The polarization of a σ bond due to electron withdrawing or electron donating effect of adjacent groups or atoms is called inductive effect.
Salient features of inductive effect
* It arises due to electronegativity difference between two atoms forming a sigma bond.
* It is transmitted through the sigma bonds.
* The magnitude of inductive effect decreases while moving away from the groups causing it.
* It is a permanent effect.
* It influences the chemical and physical properties of compounds.
The C-Cl bond in the butyl chloride, CH3-CH2-CH2-CH2-Cl is polarized due to electronegativity difference. The electrons are withdrawn by the chlorine atom. Thus the first carbon atom gets partial positive charge. In turn, this carbon atom drags electron density partially from the next carbon, which also gets partial positive charge. Thus the inductive effect is transmitted through the carbon chain.
But the inductive effect weakens away along the chain and is not significant beyond 3rd carbon atom.
The inductive effect is divided into two types depending on their strength of electron withdrawing or electron releasing nature with respect to hydrogen.
The electron withdrawing nature of groups or atoms is called as negative inductive effect. It is indicated by -I. Following are the examples of groups in the decreasing order of their -I effect:
NH3+ > NO2 > CN > SO3H > CHO > CO > COOH > COCl > CONH2 > F > Cl > Br > I > OH > OR > NH2 > C6H5 > H
It refers to the electron releasing nature of the groups or atoms and is denoted by +I. Following are the examples of groups in the decreasing order of their +I effect.
C(CH3)3 > CH(CH3)2 > CH2CH3 > CH3 > H
Why alkyl groups are showing positive inductive effect?
Though the C-H bond is practically considered as non-polar, there is partial positive charge on hydrogen atom and partial negative charge on carbon atom. Therefore each hydrogen atom acts as electron donating group. This cumulative donation turns the alkyl moiety into an electron donating group.
The stability of carbonium ions increases with increase in number of alkyl groups due to their +I effect. The alkyl groups release electrons to carbon, bearing positive charge and thus stabilizes the ion.
The order of stability of carbonium ions is :
In the same way the stability of free radicals increases with increase in the number of alkyl groups.
Thus the stability of different free radicals is:
However the stability of carbanions decreases with increase in the number of alkyl groups since the electron donating alkyl groups destabilize the carbanions by increasing the electron density.
Thus the order of stability of carbanions is:
The electron withdrawing groups (-I) decrease the negative charge on the carboxylate ion and thus by stabilizing it. Hence the acidic strength increases when -I groups are present.
However the +I groups decrease the acidic strength.
i) The acidic strength increases with increase in the number of electron withdrawing Fluorine atoms as shown below.
CH3COOH < CH2FCOOH < CHF2COOH < CF3COOH
ii) Formic acid is stronger acid than acetic acid since the –CH3 group destabilizes the carboxylate ion.
On the same lines, the acidic strength of phenols increases when -I groups are present on the ring.
E.g. The p-nitrophenol is stronger acid than phenol since the -NO2 group is a -I group and withdraws electron density. Whereas the para-cresol is weaker acid than phenol since the -CH3 group shows positive (+I) inductive effect.
Therefore the decreasing order of acidic strength is:
The electron donating groups like alkyl groups increase the basic strength of amines whereas the electron withdrawing groups like aryl groups decrease the basic nature. Therefore alkyl amines are stronger Lewi bases than ammonia, whereas aryl amines are weaker than ammonia.
Thus the order of basic strength of alkyl and aryl amines with respect to ammonia is :CH3NH2 > NH3 > C6H5NH2
The +I groups increase the electron density at carbonyl carbon. Hence their reactivity towards nucleophiles decreases. Thus formaldehyde is more reactive than acetaldehyde and acetone towards nucleophilic addition reactions.
Thus the order of reactivity follows:
E.g. The electron withdrawing nature of nitro group, -NO2 is mostly due to resonance effect rather than the inductive effect.
But there are exceptions. For example, in cases of halogens, the negative inductive effect is more dominating than positive resonance effect.
1) The inductive effect can be best described as:
a) the conjugation of σ-bonding orbital with the adjacent π-orbital.
b) the ability of atom or group to cause bond polarization
c) the transfer of lone pair of electrons from more electronegative atom to lesser electronegative atom in a molecule.
d) All of the above.
2) Which of the following statement is incorrect about the inductive effect ?
a) It is a permanent effect
b) It decreases with increases of distance
c) It involves delocalization of π-electrons
d) It involves displacement of σ-electrons
3) Which of the following shows positive (+I) inductive effect?
a) – NO2
4) Which of the following group shows negative (-I) inductive effect ?