Inductive effect
The Inductive effect in
chemistry is an experimentally observable effect of the transmission of
charge through a chain of
atoms in a
molecule by
electrostatic induction (IUPAC definition). The net
polar effect exerted by a
substituent is a combination of this inductive effect and the
mesomeric effect.
The
electron cloud in a
σ-bond between two unlike
atoms is not uniform and is slightly displaced towards the more
electronegative of the two atoms. This causes a permanent state of
bond polarization, where the more electronegative atoms has a slight negative charge(δ-) and the other atom has a slight positive charge(δ+).
If the electronegative atom is then joined to a chain of atoms, usually
carbon, the positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect, also known as the
-I effect.
Some groups, such as the
alkyl group are less electron-withdrawing than
hydrogen and are therefore considered as electron-releasing. This is electron releasing character is indicated by the
+I effect.
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. The inductive effect is permanent but feeble, as it involves the shift of strongly held σ-bond electrons, and other stronger factors may overshadow this effect.
The inductive effect may be caused by some
molecules also. Relative inductive effects have been experimentally measured with reference to hydrogen.
|
Relative strengths of the inductive effect of various molecules |
Inductive effects can be measured through the
Hammett equation.
* (a) Aliphatic carboxylic acids
The
strength of an
carboxylic acid depends on the extent of its
ionization-the more the ionization, the stronger is the acid. The strength of an acid is denoted by the numerical value of
pKa.In aliphatic acids, the electron releasing inductive effect of the methyl group, increases the electron density on
oxygen and thus hinders the breaking of the O-H bond, which consequently reduces the ionization.
Greater ionization in
formic acid when compared to
acetic acid makes formic acid(pK
a=3.75) stronger than acetic acid(pK
a=4.76). Monochloroacetic acid(pK
a=2.82) is stronger than formic acid since the electron-withdrawing effect of chlorine promotes ionization.
* (b) Aromatic carboxylic acids
In
benzoic acid, the carbon atoms which are present in the ring are
sp2 hybridised.As a result, benzoic acid(pK
a=4.20) is a stronger acid than
cyclohexane carboxylic acid(pK
a=4.87). Also, electron-withdrawing groups substituted at the
ortho and
para positions, enhance the acid strength.
* (c) Dioic acids
Since the
carboxyl group is itself an electron-withdrawing group, the dioic acids are, in general, stronger than their monocarboxyl analogues.
* IUPAC
Gold Book definition*
Important publications in organic chemistry
