Electronic Effects-I Inductive Effect If a covalent bond is formed between atoms having similar electronegativity, the bonding electrons will be shared equally between the two atoms. For example, in hydrogen molecule, the two hydrogen atoms have identical electronegativities. Therefore, the electron pair is shared equally between the two hydrogen atoms and so the molecular orbital (σ-bond) will be symmetrically distributed. Similarly, if we consider a chlorine molecule, the molecular orbital (σ- bond) formed between the two identical chlorine atoms is symmetrically distributed. This type of bond is called a non-polar covalent bond. 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 1 H-Cl However, if we consider a hydrogen chloride molecule, the electron pair is not equally shared between hydrogen and chlorine because chlorine has a greater electronegativity than hydrogen. Therefore, the electron pair shifts towards the more electronegative atom. This causes polarization in the bond in which the more electronegative atom chlorine acquires a partial negative charge. This type of bond in which unequal sharing of electron pair between two atoms of different electronegativities are involved is called a polar covalent bond. The atom which has a greater share of the paired electrons is given a symbol δ- and the atom with a smaller share is given a symbol δ+. Thus, a polar covalent bond can be represented as: 2 3 Consider a chain of carbon atoms such as C4—C3—C2— C1—X with an atom X at the terminal having higher electronegativity than carbon. A positive charge (δ+) will appear on C1 which will not remain localized. Since C1 is positively charged, it will attract the bonding electron pair between C1 and C2. Hence, C2 will also become positively charged but the magnitude of positive charged on C2 will be less than C1. Similarly, C3 will also acquire a positive charge whose magnitude will be smaller than that on C2. This effect will be transmitted through out the length of carbon chain. Therefor it is termed as Inductive Effect. This type of electron displacement along a chain of σ – covalent bonds due to the presence of a polar bond is known as Inductive Effect. This effect is a permanent effect and decreases rapidly as the distance from the source increases. 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 4 acidity of a compound and electron-donating group decrease the acidity of a compound. Thus, it can be said that, +I groups decrease acidity (or increase basicity) and –I groups increase 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. 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. 5 If R had been electron-donating, then the conjugate base would be destabilised because of inter-electronic repulsions. 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 6 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 play 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 7 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. 8 .