Inductive Effect

 

The phenomenon wherein a permanent dipole is induced in a bond by another, directly along a chain of atoms within a molecule, is termed as the inductive effect. This effect may also manifest through space via solvent molecules or ions, commonly referred to as field effects. The inductive effect, a permanent manifestation in the ground state of a molecule, is presumed to operate through single bonds, earning it the designation of permanent polarization of single bonds.

An illustrative example is found in the C1-Li bond within the chain C4-C3-C2-C1-Li, where the permanent dipole arises due to the greater electronegativity of C compared to Li. Consequently, C1 carries a δ- charge, inducing a permanent dipole in the C2-C1 bond, rendering C2 slightly negatively charged (δδ-), though less than C1. This induction of negative charge propagates down the chain, diminishing with distance from the initiating dipole, as depicted:

C4--<--C3--<--C2--<--C1--<--Li

δδδδ-  δδδ-      δδ-       δ-        δ+

Where the arrow indicate the electron displacements in the bonds and

δ- > δδ- > δδδ- > δδδδ-

The inductive effect proves significant up to the second atom (C3) but diminishes beyond the fourth atom. Another example involving the chain C4-C3-C2-C1-F demonstrates induction of positive charge due to the higher electronegativity of F compared to C. The representation is as follows:

C4-->--C3-->--C2-->--C1-->--F

δδδδ+ δδδ+     δδ+     δ+        δ-

Where δ+ > δδ+ > δδδ+ > δδδδ+

Most of the atoms or groups of atoms like F, Cl, Br, OH, etc. draw bonded electrons toward themselves, i.e. away from the carbon chain; these are known as electron withdrawing groups. And the effect is called electron withdrawing effects which are symbolically represented as – I effect. Metals, silicon and alkyl groups repeal electron and are known as electron releasing or electron repealing group; the effect is called electron releasing effect and its symbolic representation is +I. To measure the relative inductive effect of an atom or group hydrogen atom of the molecule R₃C-H has been taken as standard if an atom or group attracts electrons relative to that hydrogen atom it is said to possess a -I effect and if it push electrons relative to that hydrogen it is said to have a + I effect. Some +I and -I groups are arranged below in the order of decreasing inductive effect.

+I groups: O^-, CO₂^-, CR₃, CHR₂, CH₂R, CH₃, D

-I groups: ⁺NR₃, ⁺SR₂, ⁺NH₃, NO₂, SO₂R, CN, COOH, F, Cl, Br, I, Oar, OR, OH, Ar

The inductive effects may be directly transmitted through space or solvent molecules rather than along a chain. Such effects are often called the field effects. These are long range polar interactions. Both the inductive and field effects operate in the same direction. It is therefore, difficult to separate them. However, an attempt has been made in this respect by taking advantage of the fact that the field effect depends on the geometry of the molecule but the inductive effect depends only on the nature of the bonds. For example, in the isomeric dichloroacids 1 and 2, the inductive effect of the chlorine atoms on the position of electrons in the -COOH group and therefore on acidity should be same since the same bond intervene; but the field effect disfavours the dissociation of 1 but favours the dissociation of 2 because Cl atoms are closer in space to the -COOH group in 1 than they are in 2 and because of this, 1 is less acidic than 2.

As an example for inductive effect we can say that chloroacetic acid is more acidic than acetic acid while 2-chlorobutanoic acid is more acidic than 3-chlorobutanoic acid which in turn is more acidic than 4-chlorobutanoic acid. Also for an acetic acid if we substitute the α-hydrogen with more than one chlorine atom the pKa value further reduces which dichloro or trichloro acetic acid more acidic than chloro acetic acid. In chloroacetic acid, the electronegative Cl atom exerts its electron withdrawing inductive effect (-I) and causes displacement of O-H bonding electrons towards oxygen as a result the O-H dissociates readily to give H⁺. In acetic acid the methyl group exerts its +I effect and disables the release of proton by increasing the electron density on oxygen atom and thereby making the O-H bond strong. Chloroacetic acid is therefore a stronger acid than acetic acid.

