Following the footsteps of Gilbert N. Lewis in understanding the nature of the covalent bond, Linus N. Pauling stumbled upon an interesting observation. Pauling noticed that the nature of the chemical bond could be explained using a scale or continuum.
A permanent dipole is an inherent feature of the molecule due to the nature of the participating atoms forming the two-electron covalent bond.
Some atoms withdraw more of the bond electrons towards themselves, increasing their electron density and leaving the other end electron deficient. This unequal electron charge distribution creates positive and negative poles (denoted with δ+, δ- signs), causing the molecule to have permanent poles or a dipole.
The redistribution of electrons in an atom, bond, or molecule creates two ends (or poles), one electron-rich negative and the other electron-deficient positive; such an atom, bond, or molecule is said to have a dipole (two poles).
Inductive Effect | Resonance/Mesomeric Effect |
|---|---|
The sigma (σ) or the covalent bond electrons are pulled or pushed so that the electron density shifts towards the most electronegative atom. | The pie (π) electrons are pulled and pushed through the p-orbitals along the sigma (covalent) bond framework. |
It is a permanent effect. | It is a permanent effect. |
The atoms or group of atoms are classified based on the Inductive effect as electron-withdrawing (-I) or electron-donating (+I) relative to Hydrogen.
The common functional groups showing +I and -I effect are:
+I group | -I group |
|---|---|
O- |
Inductive, along with resonance effects, are permanent effects.
The atoms or groups of atoms causing the inductive effect are part of the molecule. Depending on the atom's nature (electron-withdrawing or donating), the groups causing the molecule's inductive effect can impact the molecule's stability and physical and chemical properties.
The Inductive effect is the outcome of Electronegativity; therefore, they are different and not the same.
The +I effect is an electron-donating (or an electron-pushing) inductive effect by an atom or group of atoms relative to Hydrogen. It means that a +I group like methyl (-CH3) will push electrons away from itself more than the hydrogen atom would if it occupied the exact position in the molecule.
The push of electrons is represented with an arrow over the bond, (>) indicating the direction of the electron flow.
Pre-Requisite Reading: Lewis Structures, Types of reactions, Using curly arrows for electron movement, Identifying Functional groups.
Pre-Requisite Reading: Resonance
Electronegativity measures on a scale of 0.8 – 4 an atom’s or group of atoms’ tendency to attract the bond electron pair towards itself, thereby creating partial negative (δ-) and positive (δ+) terminals.
An atom or group of atoms that can pull the bond electrons towards itself or push the bond electrons from itself and decreasingly transmit the effect along the sigma (σ) bonds of the carbon chain inducing permanent polarization in the molecule. Such an effect is called the Inductive effect.
Resonance theory explains various observed properties in a molecule using the electron delocalization concept and multiple Lewis structures, which a single Lewis structure cannot.
A single Lewis structure can only describe some but not all of a molecule's observed properties. Resonance theory is helpful in molecules that can be expressed using several Lewis structures, like Benzene or CO2.
Pre-Requisite Reading: What is electronegativity?
Carbon and Hydrogen do not have any significant electronegativity difference (< 0.5) and can form long chains of neutral hydrocarbon compounds like Butane, Pentane, Hexane, Heptane, Octane, etc.
Sharing resources is essential to build a harmonious world. When the resources are shared ineffectively conflicts emerges. A similar principle extends to Chemistry.
Atoms are in a state of harmony when the neighbouring atoms shares electrons. When the sharing stops, Chemical reactions triggers.
The imbalance in sharing electrons influences the molecule’s polarity, reactivity, and physical properties. The disagreement on sharing is brought about by electronegativity.
Pre-Requisite Reading: Electronegativity
A covalent bond is formed between two similar or dissimilar atoms (Ex: Carbon-Carbon and Carbon-Halogen).
The replacement of the hydrogen in acetic acid (H-CH2-COOH) with the chloro gives chloroacetic acid (Cl-CH2-COOH). Therefore, Acetic acid and Chloroacetic acid have carboxylic acid (R-COOH) as the principal functional group.
The dipole moment, a product of charge difference (q) and the distance (d) between the centres of positive and negative charges (µ = q x d), is also directional. The direction is indicated by an arrowhead, which points towards the most electronegative atom, representing the direction of the dipole moment.