.86) is a stronger acid than formic acid since the −I effect of chlorine promotes ionization. As this effect is additive trichloroacetic acid (pKa=0.66) is still stronger acid than the above three acids.
When an unsaturated carbon is conjugated with the carbonyl group the acid strength is increased. This is because with the increasing s-contribution to the hybrid orbitals the electrons are progressively drawn closer to the nucleus of the carbon resulting in the increase in the −I effect. Since the s-contributions in sp,sp2, and sp3 orbitals are respectively 50%, 33.3%, and 25%, thus the effect of hybrid orbitals is sp>sp2>sp3. This is evident from the pKa values of the following acids.
propanoic acid(4.88)
propenoic acid(4.25)
propynoic acid(1.84)

(b) Dioic acids :

Since the – COOH group is itself an electron-withdrawing group, the dioic acids are in general stronger than their monocarbonyl analogues. For example:
formic acid(3.77)
oxalic acid (1.23)
acetic acid (4.76)
malonic acid (2.83)
The electron-withdrawing effect of one COOH group over the other falls off sharply on separating the two carbonyl groups by at least two saturated carbons.

(C) Aliphatic bases :

The strength of nitrogenous bases depends on the ease of availability of the lone pair of electrons on the nitrogen atom to the proton (H+). Due to the increasing +l effect in amines, the order of base strength should be:
In the gaseous phase, the order of basicity of the amines is :
but in the aqueous medium the order of basicity of these amines is :
However, the pKa values of these amines are as follows :
Base: NH3 MeNH2 Me2NH Me3N
pK a value: 9.25 10.64 10.77 9.80
The pKa value for the base B: is a measure of the acid strength of its conjugate acid B:H Stronger the acid B:H weaker is the base B: In other words smaller the numerical value of pKa for the acid B:H the weaker is the base B: From the pKaa values, it is seen that 2∘ amine(10.77) is a stronger base than 3∘ amines (9.80). This is because the base strength of an amine in water depends not only on the ease of availability of lone pair but also on the extent of solvation of protonated amine by hydrogen bonding. The protonated 3∘ amine has one, protonated 2∘ amine has two and ammonia has three hydrogens on nitrogen for hydrogen bouncing.

Primary amine has more solvation with less +1, tertiary amine has minimum solvation and maximum +1 while ammonia still has maximum solvation but no +1 effect. Hence, 2∘ amine is a stronger base than 3∘ amines. Solvation is an important factor for determining the base strength. This is supported by the fact that the order of base strength of amines is 2>1>3>NH3.

Hydrogen bonding in 3∘ amine, Hydrogen bonding in 1∘ amine, Hydrogen bonding in 2∘ amine, Hydrogen bonding in ammonia

Electromeric effect

On the close approach of a reagent, the electronic system of an unsaturated molecule is deformed. When the reagent is removed without allowing the reaction to take place the electronic system reverts to the original ground state of the molecule. This kind of polarizability of multiple bonds is known as the ‘electromeric effect‘; E. The electronic effect causes a complete transfer of the loose π – electrons from one carbon atom to the other. Consequently, one end is positively charged and the other negatively charged which supports the reagent to attack. The shift of the electrons is shown by a curved arrow (↷) indicating the direction of the electron shift.

electromeric effectThis effect is temporary because observed at the demand of attacking reagents like E+, Nu−, and free radicals since the electrons revert to the original state upon removing the reagent. These addition and substitution reactions are examples of the electromeric effect.

When the multiple bonds are between two dissimilar elements the shift of elections takes place towards the more electronegative of the two. When the involved atom or group withdraws electrons towards itself the electromeric change is known as the −E effect and the repulsion of electrons is the +E effect. Ingold has worked out the electromeric effect of some groups which some of which are given below-
It has been shown that a carbonyl group is represented as a resonance hybrid of the forms  and II where both contribute quite effectively.
When a proton-like species comes nearer to such a system it disturbs the ground state electrons distribution and more effective polarisation will take place. This electron displacement may be superimposed on the resonance hybrid and thus the electromeric shift simply refers to the facility with which the electronic system of an unsaturated molecule is disturbed. As the electromeric change takes place on the demand of the attacking reagent., the electron displacement follows that path which facilitates the chemical reaction. It has normally been observed that both resonance and electromeric changes operate in the same direction. In the presence of the +R effect, a strong electromeric force will operate if the attacking reagent desires so but when the −E effect is the need of the reagent then with the permanently existing +R effect and −E will be negligible or totally absent.
In addition to the above typical example of a carbonyl group, there are many systems where electromeric displacements play quite an important role. Some examples are-

1 – A carbon-carbon double-bond system (ethylene) :

electron-distribution, electromeric effect

2- A carbon-carbon conjugated system (butadiene) :

3- A carbon-halogen conjugate system (Halogen substituted ethylene) :
As expected when we move across a row in the periodic table the ease of electromeric shift goes on decreasing and in this respect it is comparable to single bond availability.

Difference between inductive effect and electromeric effect:

The inductive effect differs from the electromeric effect as follows :
Inductive Effect  Electromeric Effect
1. This is a permanent effect operating in polar covalent molecules.

2. This Effect may be defined as a process of electron shift along a chain of atoms due to the presence in it of a polar covalent bond. This effect is also called as “Transmission Effect” and in short I or T.

3. This effect is shown by the symbol (–>–), the arrowhead pointing towards the more electronegative heteroatom.

4. The electron pair is partially displaced from a less electronegative atom to a highly electronegative atom. That is why δ+ charge develops on less electronegative C-atom to highly electronegative.

eg. C4δδδδ+–>–C3δδδ+–>–C2δδ+–>–C1δ+–>–Clδ-


1. This is a temporary effect operating only at the demand of attacking reagents.

2. The complete transfer of the shared pair of Π-electrons of multiple bonds to the more electronegative bonded atom due to the requirement of an attacking reagent is called the “Electromeric Effect” or E, effect.

3. The curved (↓) arrow shows the displacement of pair of electrons, the arrowhead pointing towards the atom that gains the electron pair.

4. If in an unsaturated AB molecule, the atom ‘A’ has lost its share in the electron pair and B has gained this share. As a result, ‘A’ acquires a positive charge and ‘B’ a negative charge.

A — BΘ




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