Inductive Effect

The inductive effect refers to the phenomenon wherein a permanent dipole arises in a given molecule due to the unequal sharing of the bonding electrons in the molecule. This effect can arise in sigma bonds, whereas the electromeric effect can only arise in pi bonds.

What Is the Inductive Effect?

When an electron-releasing or an electron-withdrawing species is introduced to a chain of atoms (generally a carbon chain), the corresponding negative or positive charge is relayed through the carbon chain by the atoms belonging to it. This causes a permanent dipole to arise in the molecule and is referred to as the inductive effect.

An illustration describing the inductive effect that arises in a chloroethane molecule due to the more electronegative chlorine atom is provided above.

Inductive Effect on Acidity and Basicity

Using the inductive effect, we can predict the acidity and basicity of compounds. As a generalisation, it may be said that the electron-withdrawing groups (EWG) increase the acidity of a compound, and the electron-donating group decrease the acidity of a compound.

This is because, if we take the conjugate base of the acid, that is, RCOO-, if R is electron-withdrawing, then the conjugate base is stabilised via delocalisation of the formed negative charge.

If R had been electron-donating, then the conjugate base would have been destabilised because of inter-electronic repulsions.

Thus, it can be said that +I groups decrease acidity (or increase basicity) and –I groups increase the acidity (or decrease basicity) of compounds.

For example, formic acid (HCOOH) is more acidic than acetic acid (CH3COOH) due to the +I inductive effect of the methyl group attached to the carboxylic acid group.

Note: If the Ka of acid is high, it is a strong acid, but if the PKa of acid is high, it is said to be a weak acid [pka = -log(ka)]. The same logic applies to bases.

Consider the acidity of mono-, di- and trichloroacetic acid.

It can be said that the presence of three Cl atoms makes oxygen highly electron deficient and, thereby, polarising the O-H bond the most. Therefore, the acidity order for the above compounds would be III > II > I.

Types of Inductive Effect

·        Negative inductive effect or -I effect

·        Positive inductive effect +I effect

-I Effect (Negative Inductive Effect)

When an electronegative atom, such as a halogen, is introduced to a chain of atoms (generally, carbon atoms), the resulting unequal sharing of electrons generates a positive charge which is transmitted through the chain.

This causes a permanent dipole to arise in the molecule wherein the electronegative atom holds a negative charge, and the corresponding effect is called the electron-withdrawing inductive effect or the -I effect.

+I Effect (Positive Inductive Effect)

When a chemical species with the tendency to release or donate electrons, such as an alkyl group, is introduced to a carbon chain, the charge is relayed through the chain, and this effect is called the positive inductive effect or the +I effect.

Inductive Effect on Stability of Molecules

The charge on a given atom and the charge on a group bonded to the atom plays a strong part when determining the stability of the resulting molecule as per the inductive effect.

An example of this can be observed when a group displaying the -I effect is bonded to a positively charged atom, and the positive charge on the resulting molecule is amplified, reducing its stability.

On the other hand, when a negatively charged atom is introduced to a group displaying a -I effect, the charge disparity is somewhat quenched, and the resulting molecule would be stable as per the inductive effect.

Also,

When a group displaying the -I effect is bonded to a molecule, the electron density of the resulting molecule effectively reduces, making it more likely to accept electrons and, thereby, increasing the acidity of the molecule.

When a +I group attaches itself to a molecule, there is an increase in the electron density of the molecule. This increases the basicity of the molecule since it is now more capable of donating electrons.

Applications of the Inductive Effect

Illustration 1:

Give the stability of the following canonical forms.

Structures I and III have more covalent bonds and are more stable than II and IV. Between I and III, I is more stable because the negative charge is on an electronegative element.

Between II and IV, II is more stable because of the same reason as said above.

The order is I > III > II > IV

Illustration 2:

We know that EWG increases acidity and EDG decreases acidity.

-Me group is a +I group, whereas -OMe is an +R group, so –OMe decreases the acidity more strongly than -Me.

Therefore, the order is, d>c>e>a>b

Since ka is directly proportional to acidity, the answer is a→ t, b →p, c→ s, d→ q, e→ r.

Illustration 3:

Solution: NaNH2 is a base; therefore, the most acidic proton of the substrate would react to form a conjugate base. The idea here is to find out the most acidic proton.

There are totally four protons, -COOH, -OH, nitro-substituted –OH and alkyne proton.

Since two moles of the base are used, two protons would react.

The order of acidity of the protons are

-COOH>-OH (Nitro substituted)>-OH> acetylenic proton

So the product would be,

Illustration 4: The order of acidity of the following compounds is

Solution: To find out the acidity of the compounds, remove the proton and check the stability of the conjugate base formed.

CB of structures I and II are stabilised by intramolecular hydrogen bonding (I more than II).

Between the meta and para isomer, meta would be more acidic due to the –I effect of oxygen.

Therefore, the order is I>II>III>IV

Illustration 5:

Solution: The most basic among the four is I. This is because structures II and IV are aromatic. Between I and III, I is more basic due to the presence of an oxygen atom in III, which decreases basicity by the –I effect.

Between II and IV, II would be more basic because, in IV, the lone pair on nitrogen is delocalised to make the compound aromatic. The non-availability of the lone pair for donation makes IV the least basic.

Therefore, the order is I > III > II > IV.

Inductive Effect vs Electromeric Effect

A tabular column highlighting the key differences between the electromeric and the inductive effects can be found below.

 

 

 

 

Inductive Effect	Electromeric Effect
Works on sigma bonds	Works on pi bonds
The inductive effect is permanent	The electromeric effect is a temporary effect
It doesn’t require any attacking reagent	An electrophilic attacking reagent is required for this effect to arise

 

Thus, it can be understood that the +I and -I effect play a vital role in the stability, as well as the acidity or basicity of molecules.

How to Check the Acidity of Organic and Unsaturated Compounds?

To check the acidity of an organic compound, remove the proton and then check the stability of the resulting conjugate base so formed. More the stability of the conjugate base, the stronger the acid.

To check for acidity among unsaturated compounds, check the hybridization of the carbon involved. The more the s-character on the carbon, the more its electronegativity, and hence, more the acidity.

Therefore, the most acidic amongst alkynes, alkenes and alkanes is Alkynes > Alkenes > Alkanes

If there is competition among two groups that are electron withdrawing via resonance and via induction, preference is given to the resonance because it affects the whole molecule.

Print or Download this article: Inductive Effect