Inductive Effect

ELECTRONIC DISPLACEMENT IN COVALENT BONDS

The following four types of electronic effects operates in covalent bonds

(i) Inductive effect (ii) Electromeric effect

(iii) Resonance and mesomeric effect (iv) Hyperconjugation

Inductive Effect (Polar Nature Of Covalent Bonds)

The displacement of an electron (shared) pair along the carbon chain due to the presence of an electron withdrawing or electron releasing groups in the carbon chain is known as inductive effect (I – effect).

$\bullet$ It is a permanent effect which is transmitted along the chain.

C……> ……C…>….C……>G (G – Functional group)

$\bullet$ This permanent polarity is due to electron displacement due to difference in electronegativities.

$\bullet$ This effect weakens steadily with increasing distance from the substitution

(electron – withdrawing or electron – donating group) and actually diminishes down after three carbon atoms.

There are two types of inductive effect i.e. – I effect and +I effect.

Negative Inductive effect ($-$ I Effect)

If the substituent attached to the end of the carbon chain is electron withdrawing (X). The effect is called – I effect.

$-$I effect decreases as one goes away from groups (electron attacking)

C₁($\delta$+) > C₂($\delta$$\delta$+) > C₃ ($\delta$$\delta$$\delta$+) and other third carbon charge is negligible.

$-$I effect is in order.

NO₂ > F > COOH > Cl > Br > I > OH > C₆H₅

Due to.- effect (electron – with drawing nature) electron density decreases, hence basic nature is decreased and acidic nature is increased.

Chloroacetic acid is stronger than acetic acid since Cl shows (-I) effect, electron – density is decreased and O – H bond is weakens causing ionisation of (-COOH) to a greater extent than $C{H}_{3}COOH.$

$N{H}_3$is a base due to lone pair on nitrogen. Phenyl group is electron – withdrawing. What happens to electron – density of nitrogen in aniline, Naturally electron – density is decreased. Hence aniline is weaker base than$N{H}_3$.

Similarly acidic nature of phenol is greater than $H_{2}O$due to electron – withdrawing nature of phenyl group.

Positive inductive effect (+I effect):

If the substituent attached to the end of the carbon chain is electron – donating, the effect is called +I effect.

This is due to electron – releasing (Y). It develops a negative charge on the chain.

+I effect also decreases as we go away from group Y (electron – releasing)

$C_1(\delta -)>C_2(\delta \delta -)>C_3(\delta \delta \delta -)$

So +I effect is in the order of

${(C{H}_3)}_{3}C->{(C{H}_3)}_{2}CH->C{H}_{3}C{H}_2->C{H}_3-$

Due to electron – releasing group electron density is increased, hence basic nature is also increased and naturally acidic nature is decreased, thus

Note:

Inductive effect is a permanent effect operating in the ground state of the organic molecules and hence is responsible for high melting point, boiling point and dipole moment of polar compounds.

Illustration 1. Take the following isomeric $\alpha ,\beta ,\gamma$ chlorobutyric acid

Arrange the acids increasing order of acid strength.

Solution: As we have stated, as we go away from the source, electron - withdrawing tendency decreases, hence acidic nature also decrease. Thus

(III) < (II) < (I)

Illustration 2. Which of the following carbonyl compound is more acidic?

Solution. (b) The acidity of $\alpha$-hydrogen depends on +ve charge on the carbon atom to which it is attached. In an aldehyde the carbonyl carbon has more positive charge and hence more –I effect.