Dr Anju K. Gupta Associate Professor Department of A.N. College, Patna Lecture note for B.Sc. (I) Subsidiary

Inductive Effect

If a is formed between having similar , the bonding will be shared equally between the two atoms. For example, in , the two hydrogen atoms have identical . Therefore, the 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.

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 . This causes polarization in the bond in which the more electronegative atom chlorine acquires a partial positive 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:

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Dr Anju K. Gupta Associate Professor Department of Chemistry A.N. College, Patna Lecture note for B.Sc. (I) Subsidiary

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. 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.

The groups producing inductive effect can be classified as electron-withdrawing (-I) or electron-donating (+I) groups relative to hydrogen. Any atom or group which attracts electrons more strongly than hydrogen is said to have negative inductive effect (-I effect). On the other hand, atoms or groups which attract electrons less strongly than hydrogen are said to have positive inductive effect (+I effect).

Examples of –I groups arranged in decreasing order of negative inductive effect:

Examples of +I groups arranged in decreasing order of positive inductive effect:

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