Infrared spectroscopy

Question 1

What is infrared spectroscopy?

Answer 1

• Identifies FUNCTINAL GROUPS in organic compounds
• Covalent bond naturally VIBRATE
• Infrared radiation of the CORRECT FREQUENCY is ABSORBED by covalent bonds causes them to either:

STRETCH ~ where the distance between the atoms increase or decrease

BEND ~ where the angle between the bond increases or decreases.

Question 2

A TYPICAL infrared spectrum

Answer 2

• Certain frequencies are ABSORBED by bonds in the FUNCTIONAL GROUPS when IR is passed through the sample.
• PEAKS are produced at particular frequencies.
• Measured in WAVENUMBERS cm-1
• The peaks relate to the particular BONDS in the functional groups in the molecule.

Question 3

The fingerprint region

Answer 3

• The peaks found BELOW 1500cm-1
• UNIQUE to the compound
• Used to identify a molecule by COMPARING the spectrum to published spectra or a spectral database.

Question 4

C-O functional group & wavenumber

Answer 4

• alcohols
• carboxylic acids

1000 - 1300

Question 5

C=O functional group & wavenumber

Answer 5

• aldehydes
• ketones
• carboxylic acids

1630-1820
SHARP PEAK

Question 6

C-H functional group & wavenumber

Answer 6

• any organic compound

2850-3100

Question 7

O-H functional group & wavenumber (carboxylic acids)

Answer 7

• Carboxylic acids

2500-3300
- very BROAD peak due to hydrogen bonding

Question 8

O-H functional group & wavenumber (alcohols)

Answer 8

• alcohol

3200-3600
- BROAD peak due to hydrogen bonding

Question 9

USES of IR spectroscopy

Answer 9

MONITORING THE DEGREE OF UNSATURATION OF POLYMERS:

• The C=C double bond produces a peak at 1620-1680cm-1
• Its absence confirms that the polymer is saturated and addition polymerisation has fully taken place.

FORENSIC SCIENCE:

• The IR spectrum of an illegal drug
• Matched to the IR spectrum of an unknown sample taken from clothing or equipment at a crime scene