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 _________.
4-octyne has symmetrically substituted triple bond while 1-octyne does not
Understanding IR Spectra of Octynes
Infrared (IR) spectroscopy is a technique used to identify functional groups in a molecule by analyzing the absorption of infrared light. Molecular vibrations, such as stretching and bending of bonds, absorb specific frequencies of IR light. For a vibration to be IR active and show a peak in the spectrum, it must cause a change in the dipole moment of the molecule.
Analyzing the Structure of 1-Octyne and 4-Octyne
Let's look at the structures of 1-octyne and 4-octyne:
- 1-Octyne: CH${_3}$CH${_2}$CH${_2}$CH${_2}$CH${_2}$CH${_2}$C$\equiv$CH. This is a terminal alkyne, meaning the triple bond is at the end of the carbon chain. The carbon atoms involved in the triple bond are attached to an alkyl chain on one side and a hydrogen atom on the other.
- 4-Octyne: CH${_3}$CH${_2}$CH${_2}$C$\equiv$CCH${_2}$CH${_2}$CH${_3}$. This is an internal alkyne, meaning the triple bond is located within the carbon chain. The carbon atoms involved in the triple bond are each attached to alkyl chains (propyl groups) of approximately similar size and electron-donating ability.
IR Activity of C$\equiv$C Stretch
The stretching vibration of the C$\equiv$C triple bond typically gives rise to an absorption peak in the IR spectrum in the range of 2100-2260 cm${^{-1}}$. However, the intensity of this peak depends on whether the vibration causes a significant change in the molecule's dipole moment.
- 1-Octyne: In 1-octyne, the triple bond is between a carbon atom bonded to an alkyl group and a carbon atom bonded to a hydrogen atom. The alkyl group and the hydrogen atom have different electronic properties, making the C$\equiv$C bond somewhat polar. The stretching vibration causes a change in this polarity and, therefore, a change in the dipole moment. This results in a visible absorption peak for the C$\equiv$C stretch in the IR spectrum of 1-octyne.
- 4-Octyne: In 4-octyne, the triple bond is located symmetrically within the carbon chain. The molecule has a degree of symmetry around the triple bond axis. When the C$\equiv$C bond stretches symmetrically, the dipole moment of the molecule does not change significantly (ideally, if perfectly symmetrical, the change is zero). Because there is little to no change in dipole moment during the vibration, the C$\equiv$C stretch is IR inactive or very weak in the spectrum of 4-octyne.
Therefore, the absence of the C$\equiv$C stretch absorption peak in the IR spectrum of 4-octyne, compared to 1-octyne, is due to the symmetrical substitution of the triple bond in 4-octyne, which results in no significant change in dipole moment during the vibration, unlike the asymmetrically substituted triple bond in 1-octyne.
Important Questions from IR Spectroscopy
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
Consider the following statements about Infrared (IR) spectroscopy.
A. It is used to determine the band gap, the band structure, and the charge carrier concentration of a compound.
B. It is used to identify the functional group(s) of a compound.
C. It is used to characterize different stretching and bending modes of vibration in molecules.
D. Heteronuclear diatomic molecules are IR-active.
The correct statements are
