Question

How do you predict IR spectra?

Answer 1

Hint: IR means Infrared spectroscopy. In IR spectroscopy the infrared rays from the electromagnetic radiation are going to be used to determine the structure of the organic compounds. In IR spectroscopy there is an involvement of vibration of the organic molecules.

Complete answer:
- In the question it is asked how we can predict IR spectra.
- By using IR spectroscopy we will get the details of the functional groups which are present in the organic compounds.
- The IR spectroscopy also called vibrational spectroscopy.
- There are two types of vibrations in IR spectroscopy. They are stretching and bending.
- Based on the stretching and bending vibrations IR spectrum is going to be divided into two regions.
- The regions which are present in the IR spectrum are the functional group region and fingerprint region.
- In the functional group region we can see the stretching of the functional groups which are present in the organic compounds.
- Based on the stretching we can find the functional groups which are present in the compounds by using IR spectroscopy.
- The stretching frequencies of various functional groups are tabulated as below.

Functional Group	Frequency (1/cm)
water OH Stretch	3700-3100
alcohol OH stretch	3600-3200
carboxylic acid OH stretch	3600-2500
N-H stretch	3500-3350
stretch	~3300
=C-H stretch	3100-3000
-C-H stretch	2950-2840
-C-H aldehyde	2900-2800
stretch	~2250
stretch	2260-2100
C=O aldehyde	1740-1720
C=O anhydride	1840-1800, 1780-1740
C=O ester	1750-1720
C=O ketone	1745-1715
C=O amide	1700-1500
C=C alkene	1680-1600
C=C aromatic	1600-1400
CH2 bend	1480-1440
CH3 bend	1465-1440, 1390-1365
C-O-C stretch	1250-1050 several
C-OH stretch	1200-1020
NO2 stretch	1600-1500 and 1400-1300
C-F	1400-1000
C-Cl	800-600
C-Br	750-500
C-I	~500

- By using the above stretching and bending vibrations we can find the presence of the type of functional groups.

Note:
IR spectroscopy is going to play a big role in the structural elucidation of the organic compounds. IR spectroscopy vibrational frequency is going to consider while determining the unknown structure of the organic compounds.