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Electronic Effects Affecting a Covalent Bond

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  • Things to remember

Inductive effect

It involves (sigma) electrons. The electrons which form a covalent bond are seldom shared equally between the two atoms. This is because of different electronegative values, that is the different power of attracting the electrons in the bond, i.e a more electronegative atom acquires a small negative charge (δ-) and the less electronegative atom acquires a small positive charge (δ+).

Consider the carbon-chlorine bond, as chlorine(3.0) is more electronegative, it will become negatively charged with respect to the carbon atom (2.5).

a

The inductive effect (I-effect) refers to the polarity produced in a molecule as a result of higher electronegativity of one atom compared to another. An inductive effect is transmitted along a chain of carbon atoms, although it tends to be insignificant beyond the second carbon. It is a permanent effect.

Types of inductive effect -

(a) -I effect: It is due to the presence of electron withdrawing group. For example - NO2, F, C6H5, Cl e.t.c.

-NO2 > -CN > -COOH > F > Cl > Br > I } Decreasing order of -I effect.

(b) +I effect: It is due to the presence of electron donating group or pumping group. For example-

R3C- (3º alkyl) > R2CH (2ºalkyl) > RCH2 (1ºalkyl) > R (alkyl) } decreasing order of +I effect.

  • Application of inductive effect:

Trichloroacetic acid (trichloro ethanoic acid) is more acidic than ethanoic acid.

a

In the case of trichloro ethanoic acid, there is three electron withdrawing group (Cl), which causes less electron density in acid due to -I effect and it loses a proton (H+) easily. So, it is stronger acid. But in the case of ethanoic acid, there is one electron donating group (-CH3), which causes more electron density due to +I effect and it loses proton with difficulty. So, it is less acidic than trichloro ethanoic acid,

Mesomeric effect

It involves π electrons of the double or triple bond (multiple bonds).

M- effect refers to the polarity produced in a molecule as a result of interaction between two π bonds or a π bond and lone pair of electrons.

It is also a permanent effect. It also transmits along a chain of carbon atoms similar to inductive effect.

Consider a carbonyl group (C=O), in which oxygen atom is more electronegative than the carbon atom. As a result, the π electrons of the carbon, the double bond of oxygen gets displaced forward. This gives the following structure.

a

The mesomeric effect is represented by a curved arrow. The head of the arrow indicates the movement of a pair of π electrons.

There are two types of mesomeric effect -

(a) +M effect: Atoms which lose electrons towards a carbon atom are said to have a +M effect. For example - Cl, -Br, OH etc.

source: www.cliffsnotes.com
source: www.cliffsnotes.com

(b) -M effect: Those atoms or group which draws electrons away from a carbon atom are said to have a -M effect. For example- NO2, C≡N, -CO etc.

It is also a permanent effect.

Electromagnetic effect

Electromagnetic effect refers to the polarity produced in a multiple bonded compound as it is approached by a reagent (electrophile or nucleophile).

When a double or a triple bond is exposed to an attack by an electrophile E+ (a reagent) the two π electrons which form the π bond are completely transferred to one atom or the other. The electrometric effect is represented as -

a

The curved arrow shows the displacement of the electron pair. 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 gains a negative charge.

Thus, the electrometric effect is a temporary effect. It takes place only in the presence of a reagent.

Reference:

Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.

Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).



  • Inductive effect  involves (sigma) electrons. The electrons which form a covalent bond are seldom shared equally between the two atoms. 
  • -I effect is due to the presence of electron withdrawing group. 
  • +I effect is due to the presence of electron donating group or pumping group.
  • Mesomeric effect involves π electrons of the double or triple bond (multiple bonds).
  • Atoms which lose electrons towards a carbon atom are said to have a +M effect. 
  • Those atoms or group which draws electrons away from a carbon atom are said to have a -M effect.
  • Electromagnetic effect refers to the polarity produced in a multiple bonded compound as it is approached by a reagent (electrophile or nucleophile),
.

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