Q.2. (b) Explain frequency shift due to inductive and mesomeric effects in IR spectroscopy
(b)Frequency shift due to inductive: The inductive effects solely depends upon the ‘intrinsic’ tendency of a substituent to either release or withdraw electrons- by definition, its electronegativity acting either through the molecular chain or through space. This effect usually weakens steadily with increasing distance from the substituent
The inductive effects shall now be discussed specifically with regard to the various functional moieties such as: amides, acyl chlorides, alkyl esters and aryl esters:
Amides:(q), the +M effect causes weakening of the C=O bond, leading to the corresponding ketone (p). In this particular instance, the –I effect of nitrogen is being dominated by +M effect.
Acyl chlorides: (r), the –I effect of CI is more effective than +M effect, thereby causing an opposite shift (to higher frequency).
Alkyl esters: (x), it has been observed that a conflict between I and M effects invariably takes place in the case of esters. Here, the non-bonding electrons residing on oxygen enhance the +M conjugation thereby decreasing the C=O frequency.
Aryl esters: (y), Here the non-bonding electrons located on oxygen are partially drawn into the benzene ring and thereafter their conjugation with C=O is minimized. The net effect would be that –I effect of oxygen becomes dominant and consequently C=O moves to a higher frequency.
Field effects: It has been observed that two functional groups often influence each other’s vibrational frequencies by a through-space interaction that may be either steric and/or electrostatic in nature. A typical example of ortho-chlorobenzoic acid esters is shown below:
In the above instance, the field effect shifts the C=O frequency in the rotational isomer (k) and not in the isomer (1). As both isomers are usually found to be present together, therefore, two C=O str. absorptions are observed in the spectrum of this compound.
Frequency shift due to Mesomeric (or Resonance) Effect: Whenever a molecule can be represented by two or more structures that differ only in the arrangement of electrons- that is, by structures that have the same arrangement of atomic nuclei- there is resonance. It may be further explained with the help of the following typical examples:
-OMe: Electron Releasing Moiety
-NO2: Electron withdrawing Moiety
In (b) above, the presence of a phenyl ring increases the mesomeric shift thereby lowering C=O str. frequency.
+M Group: p-OMe (an electron releasing function)- its presence as depicted in (c) above will further lower frequencies due to enhanced mesomeric effect.
-M Group: p-NO2 (an electron withdrawing function)- its presence as shown in (d) above will further increase frequencies due to decreased mesomeric effect.
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