mesomeric effect and inductive effect
The mesomeric effect or resonance effect in
chemistry is a property of substituents or functional groups in a chemical
compound. The effect is used in a qualitative way and describes the electron
withdrawing or releasing properties of substituents based on relevant resonance
structures and is symbolized by the letter M. The mesomeric effect is negative
(-M) when the substituent is an electron-withdrawing group and the effect is
positive (+M) when based on resonance and the substituent is an electron
releasing group.
Examples of -M substituents: acetyl (IUPAC
ethanoyl) - nitrile - nitro
Examples of +M substituents: alcohol -
amine-benzene
The net electron flow from or to the substituent is
determined also by the inductive effect. The mesomeric effect as a result of
p-orbital overlap (resonance) has absolutely no effect on this inductive
effect, as the inductive effect is purely to do with the electronegativity of
the atoms and their topology in the molecule (which atoms are connected to
which).
The concepts of mesomeric effect, mesomerism and
mesomer were introduced by Ingold in 1938 as an alternative to the Pauling's
synonymous concept of resonance.[1] "Mesomerism" in this context is
often encountered in German and French literature but in English literature the
term "resonance" dominates.
Mesomerism in conjugated systems :
Mesomeric effect can be transmitted along any
number of carbon atoms in a conjugated system. This accounts for the resonance
stabilization of the molecule due to delocalization of charge.
The electron withdrawing or releasing effect attributed
to a substituent through delocalization of p or π electrons, which can be
visualized by drawing various canonical forms, is known as mesomeric
effect or resonance effect. It is symbolized by M or R.
Negative
resonance or mesomeric effect (-M or -R): It is
shown by substituents or groups that withdraw electrons by delocalization
mechanism from rest of the molecule and are denoted by -M or -R. The electron
density on rest of the molecular entity is decreased due to this effect.
E.g. -NO2, Carbony group (C=O), -C≡N,
-COOH, -SO3H etc.
Positive
resonance or mesomeric effect (+M or +R): The
groups show positive mesomeric effect when they release electrons to the
rest of the molecule by delocalization. These groups are denoted by +M or +R.
Due to this effect, the electron density on rest of the molecular entity is
increased.
E.g. -OH, -OR, -SH, -SR, -NH2, -NR2 etc.
1) The negative resonance
effect (-R or -M) of carbonyl group is shown below. It withdraws electrons by
delocalization of π electrons and reduces the electron density particularly on
3rd carbon.
2) The negative mesomeric
effect (-R or -M) shown by cyanide group in acrylonitrile is illustrated below.
The electron density on third carbon decreases due to delocalization of π
electrons towards cyanide group.
Because of negative
resonance effect, the above compounds act as good micheal
acceptors.
3) The nitro group, -NO2,
in nitrobenzene shows -M effect due to delocalization of conjugated π electrons
as shown below. Note that the electron density on benzene ring is decreased
particularly on ortho and para positions.
This is the reason
for why nitro group deactivates the benzene ring towards electrophilic
substitution reaction.
4) In phenol, the -OH group
shows +M effect due to delocalization of lone pair on oxygen atom towards the
ring. Thus the electron density on benzene ring is increased particularly on
ortho and para positions.
Hence phenol is more
reactive towards electrophilic substitution reactions. The substitution is
favored more at ortho and para positions.
5) The -NH2 group in aniline also
exhibits +R effect. It releases electrons towards benzene ring through
delocalization. As a result, the electron density on benzene ring increases
particularly at ortho and para positions. Thus aniline activates the ring
towards electrophilic substitution.
It is also worth mentioning
that the electron density on nitrogen in aniline decreases due to
delocalization which is the reason for its less basic strength when compared to
ammonia and alkyl amines.
Inductive effect
In
chemistry and physics, the 'Inductive Effect' is an experimentally observable
effect of the transmission of charge through a chain of atoms in a molecule.
The permanent dipole induced in one bond by another is called inductive 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
atom 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 and is indicated by
the +I effect. In short, alkyl groups tend to give electrons, leading to
induction effect.
As the induced change in polarity is less
than the original polarity, the inductive effect rapidly dies out. Therefore,
the effect 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.
Relative inductive effects have been
experimentally measured with reference to hydrogen:
(Decreasing order of - I effect or
increasing order of + I effect)
—NR3 > —NO2 > —SO2R > —CN >
—COOH > —F > —Cl > —Br > —I > —OR > —COR > —OH > —C6H5 > —CH=CH2 > —H
Also the inductive effect is dependent on
the distance between the substituent group and the main group that react. That
is, as the distance of the substituent group increases the Inductive effect
weakens or decreases.
Inductive effects can be measured through
the Hammett equation.
The inductive effect can also be used to
determine the stability of a molecule depending on the charge present on the
atom and the groups bonded to it. 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. But, contrary to the above two cases, 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 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
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.
The inductive effect also plays a vital role
in deciding the acidity and basicity of a molecule. Groups having +I 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.
Aliphatic
carboxylic acids. The
strength of a carboxylic acid depends on the extent of its ionization: the more
ionized it is, the stronger it is. As an acid becomes stronger, the numerical
value of its pKa drops. 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 (pKa=3.75) stronger than acetic acid (pKa=4.76).
Monochloroacetic acid (pKa=2.82), though, is stronger than formic acid, since
the electron-withdrawing effect of chlorine promotes ionization.
Aromatic
carboxylic acids. In
benzoic acid, the carbon atoms which are present in the ring are sp2
hybridised.As a result, benzoic acid(pKa=4.20) is a stronger acid than
cyclohexane carboxylic acid(pKa=4.87). Also, electron-withdrawing groups
substituted at the ortho and para positions, enhance the acid strength.
Dioic
acids. Since
the carboxyl group is itself an electron-withdrawing group, the dioic acids
are, in general, stronger than their monocarboxyl analogues.
In the so-called Baker–Nathan effect the observed order
in electron-releasing alkyl substituents is apparently reversed.
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