Montgomery College - Takoma Park/Silver Spring Campus CH 203 Organie Chemistry I

Question

Montgomery College - Takoma Park/Silver Spring Campus CH 203 Organie Chemistry INFRARED SPECTROSCOPY For discussion of infrared spectroscopy (IR), refer to the chapter containing IR in the text book. Pre Lab Questions 1. Define the following terms: a. Wavenumber: b. Absorbance: c. Percent transmittance: 2. For each of the following functional groups, give the wave number range where a peak would show up (some may have more than one characteristic peak): a. Alcohol b. Carboxylic acid c. Sp' C-H d. Sp C-H e. Aldehyde f. Alkene g. Amine

Explanation / Answer

1)

Wavenumber: The wavenumber is the number of waves in one centimeter and has the units of reciprocal centimeters (cm-1).
As per Plank's equation:
E    = hc/l
   = hcn
Absorbance: In IR spectroscopy, molecule absorbs particular wavelength of light (photons) to show molecular vibration. Thus, amount of light (wavelength) absorbrbed by particular vibration of a molecule is called as absorbance.

% Transmittance: Intensity is measured as the percent transmittance of the IR radiation with respect to the reference. In other words, a 100% transmittance means that the sample absorbed the same amount of radiation as the reference. A 0% transmittance means that the sample absorbed all of the radiation.

2)

a) Alcohol: -OH : 3200-3500 cm-1

b) Carboxylic acid: carbony (C=O) : 1715-1730 cm-1 and -OH : 3300-3500cm-1 broad

c) sp3 C-H : 2850-3000cm-1

d) sp2 C-H: 3000-3100 cm-1

e) Aldehyde: Carbonyl (C=O) : 1705-1730cm-1 and C(O)-H: 2700 - 2850cm-1 two peaks.

f) Alkene: C=C : 3000-3100 C-H; 1630-1670 : C=C

g) Amine: N-H: 3300-3500cm-1; C-N: 1000-1250cm-1.

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