Infrared Spectroscopy: Analyse the functional groups of benzoic acid

IR spectroscopy deals with the interaction of infrared radiation with matter.

It is a light with a longer wavelength and lower frequency than visible light.

Typical IR wavelengths range from 8x10-5 cm to 1x10-2 cm, and this corresponds to energies of around 1-10 kcal.

This energy is sufficient to make atoms vibrate, but not enough to cause electronic transitions.

1. 1 INSTRUMENTAL ANALYSIS LAB. Mr Haydar A.M.S BSc, MSc Faculty of Science Chemistry Department Email: [email protected]

2. 2 Infrared Spectroscopy: Analyse the functional groups of benzoic acid Lab 7

3. Aim of the experiment 3 To analyze the functional groups of benzoic acid using infrared spectroscopy

4. 4 Introduction IR spectroscopy deals with the interaction of infrared radiation with matter . It is a light with a longer  wavelength and lower  frequency   than visible light. Typical IR wavelengths range from 8x10 -5 cm to 1x10 -2 cm, and this corresponds to energies of around 1-10 kcal. This energy is sufficient to make atoms vibrate, but not enough to cause electronic transitions.

5. 5 IR Spectroscopy Symmetrical stretching Antisymmetrical stretching Scissoring Rocking Wagging Twisting

6. 6 IR Spectroscopy Heavier atoms vibrate more slowly than lighter ones, so a C-D bond will vibrate at a lower frequency than a C-H bond . Thus, stronger bonds generally vibrate faster than weaker bonds. So O-H bonds which are stronger than C-H bonds vibrate at higher frequencies.

7. IR Spectrum A plot of % transmittance ( %T) vs vibrational frequency in wavenumbers (cm -1 ) 7 λ = wavelength υ = frequency  c = speed of light in a vacuum

8. 8 Increasing energy Increasing frequency IR Spectrum

9. 9 IR Spectrum

10. 10 IR Spectrum

11. 11 Instrumentation

12. 12 Sample Analysis of Solid Compounds KBr is used mostly in IR Spectroscopy techniques In  IR spectroscopy, samples are analyzed by grinding with powdered potassium bromide and pressing into a disc . Potassium bromide is transparent from the near  UV  to long-wave  IR wavelength  (0.25-25 µm) and has no significant  optical absorption  lines in its high transmission region. It is used widely as infrared optical windows and components for general spectroscopy because of its wide spectral range.

13. 13 Sample Analysis of Benzoic acid

14. 14 Procedure Transfer some KBr out of the desiccator into a mortar and grind it into a very fine powder using a clean pestle . Add about 1 to 2 % of your sample (Benzoic acid), mix and grind it well again. The mixture is now ready for placement in a stainless steel die that should have already been assembled as per the following figure.

15. 15 Procedure KBr die – Side View KBr die – Cross Section ram upper pellet lower pellet KBr+sample

16. 16 Procedure Apply 8 to 10 tons of pressure for 3 to 5 minutes. Leave the vacuum on during this step also . Slowly bleed pressure from the die by slowly opening the bleed screw and remove the disk from the die Using tweezers transfer the pressed KBr disk to the appropriate disk holder for IR analysis. Avoid contact with your fingers as the disk may pick up moisture.