In IR spectrum, recorded neat, a compound shows a strong and broad band at 3300 cm−1 . The band becomes sharp and shifts to 3600 cm−1 when the spectrum is recorded in CCl 4 at high dilution. This proves that the compound has
IR Spectrum and OH Group Identification
The question describes the behavior of a specific absorption band in the infrared (IR) spectrum of a compound. The band is observed at approximately 3300 cm\(^{-1}\) when the compound is measured neat (as a liquid or solid without solvent) and shifts to about 3600 cm\(^{-1}\) and becomes sharp when measured in CCl\(_4\) solvent at high dilution.
Understanding IR Band Changes with Dilution
IR spectroscopy is a powerful tool for identifying functional groups in a molecule. The stretching vibration of the O-H bond typically appears in the region of 3200-3650 cm\(^{-1}\).
- A broad band in the 3200-3400 cm\(^{-1}\) range is characteristic of O-H groups involved in hydrogen bonding. Hydrogen bonding weakens the O-H bond and lowers its stretching frequency. The broadness arises from the variety of hydrogen-bonded states present.
- A sharp band in the 3600-3650 cm\(^{-1}\) range is characteristic of "free" O-H groups, which are not involved in hydrogen bonding. The O-H bond is stronger when not hydrogen-bonded, resulting in a higher stretching frequency.
The observation that the band changes from broad at 3300 cm\(^{-1}\) (neat) to sharp at 3600 cm\(^{-1}\) (dilute) is crucial.
Hydrogen Bonding: Intermolecular vs Intramolecular
Hydrogen bonding can occur in two main ways:
- Intermolecular hydrogen bonding: This occurs between two different molecules. In the neat liquid state, molecules are close together, allowing for extensive intermolecular hydrogen bonding.
- Intramolecular hydrogen bonding: This occurs within the same molecule. This is possible when a molecule contains both a hydrogen bond donor (like O-H) and an acceptor within a suitable distance and orientation.
Now let's consider the effect of dilution in an inert solvent like CCl\(_4\):
- If the hydrogen bonding is intermolecular, adding a large amount of inert solvent separates the molecules. This disrupts the intermolecular hydrogen bonds, leading to a decrease in hydrogen bonding. At high dilution, most O-H groups will be free from intermolecular bonding.
- If the hydrogen bonding is intramolecular, adding solvent does not significantly affect the bonding within the same molecule. The intramolecular hydrogen bond will persist even at high dilution.
Analyzing the Observed Shift
The shift from a broad band at a lower frequency (3300 cm\(^{-1}\)) to a sharp band at a higher frequency (3600 cm\(^{-1}\)) upon dilution indicates that the hydrogen bonding present in the neat liquid is being broken by the solvent. This disruption of hydrogen bonding results in the formation of free O-H groups.
This behavior is consistent with the presence of intermolecular hydrogen bonding. In the neat state, intermolecular H-bonds are abundant, causing the O-H stretch to appear broad and at lower frequency. Upon high dilution, these intermolecular H-bonds are broken, and the O-H groups primarily exist as free oscillators, resulting in a sharp band at higher frequency.
If the hydrogen bonding were intramolecular, the band position and shape would not change significantly upon dilution, as the internal bond is not affected by separating the molecules.
Therefore, the observed change strongly proves that the compound has an O-H group which is involved in intermolecular hydrogen bonding.
Important Questions from IR Spectroscopy
In an IR spectra of 1-octyne and 4-octyne, the IR-spectra of 4-octyne does not have C ≡ C stretch absorption peak. The reason for this observation is that _________.
An IR-spectra is found to have a medium adsorption peak near 3400cm-1. This corresponds to which organic compound?
The number of CO bands for isomers from sets (i) and (ii) in their IR spectra
Set (i): Trigonal bipyramidal isomers, axial‐Fe(CO)4L (A)and equatorial‐Fe(CO) 4 L(B)
Set (ii): Octahedral isomers, fac‐Mo(CO)3L3 (C) and mer‐Mo(CO)3L3(D)
are
Match List I with List II
List I
List II
functional groups
respective approximate symmetric and asymmetric stretching frequencies
A.
N - H bonds of R - NH2
I.
1790 and 1810
B.
N - O bonds of R - NO2
II.
3300 and 3400
C.
C = O bonds of anhydride
III.
1350 and 1550
Choose the correct answer from the options given below:
The molecule that can absorb in the infra-red among the following is
