Infrared spectroscopy

Question 1

Do all molecules absorb infrared radiation?

Answer 1

Yes

Question 2

How does the absorption of infrared radiation affect the molecule?

Answer 2

-This absorbed energy makes bonds vibrate with either a stretching or bending motion.

Question 3

What is the amount of vibration each a bond has (when absorbing infrared radiation) determined by?

Answer 3

The amount of vibration depends on: - The bond strength - The bond length - The mass of each atom involved in the bond.

Question 4

What frequency do most bonds vibrate at?

Answer 4

Between 300 and 4000 cm^-1, in the infrared part of the electromagnetic spectrum.

Question 5

Explain how infrared spectrum works in analysing a molecule.

Answer 5

1) In a modern infrared spectrometer, a beam of infrared radiation is passed through a sample of the material under investigation. 2) This beam contains the full range of frequencies present in the infrared region. 3) The molecule absorbs some of these frequencies and the emerging beam is analysed to plot a graph of transmittance against frequency. The frequency is measured using wavenumbers, with units of cm^-1

Question 6

What do the peaks of the following graphs show?

Answer 6

Each peak represents the absorbance of energy from infrared radiation that causes the vibration of a particular bond in the molecule under investigation.

Question 7

What are the following functional groups you need to be able to indentify?

Answer 7

• The O-H(hydroxyl) group in alcohols
• The C=O(carbonyl) group in aldehydes and ketones
• The COOH (carboxyl) group in carboxylic acids.

Question 8

What is the wavenumber(in cm^-1) for O-H(hydroxyl) group?

Answer 8

2500-3300 for carboxylic acids

3200-3550 for hydrogen bonded alcohols.

Question 9

What is the wavenumber(in cm^-1) for C=O(carbonyl) group in aldehydes and ketones?

Answer 9

1640-1750

Question 10

Why do most organic compounds produce a peak at approximately 3000cm^-1?

Answer 10

Most organic compounds produce a peak at approximately 3000cm^-1due to the absorption by C-H bonds.

Question 11

What sort of organic compound does the following infrared spectrum show?

Answer 11

• It shows the infrared spectrum for an alcohol.
• This is because there is a peak at 3230-3500 which represents an O-H bond, only present in alcohols.

Question 12

What does the following spectrum of a organic compound tell you what it is?

Answer 12

The peak at 1680-1750cm^-1 represents a C=O bond in aldehydes and ketones.

Question 13

What does the following infrared spectrum tell you about the organic compound?

Answer 13

Here we have two characteristic peaks:

• The peak at 2500-3300 cm^-1 is very broad and indicates the presence of the O-H group in a carboxylic acid.
• The strong sharp peak at 1680-1750 cm^-1 represents the C=O group in a carboxylic acid.

Question 14

Give some everyday uses of infrared spectroscopy.

Answer 14

• Monitoring the degree of unsaturation in polymers
• Quality control in perfume manufacture
• drug analysis
• Uses as one of the main methods for testing the breath of suspected drunken drivers for ethanol.