What is IR Spectrum of Anisole?

Interpretation of anisole using IR spectrum obtained from IR analysis.

IR Spectroscopy

Infrared spectroscopy for purified substances is performed to functional groups present. Infra-Red (IR), as observed in UV spectroscopy, does not contain enough energy to lead to electronic conversion. The electromagnetic energy in a molecule in the IR-region absorbs vibrational or rotating changes resulting in production of net changes in the molecular dipole moment. IR-active compounds include HCl and CO. IR bandwidth ranges from${\text{4000 cm}}^{\text{-1 }}{\text{to 400 cm}}^{\text{-1}}$.

IR spectroscopy depending on Hooke's Law, make the assumption that when a bond connects two atoms or particles, thus the vibration frequency may be described as follows:

Where, 𝜅 is bond force constant, ν is the frequency, 𝜇 is reduce mass where m1 and m2 are the atom masses

 Fundamental Vibration

The vibrations occur as the particle is transferred between ground level to further lower level. The basic frequencies for linear or non-linear compounds are calculated by the following formulas:

Linear molecules will have degree of freedom as $3\text{n}-5$, whereas, non-linear molecules will have degree of freedom as $3\text{n}-6$, where “n” is given as the number of atoms in the molecule.

The fundamental vibrations can of various types as follows:

• Stretching vibration: Proximity among two atoms increases and decreases, yet the angle of connection is still fixed.
• Types of stretching vibrations
  • Symmetric stretching vibration: In this case both the atoms are stretched or compressed in the matching path.
  • Asymmetric stretching vibration: In this kind of vibration only one atom undertakes stretching while the other atom experiences compression.

• Bending vibrations: Proximity connecting two atoms persists, but the angle of the connection varies.

Types of bending vibrations

• In plane bending vibrations:
  • Scissoring: Just like scissors both the atom will shift towards each other.
  • Rocking: Both the atoms will shift in one similar direction, that is, either to the left side or to the right side.

• Out of plane bending vibrations:
  • Wagging: In this vibration, both the atom will move up and down keeping the central atom permanent.
  • Twisting: In this vibration, one atom moves up and another atom moves down with a stable central atom.

Group Frequencies

Detailed information on infrared absorption for general functional groups can be viewed in the given tabular column. The following table can be used as reference for the most frequent functional groups a data collection.

Aromatic Ethers

There are two types of aromatic ethers. One is phenolic ethers and other is diaryl ethers. These aromatic ethers can be distinguished from alkyl ethers using IR spectroscopy. The diaryl and phenolic ethers show IR absorption at .

Phenolic ethers

Usually, the phenolic ethers were prepared by heating an alkyl halide with sodium phenoxide in the presence of ethanol. Another method is by treating the phenol with alkyl sulfate in the presence of an alkali. The two major phenolic ethers with many applications are anisole and phenetole. Anisole is also known as methyl phenyl ether or methoxy benzene. Phenetole is also known as ethyl phenyl ether or ethoxy benzene. Anisole and phenetole are unaffected by most of the acids and alkalis.

Anisole

Anisole is a colorless liquid with an anise seed scent, and many of its derivatives can be found in natural and synthetic scents. The molecule is primarily synthesized and serves as a precursor to other synthetic chemicals. It's a form of ether.

Anisole reacts quicker than benzene, which reacts quicker than nitrobenzene in electrophilic aromatic substitution processes. Because the methoxy group is an ortho/para directing group, electrophilic substitution happens more commonly at these three positions. The impact of the methoxy group, which makes the ring more electron-rich, is evident in anisole's increased nucleophilicity over benzene. Despite the oxygen's electronegativity, the methoxy group has a greater impact on the ring's pi cloud as a mesmeric electron donor than as an inductive electron withdrawing group.

Applications of Anisole

 Anisole is an important precursor in many industrial productions. Some of them are given below;

• Pharmaceutical applications.
• Precursor in perfume industry.
• Precursor in insect pheromone synthesis.
• Trinitro anisole synthesis.

Structure of Anisole

Anisole is often referred to as methoxybenzene. It has a molecular formula${\text{C}}_{\text{7}}{\text{H}}_{\text{8}}\text{O}$. It has IUPAC nomenclature as methyl phenyl ether. At room temperature, it is in a liquid state. It has an aromatic smell and hence is utilized as imitation aromas and fragrances. Anisole has five $\text{sp² C-H}$ links between ${\text{2960 and 2838 cm}}^{-1}$, three $\text{C = C}$ aromatic linkages at ${\text{1600-1500 cm}}^{-1}$, one $\text{C-O}$ linkage at $1249{\text{ cm}}^{-1}$, one aliphatic $\text{C-O}$ bond at $1100{\text{cm}}^{-1}$and also three ${\text{sp}}^{\text{3}}\text{ C-H}$ aromatic linkages. Therefore, the associated frequencies in its FT-IR spectra should be used as reference and examined for analysis.

Infrared Spectroscopy of Anisole

Since anisole is a fluid at room temperature, the FT-IR range is observed across two KBr sheets. This approach is known as neat liquid since there is no extra solvent involved in the process.

The $\text{C-O}$ bonds have two pinnacles, one because of the asymmetrical $\text{C -O - C}$ at approximately $1250{\text{ cm}}^{-1}$, and the other at $~1040{\text{ cm}}^{-1}$ because of symmetrical stretch. Asymmetric bending is stronger since the double bonding characteristic increases owing of the resonance. The double bond feature gives increased bonding strength and so improves the elongation frequency.

Rules for Infrared Spectroscopy

1. The first rule is to locate the high end of wavenumber$\left({\text{>1500cm}}^{-\text{1}}\right)$in the spectrum and focus on all the major bands that are visible.

2. The correlation table as reference should be prepared for each of the band that has been shortlisted.

3. To validate or develop putative structural features, employ the lower wave-number edge of the spectral range.

4. Avoid the expectations to label each and every peak in an obtained spectrum.

5. Keep various articles and references for 'cross-checking' wherever required.

6. All the positive and negative results which are viewed should be explored cautiously.

7. There might be considerable consideration regarding band strengths. They can differ greatly in the very same cluster under specific situations.

8. One should be alert while viewing small wavenumber changes as few bands may be sensitive towards certain solvent.

9. And lastly once the band is measured remember to subtract the bands observed due to solvent used.

Applications of Infra-red spectroscopy

Infra-red spectroscopy has the following applications:

• Distinct functional group classification can be done.
• Intermolecular and intramolecular hydrogen binding differentiation can be done.
• Chemical purity is identified, and extra bands in the IR spectra are noticed where the component is impure.
• Cis and trans isomers of a compound can also be identified.

Common Mistakes

The most common mistake while studying infrared spectra of anisole is not illustrating the $\text{C-H}$ region correctly which fails to determine what type of compound such as alkane, alkene, aromatic, or mixed is present in the given sample compound.

Context and Applications

This topic is useful for Bachelors and Masters in Chemistry.