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# IR spectropscopy: Practical Problems

### Introduction

In molecules, bonds between atoms vibrate with both stretching and bending motions. Each stretching and bending vibration of a given bond occurs  with a distinct frequency which can be obtained by IR spectrometer. An IR spectrometer is an instrument that uses infrared radiation passing through a compound and gives us IR spectrum due to the absorption of energy at a distinct wavelength of each kind of bond.  By interpreting IR spectrum, we can determine the wavelengths or wavenumbers  to find what kinds of bonds a compound has.

### Methods

1. locate the key peaks, especially from 4000 to 1400 cm-1

2. obtain the type of bond based on the table

3. determine the functional group (sometimes structure) of the compound

### Frequencies of important IR stretching vibration

 Type of bond Wavenumber (cm-1) sp3 C-H (alkane) 2960-2850 sp2 C-H (alkene) 3100-3020 sp C-H (alkyne) ~3300 C-H (aldehyde) ~ 2820 and ~ 2720 N-H (primary) 3500-3300(double peaks, broad) N-H (secondary) 3500-3300(single peak,broad) O-H (carboxylic acid) 3300-2500 (very broad) O-H ( alcohol) 3650-3200 C-N 1230-1020 C=N 1650-1550 C≡N 2260-2220 C-O 1250-1050 C=O 1780-1650 C=C 1680-1600 C=C (acromatic) ~ 1600 and ~1500 C≡C 2260-2100

### Examples

#### 1.

In this spectrum, we notice the high peaks at 2934 cm-1 and 1704 cm-1. Compare these wavenumbers to the  table, the 2934 cm-1 peak is the sp3 C-H stretch, and the 1704 cm-1 peak is the C=O stretch. Therefore, this compound is ketone. The spectrum is actually got from cyclohexanone sample.

#### 2.

In this spectrum, there are 3 high peaks. The peak at 2925 cm-1 is the sp3 C-H stretch, the peak at 1739 cm-1 is the C=O stretch, and the peak at 1230 cm-1 is the C-O stretch. Thus, this compound is ester. This is the spectrum of octyl acetate.

#### 3.

This spectrum is more complicated. The broad and double peak at 3387 cm-1 is the primary N-H cm-1 stretch. The peak at 3175 cm-1 is the sp2C-H stretch. The very boad peak from 3500 cm-1 to 2500 cm-1 is the O-H stretch. The peak at 1673 cm-1 is the C=O stretch. The peak at 1624 cm-1 is the C=C acromatic stretch. The peak at 1246 cm-1 is the C-O stretch. So this compound has many functional groups which are amine, ketone, alcohol and benzene. The spectrum is from salicylamide sample.

### References

1. Buice, Paula Yurkanis. Organic Chemistry. fifth edition. Pearson: New Jersey, 2007.
2. All images are provided by the author.

### Problems

Figure 1 - C6H12O2

Figure 2

Figure 3

1. For figure 1, find the wavenumber of the sp3 C-H stretch and the C-O stretch

2. For figure 1, determine the type of bond at 1733 cm-1

3. The compound in figure 1 is alcohol, carboxylic acid or ester?

4. Determine the functional group of the compound in figure 2

5. The compound in figure 3 has formula C7H8, determine its structure, explain

1. 2900 cm-1, and 1180 cm-1

2. C=O stretch

3. ester

4. this compound has a broad spectrum at 3347 cm-1 , so it is alcohol

5. this spectrum has a high peak at 1494 cm-1 which is the C=C acromatic stretch. In addition, it has a peak at 3025 cm-1, and another about 2850 cm-1 which are sp2 C-H and  sp3 C-H respectively. Therefore, it is toluene.

### Contributors

• Name #1 here (if anonymous, you can avoid this) with university affiliation

10:15, 2 Oct 2013

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