B4 - Infrared Spectrometry

Question 1

What happens when bonds absorb infrared radiation (IR)?

Answer 1

They bend or stretch more (atoms are already in constant motion in a molecule).

Question 2

What are two types of vibrations in a bond?

Answer 2

A stretch - a rhythmic movement along the line between two atoms so that the distance between the two atomic centres increases and decreases.
A bend - results in a change in bond angle.

Question 3

The amount that a bond stretches or bends depends on what two things?

Answer 3

• The mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms.
• The strength of the bond - stronger bonds vibrate faster than weaker bonds.

Question 4

Any particular bond can only absorb radiation that has the same what?

Answer 4

The same frequency as the natural frequency of the bond.

Question 5

What scale is used for the frequency of bonds?

Answer 5

Wavenumber, which is proportional to frequency.

Question 6

In brief terms about infrared radiation and atmospheric gases, what causes global warming?

Answer 6

The Earth’s surface absorbs much of the visible and IR radiation from the sun, but some is re-emitted from the Earth’s surface in the form of longer-wavelength IR radiation. Greenhouse gases absorb this longer-wavelength IR radiation because it has the same frequency as the natural frequency of their bonds. Eventually the bonds in these molecules re-emit this energy as radiation that increases the temperature of the atmosphere close to the Earth’s surface.

Question 7

What is infrared spectroscopy used for?

Answer 7

Organic chemists use it as a means of identifying the functional groups present in organic molecules.

Question 8

How are functional groups of organic molecules identified by infrared spectroscopy?

Answer 8

1. The sample is placed inside an IR spectrometer
2. A beam of IR radiation is passed through the sample
3. The molecule absorbs some of the IR frequencies, and the emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample
4. The IR spectrometer is usually connected to a computer that plots a graph of transmitting against wavenumber

Question 10

What is the fingerprint region of the spectrum?

Answer 10

A number of unique peaks which can be used to identify the particular molecule under investigation.

Question 11

What is the wavenumber for O—H groups in alcohols?

Answer 11

3200-3600 cm-1

Question 12

What is the wavenumber for C==O groups in aldehydes, ketones and carboxylic acids?

Answer 12

1630-1820 cm-1

Question 13

What is a characteristic that all organic compounds have, so why do you need to be careful?

Answer 13

All organic compounds produce a characteristic peak between 2850 and 3100 cm-1 from the presence of C-H bonds. This is often confused with the O-H peak in alcohols, so you will need to take care.

Question 14

What are some features of an infrared spectrum of an alcohol?

Answer 14

• Absorbance peak within the range 3200-3600 cm-1 caused by the O—H bond
• Peak between 1000-1300 cm-1 caused by the C—O bond - though this peak is often difficult to assign

Question 15

What are some features of an infrared spectrum of an aldehyde or ketone?

Answer 15

• Key absorbance peak within the range 1630-1820 cm-1 caused by the C==O bond. This peak typically absorbs close to 1700 cm-1

Question 17

What are some features of an infrared spectrum of a carboxylic acid?

Answer 17

• Key absorbance peak within the range 1630-1820 cm-1 caused by the C==O bond
• A broad peak at 2500-3330 cm-1 caused by the O—H group
• Like aldehydes and ketones, the C==O peak typically absorbs close to 1700 cm-1
• Peak at 1000-1300 cm-1 that represents the C—O bond - this isn’t always reliable as it is in the fingerprint region

Question 18

What indicates the presence of a carboxylic acid in an organic molecule?

Answer 18

A C==O peak at around 1700 cm-1, together with a very broad O—H absorption within the range 2500-3000 cm-1

Question 19

What are some real-life applications of infrared spectroscopy?

Answer 19

• Remote sensors can detect and measure carbon monoxide, carbon dioxide and hydrocarbons in busy town centres or by motorways to monitor localised pollution.
  -IR-based breathalysers pass a beam of IR radiation through the captured breath and detect IR absorbance of compounds in the breath. Characteristic bonds present in ethanol are detected. The more IR radiation absorbed, the higher the reading, and the more ethanol in the breath.

Question 20

What is a typical sequence that chemists use for identification of an organic molecule?

Answer 20

1. Elemental analysis
2. Mass spectrometry
3. Infrared spectroscopy