 This fact may also be explained in terms of stability of the conjugate bases of these acids. The more the conjugate base is stable, the more the acid will be acidic. The electron-attracting Cl atom disperses the negative charge of the conjugate base of chloroacetic acid and thereby stabilized it. In acetic acid, on the other hand, the negative charge of the carboxylate ion is intensified by the electron donating CH₃ group and so the anion is destabilized. It does follows that chloroacetic acid which yields a stable conjugate base is more acidic than acetic acid.

The inductive effect decreases rapidly as the distance from the source increase in 2-, 3- and 4- cholorobutanoic acid the electron attracting Cl atom gradually moves away from the -COOH group because of this the anions become progressively less stable and the acid become progressively less acidic.

The introduction of chlorine atom at the carbon α to the -COOH group causes and increase in acidity because the electron withdrawing chlorine atom stabilizers the carboxylate and Ion by dispersing the negative charge with increasing the number of Chlorine atoms at the Alpha carbon the strength of - I effect progressively increases and so also the acid strength.

Let us look at more examples; Methylamine is more basic than trifluoromethylamine whereas the tertiary amine (CF₃)₃N has practically no basic character. The availability of unshared pair of electrons or nitrogen or protonation determines the basic strength of nitrogenous compounds. The –CH₃ group in methylamine pushes electrons towards the nitrogen atom and makes the lone pair of electrons on nitrogen easily available in methylamine. On the other hand, the electron withdrawing (-I) fluorine atoms pull the electrons from nitrogen through carbon and there by decreases the availability of the unshared pair considerably. This explains why methylamine is more basic than trifluoromethylamine.

 The difference in basic strength may also be explained on the basis of the stability of the ammonium ion the conjugate acid of the event.

The more the ammonium ion is stable the more is the tendency of the amine to take up a proton, i.e., the more the amine is basic. The electron-releasing –CH₃ group in methylamine stabilizers the corresponding ammonium ion by spreading the positive charge, whereas the electron withdrawing –CF₃ group in tetrafluoromethylamine destabilizes the corresponding ammonium ion by intensifying the positive charge and so, methylamine is more basic than trifluoromethylamine.

Due to the presence of three strongly withdrawing –CF₃ groups in the availability of an shared pair of electrons or nitrogen is decreased markedly and because of this, tetrafluoromethylamine practically shows no basic character.

The electrons in an s orbital are closer to the carbon nucleus than the p electrons. Therefore, the greater s character of a hybrid orbital has, the greater are the bonding electrons are drawn closer to the carbon nucleus, i.e., the more electronegative is that carbon atom. The sp, sp² and sp³ hybrid orbitals possess 50%, 33.3%, 25% s character, respectively. Consequently, sp hybridised carbon in acetylene is more electronegative than the sp² hybridized carbon in ethylene, which in turn is more electronegative than the sp³ hybridized carbon in ethane. Hence, release will be progressively favoured in ethane, ethylene, and acetylene, i.e., the order of acidity is in this case CH≡CH> CH₂=CH₂> CH₃CH₃. This order may also be explained on the basis of the stability of the carbanion, i.e., the conjugate base of carbon acid. The greater the carbanion is stable, the greater the hydrocarbon is acidic. An increase in the amount of s character of the hybrid bonding orbital occupied by the unshared electron pair causes an increase in carbanion stability and that is due to the fact that the electron pair in an orbital having good s character is held more tightly by the nucleus and hence of lower energy. Thus, order of stability of the corresponding carbanion is HC≡C^- > CH₂=CH^- > CH₃-CH₂^- and therefore, the order of acidity is CH≡CH (pKa = 25) > CH₂=CH₂ (pKa = 36) > CH₃-CH₃.(pKa = 42).

So, so from this order we can also observe the order of electronegativity for various hybridized carbon and that is Csp > Csp² > Csp³. The α-carbons of propanoic, propenoic, and propynoic acids are sp³, sp² and sp hybridised respectively. So the conjugate base of propynoic acid is more stable than the conjugate base of propenoic acid, which in turn is more stable than the conjugate base of propanoic acid.

Understanding the inductive effect is essential for predicting and explaining the behavior, stability of organic molecules, acidity and basicity of certain compounds etc. Therefore, this is one of the foundation  for understanding organic chemistry.

Reference

1) Jonathan Clayden, Nick Greeves, Stuart Warren, organic chemistry book second edition.