Question

define polarisation in bond

Answer 1

A polar bond is a covalent bond in which there is a separation of charge between one end and the other - in other words in which one end is slightly positive and the other slightly negative. Examples include most covalent bonds. The hydrogen-chlorine bond in HCl or the hydrogen-oxygen bonds in water are typical.

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Answer 2

From our previous discussions, you all know that a covalent bond is formed by equal sharing of electrons between the bonding atoms. However, this requires the two centers sharing the electrons to possess opposite charges. This is made possible by electron displacement that is caused by certain effects. Some of these effects are permanent while others are temporary.


The effects that remain permanently within the molecule are termed as polarization effects and are permanent in nature. They include the inductive effect and the mesomeric effect. The temporary effects which are seen only in the presence of an attacking reagent and disappear with the removal of the attacking reagent are termed as polarisability effects. These temporary effects include the electromeric and the inductomeric effects. Let us discuss about inductive effect in this session.It is an effect that causes permanent displacement of electrons involved in covalent bond formation towards a more electronegative element.


For example, formation of a covalent bond between a carbon atom and the halogens which are highly electronegative causes the electrons involved in the covalent bond formation to be pushed towards the halogens. This imparts a small negative charge on the halogen and small positive charge on carbon. Presence of the small negative and positive charges on these atomic centers makes them liable for attack by other charged groups or atoms.


On the other hand, if a covalent bond is formed between carbon and an atom that is electropositive in nature, then the electrons involved in the bond formation will be pushed towards carbon imparting a small negative charge on carbon.


This inductive effect transfers to the next atom in case of a chain of carbon atoms. For example, if four carbon atoms are linked to the highly electronegative atom chlorine, the small positive charge is transferred to all the carbon atoms due to inductive effect.


On the other hand, if a covalent bond is formed between carbon and an atom that is electropositive in nature, then the electrons involved in the bond formation will be pushed towards carbon imparting a small negative charge on carbon.


Cδδδδ+-Cδδδ+-Cδδ+-Cδ+-Cl-δ.


As can be seen, the degree of the positive charge decreases as the distance between the carbon and the electronegative chlorine atom increases.


The inductive effect can be either positive or negative. The –I effect is said to be exerted by atoms that can attract electrons more strongly than hydrogen atoms. Examples of groups capable of exerting negative inductive effect include F, Cl, Br, I, OH, OCH3 etc. These groups are termed as electron attracting or with-drawing groups.


The +I effect is said to be exerted by atoms that attract electrons less strongly than hydrogen. They can be said as electron repelling or electron releasing groups. Examples of groups showing the +I effect include the (CH3)3C , (CH3)2CH, CH3 CH2, CH3.


The reactive of alkyl halides, carboxylic acids and amines can best be explained by the inductive effect exerted by halide(F, Cl, Br and I), carboxylic(COOH) and amino groups.