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Inductive Effect

If the atoms in a covalent bond have similar electronegativity (similar attractions for electrons), the bonding electrons are shared equally. Consider, for example, the hydrogen and chlorine molecule. Since two hydrogen atoms or two chlorine atoms obviously have identical electronegativities, the electron pair is shared equally and the molecular orbital (σ bond) is symmetrically distributed.

There is no positive and negative end. Such a bond is said to be a non-polar bond.

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However, in the covalent bond between hydrogen and chlorine, the electron pair is not shared equally because chlorine has a greater electronegativity than hydrogen and therefore, the electron pair shifts towards the more electronegative atom. This causes polarisation in the bond. The more electronegative atom acquires a partial negative charge (δ) and the counterpart acquires a partial positive charge (δ+). This type of bond, which involves the unequal sharing of electron pair between two different atoms of different electronegativities, is called polar covalent bond. Polar covalent bond is intermediate between an ionic and covalent bond. In that, although one atom has a greater share of the paired electrons than the other, the electrons are not actually transferred, a contrast to the ionic bond. The symbol δ (delta) is used to signify that only partial charges are formed and not full ions in contrast to ionic compounds.

A polar covalent bond can be represented as:

Definition

An inductive effect is an electronic effect due to the polarisation of σ bonds within a molecule or ion. This is typically due to an electronegativity difference between the atoms at either end of the bond. The more electronegative atom pulls the electrons in the bond towards itself creating some bond polarity for example the O-H and C-Cl bonds in the following examples:

This effect is a permanent effect and decreases rapidly as the distance from the source (X) increases. It is important to note that the electron pairs, although permanently displaced, remain in the same orbital. Inductive effect operates through σ bonds.

Types of Inductive Effect

There are two types of Inductive effect.

Negative Inductive Effect

Any atom or group which attracts electrons more strongly than hydrogen is said to have negative inductive effect (-I effect). Following groups have been arranged in decreasing order of negative inductive effect:

I groups are electron-withdrawing groups

Positive Inductive Effect

The atoms or groups which attract electrons less strongly than hydrogen are said to have positive inductive effect (+I effect). The following groups have been arranged in decreasing order of +I effect :

+I groups are electron-donating groups.

Applications of Inductive Effect

i. Reactivity of Compounds

The presence of say a halogen atom in the molecule of alkyl halide creates a centre of low electron density on adjacent carbon. Such a carbon is readily attacked by the negatively charged reagents.

ii. Creation of Dipole Moment

As the inductive effect increases, the dipole moment increases. This is because the dipole moment is the product of distance and charge. Greater the power of the group to attract the shared pair of electrons, greater the dipole moment. The unit of dipole moment is Debye (D).

iii. Acidity of Carboxylic Acids (II) is approximately 100 times stronger acid than (I).

The decreasing order of acid strength for some of derivatives of CH3COOH is shown:

                                                                  Cl3CCOOH ˃ Cl2CHCOOH ˃ ClCH2COOH ˃ CH3COOH iv. Relative Acid Strength of Fluoro, Chloro, Bromo and Iodoacetic Acid

The acidity increases with the increase in the electronegativity of the halogen present which helps in the release of protons. Thus, the strength of halogenated acids follows the order.

v. Distance of Halogen Atom

Inductive effect decreases with the increase in distance of halogen atom from the carboxylic group, hence the strength of halogenated acids follows the order:

vi. Relative Acid Strength of Formic Acid and Acetic Acid

Methyl group has an electron releasing inductive effect (+I effect). It reduces the release of protons from –O–H group. Therefore, acetic acid is a weaker acid than formic acid.

vii. Comparison of Water, Phenol and Ethanol in Acid Strength Phenyl group has – I effect while methyl group has +I effect. So the order is;

Phenol is more acidic than water because of losing an H+ ion, it forms a phenoxide ion which is stabilized by resonance, while if water loses H +, it forms hydroxide, which is not stabilized by resonance. In methanol, methyl group is electron donating; it is creating more negative charge on oxygen, so it is difficult to remove H+.

viii. Rate of Nucleophilic Addition

The electron withdrawing – I effect decreases the electron availability on carbonyl carbon and therefore increases the rate of nucleophilic addition. –CCl3 has much greater – I effect than –H and –CH3 groups. Consider the following carbonyl compounds.

ix. Basic Strength of Amines

Due to + I effect of alkyl groups , the N- atom becomes strongly electronegative, so lone pair of electrons on N – atom in amines is more easily available than in NH3. Hence amines are stronger base than Ammonia. The relative basic character of amines is not as follows-

t- amine > sec. amine > pri. amine > NH3

Due to steric effect the correct order of decreasing basic character of amines is:

x. Relative Stabilities of Carbocations

Greater the number of alkyl groups attached to positive carbon, greater the dispersal of charge and hence greater is the stability.

xi. Relative Reactivity of Toulene and Benzene in Aromatic Substitution

Alkyl group has an electron releasing inductive effect or +I effect. Therefore, toluene with higher electron density than benzene and has greater reactivity in electrophilic substitution reactions.

xii. Relative Reactivity of Nitrobenzene and Benzene in Electrophilic Aromatic Substitution Reactions

–NO2 is strongly electron withdrawing group. It decreases the electron availability on ring and decreases the reactivity for benzene ring for the attack of an electrophile.

Field Effect

The field effect arises due to the presence of polar bonds in a molecule and it is transmitted not through bonds but through the environment (space) around the molecules. It is often very difficult to measure the inductive and field effects separately. However, it has been done in many cases, because the field effect depends on the geometry of the molecule but the inductive effect depends only on the nature of the bonds.

The distance of two Cl-atoms from –COOH group through the bonds is same in (1) and (2). We expect the same inductive effect with respect to the position of electrons associated with –COOH group. So, the two compounds should exhibit the same acid strength. When the chlorines are pointed over the carboxylic acid group in (1), the pKa is higher because loss of a proton is less favorable due to the increase in negative charge in the area. Loss of a proton results in a negative charge which is less stable if there is already an inherent concentration of electrons. This can be attributed to a field effect because in (2) the same compound with the chlorines pointed away from the acidic group, the pKa is lower so (2) is stronger acid than (1).

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