Inductive effect and field effect in organic chemistry

What is inductive effect ?

When a hetero atom of different electronegativities is attached to the ending carbon of a carbon   chain , then a permanent displacement of covalently bonded electrons  occurs  toward the more electronegative atom, which create a permanent polarization in molecule.

So,inductive effect  may be defined as  the  permanent but partial displacement of electron along the bond in a molecule actually toward the more electronegative atom due to electronegative difference of the atom forming the covalent bond.

                                                               n-propyl chloride

 

This inductive effect is sometimes reffered to as a transmission effect, since it takes place by a displacement of the intervening electrons in the molecule.

For example, when Cl –atom  is attached to the ending carbon of a carbon chain , the displacement of electron of C- Cl bond occurs toward the more electronegative chlorine atom .

 Again, if Li atom is attached to the ending carbon of a carbon chain , the displacement of electron of C- Li bond occurs toward the carbon atom .

In both cases, a permanent polarity arises in the molecules are as follows,

Influence of Inductive effect on physical and chemical properties of a molecule.

As a result of inductive effect there are many physical and chemical properties  of organic molecule changes. For example, strength of acid and base , dipole moment etc changes due to inductive effect.

What is field effect ?

There is another effect almost similar to inductive effect which operate either through the space surrounding the molecule or in solution through solvent molecule ,is called field effect.

However, in many cases field effect can not be distinguish from inductive effect.

Why  dipole moment of cis-2-butene is greater than trans-2-butene?

Dipole moment of cis-2-butene is greater than trans-2-butene due to inductive effect of methyl group which acts in the same direction for cis-2-butene but for trans it acts in opposite direction.

Why formic acid is more stronger than acetic acid ?

The strength of acid depends on the stability of anion of that acid. On ionization, formic acid gives formate anion and acetic acid gives acetate anion. Both the formate and acetate anion have two equivalent resonating structure .

 But in case of acetic acid , methyl  group has +I effect . As a result , the density of electron on oxygen atom increases. Hence, the stability of acetate anion is less than formate anion.

Consequently, the formic acid is more stronger than acetic acid.

Why chloro acetic acid is more stronger than acetic acid?

The strength of acid depends on the stability of anion of that acid. On ionization, acetic  acid gives  acetate  anion and chloro acetic acid gives chloro acetate anion. 

Both the acetate  and chloro acetate anion have two equivalent resonating structure . But in case of  chloro acetic acid , chlorine atom  has -I effect . 

As a result , the density of electron on oxygen atom decreases. Hence, the stability of chloro acetate anion  is greater  than  acetate  anion.

Consequently, the chloro acetic acid  is more stronger than acetic acid.

Why methyl amine is stronger base  than ammonia ?

The strength of base depends on the electron donating power of the concerned  base. In case of methyl amine, the density of electron on N-atom increases due to the +I effect of methyl group and a repulsive force arises among the electronic charge.

 As a result of this, the tendency for donating of electron of methyl amine is increases. Hence,  basicity of methyl amine increases.

For the above reason, methyl amine is stronger base than ammonia.

Compare the basicity of primary , secondary and tertiary amine  in aprotic solvent.

The strength of basicity of primary, secondary and tertiary amine  depends on their  electron donating power. Primary amine,secondary amine and tertiary amine  contain one ,two and three alkyl group respectively. 

Now, alkyl groups have  +I  effect . So the density of electron on N-atom is highest for tertiary amine and lowest for primary amine and hence the repulsive force among the electronic charge is the same order. 

Therefore, the tendency for donating of electron of tertiary amine is greater than the primary and secondary amine.

Consequently , in aprotic solvent the basicity order of primary , secondary and tertiary amine are as follows,  

Compare the basicity of primary , secondary and tertiary amine  in aqueous medium.

The basicity of primary , secondary and tertiary amine  in aqueous solution depends on the stability of their respective the conjugate acid . 

Now, the stability of conjugate acid of different amine depends on two factors , one is inductive effect of alkyl group  and the another is hydrogen bonding. 

Considering this two factor ,it has been experimentally found that the conjugate acid of secondary  amine is more stable  than the other two.

Hence, in aqueous solution  the secondary amine behave as a stronger base than primary and tertiary.

Depending on stability of conjugate acid , the basicity order of primary , secondary and tertiary amine  in aqueous medium are as follows,

  

Practice problem:

Why chloramine is weak base than ammonia ? 
What is inductive effect ? give example .
What is field effect ?

Click here: Geometrical or cis-trans isomerism