Question

-7. MODULE 2 ANALYTICAL METHODS AND SEPARATION TECHNIQUES 2. (a) Explain the origin of infrared (IR) absorption by compounds. 13 marks] (b) State the properties of compounds which absorb IR radiation. [2 marks]

Answer 1

2(a) Infrared spectroscopy involves the interaction of infrared radiation with matter. it can be used to identify and study the chemical compound. Infrared spectroscopy exploits the fact that molecules absorb frequencies that are characteristic of their structure. These absorptions occur at resonant frequencies, i.e. the frequency of the absorbed radiation matches the vibrational frequency. The energies are affected by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling.Absorbing groups in the infrared region absorb within a certain wavelength region. The absorption peaks within this region are usually sharper when compared with absorption peaks from the ultraviolet and visible regions. In this way, IR spectroscopy can be very sensitive to determination of functional groups within a sample since different functional group absorbs different particular frequency of IR radiation. Also, each molecule has a characteristic spectrum often referred to as the fingerprint. A molecule can be identified by comparing its absorption peak to a data bank of spectra. IR spectroscopy is very useful in the identification and structure analysis of a variety of substances, including both organic and inorganic compounds. It can also be used for both qualitative and quantitative analysis of complex mixtures of similar compounds.

(b)

1.   It can experience a change in dipole moment, whether it is induced or permanent
2. It should have resonant frequencies that are in the infrared frequency range of 100−4000 cm^−1.
3. It should be not overly soluble in a nonpolar solvent.

(e)

(i) The retention factor, or Rf, is defined as the distance traveled by the compound divided by the distance traveled by the solvent

Rf  value of component A = x/z= 2.4/6.8 = 0.3529

Rf  value of component B= y/z= 5.1/6.8 = 0.75

(ii)

• The difference in Rf values is due to the difference in polarity of components A and B.

The compound with the larger Rf is less polar because it interacts less strongly with the polar adsorbent on the TLC plate

• The difference in mobility of the components A and B
• Size difference of the component A and B

(iii) Rf value can be affected by a number of different factors such as layer thickness, moisture on the TLC plate, vessel saturation, temperature, depth of mobile phase, nature of the TLC plate, sample size, and solvent parameters.