Inductive effect

In

It is present in a σ (sigma) bond, unlike the electromeric effect which is present in a π (pi) bond.

The

.

Bond polarization

, where the more electronegative atoms has a fractional negative charge (δ^–) and the less electronegative atom has a fractional positive charge (δ⁺).

For example, the water molecule

results in a net dipole moment for the molecule. A polar bond is a covalent bond in which there is a separation of charge between one end and the other - in other words in which one end is slightly positive and the other slightly negative. Examples include most covalent bonds. The hydrogen-chlorine bond in HCl or the hydrogen-oxygen bonds in water are typical.

Inductive effect

The effect of the sigma electron displacement towards the more electronegative atom by which one end becomes positively charged and the other end negatively charged is known as the inductive effect. The -I effect is a permanent effect & generally represented by an arrow on the bond.^{[citation needed]}

However, some groups, such as the alkyl group, are less electron-withdrawing than hydrogen and are therefore considered as electron-releasing/ electron-donating groups. This is electron-releasing character and is indicated by the +I effect. In short, alkyl groups tend to give electrons, leading to the induction effect. However, such an effect has been questioned.^[2]

As the induced change in polarity is less than the original polarity, the inductive effect rapidly dies out and is significant only over a short distance. Moreover, the inductive effect is permanent but feeble since it involves the shift of strongly held σ-bond electrons and other stronger factors may overshadow this effect.

Relative inductive effects

Relative inductive effects have been experimentally measured through the resulting pK_as of a nearby carboxylic acid group (see § Carboxylic acids). In increasing order of -I effect or decreasing order of +I effect, common functional groups are:^{[citation needed]}

–NH₃⁺ > –NO₂ > –SO₂R > –CN > –SO₃H > –CHO > –COR > –COOH > –COCl > -CONH₂ > –F > –Cl > –Br > –I > –OH  > -OR > -NR₂ > –NH₂ > –C₆H₅ > –CH=CH₂ > –H.

Hydrogen subsituents also exhibit an isotope effect: relative to the same order,

–T > –D  > –H,

where H is hydrogen, D deuterium, and T tritium.

The strength of inductive effect is also dependent on the distance between the substituent group and the main group that react; the longer the distance, the weaker the effect.

Inductive effects can be expressed quantitatively through the Hammett equation, which describes the relationship between reaction rates and equilibrium constants with respect to substituent.

Fragmentation

The inductive effect can be used to determine the stability of a molecule depending on the charge present on the atom and the groups bonded to the atom. For example, if an atom has a positive charge and is attached to a -I group its charge becomes 'amplified' and the molecule becomes more unstable. Similarly, if an atom has a negative charge and is attached to a +I group its charge becomes 'amplified' and the molecule becomes more unstable. In contrast, if an atom has a negative charge and is attached to a -I group its charge becomes 'de-amplified' and the molecule becomes more stable than if the I-effect was not taken into consideration. Similarly, if an atom has a positive charge and is attached to a +I group its charge becomes 'de-amplified' and the molecule becomes more stable than if the I-effect was not taken into consideration. The explanation for the above is given by the fact that more charge on an atom decreases stability and less charge on an atom increases stability.

Acidity and basicity

The inductive effect also plays a vital role in deciding the acidity and basicity of a molecule. Groups having +I effect (Inductive effect) attached to a molecule increases the overall electron density on the molecule and the molecule is able to donate electrons, making it basic. Similarly, groups having -I effect attached to a molecule decreases the overall electron density on the molecule making it electron deficient which results in its acidity. As the number of -I groups attached to a molecule increases, its acidity increases; as the number of +I groups on a molecule increases, its basicity increases.

Applications

Carboxylic acids

The

drops.

In acids, the electron-releasing inductive effect of the alkyl group increases the electron density on oxygen and thus hinders the breaking of the O-H bond, which consequently reduces the ionization. Due to its greater ionization, formic acid (pK_a=3.74) is stronger than acetic acid (pK_a=4.76). Monochloroacetic acid (pK_a=2.82), though, is stronger than formic acid, due to the electron-withdrawing effect of chlorine promoting ionization.

In

positions can enhance the acid strength.

Since the

are, in general, stronger acids than their monocarboxyl analogues. The Inductive effect will also help in polarization of a bond making certain carbon atom or other atom positions.

Comparison between inductive effect and electromeric effect

Inductive Effect	Electromeric Effect
The polarization of a single σ covalent bond due to the electronegativity difference.	Transfer of shared π-bond electron pairs to one atom under the influence of a strong external field.
Permanent effect.	Temporary effect.
Always observed.	Only observed in the presence of an electrophilic reagent.
Induced charges are partial charges (δ+ or δ−)	Induced charges are integers (+1, -1)

See also

• Mesomeric effect
• Pi backbonding
• Baker–Nathan effect: the observed order in electron-releasing basic substituents is apparently reversed.

References

• Stock, Leon M. (1972). "The origin of the inductive effect". Journal of Chemical Education. 49 (6): 400.
  ISSN 0021-9584
  .

External links

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