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ChemWiki: The Dynamic Chemistry E-textbook > Wikitexts > UC Davis > UCD Chem 124A: Kauzlarich > ChemWiki Module Topics > IR spectropscopy: Practical Problems

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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

Answers:

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.

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Last Modified
10:15, 2 Oct 2013

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