Electronic Effects Affecting a Covalent Bond

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).

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.

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.

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.

(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 -

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).