Module 4: Mass Spectrometry + Infrared Spectroscopy

Question 1

What happens to an organic compound when placed into a mass spectrometer:

Answer 1

The organic compound loses an electron and forms a positive ion.

Question 2

On a mass spectrum what is the name of the clear peak with the highest (m/z)?

Answer 2

The M+ peak

Question 3

What is the M+ peak?

Answer 3

The positive ion formed when a molecule loses an electron.

Question 4

What does the m/z of a M+ peak tell you about the organic molecule?

Answer 4

The m/z of the M+ peak displays the molecular mass of the organic compound.

Question 5

Why does a small peak after the M+ peak exist?

Answer 5

Because 1.1% of carbon atoms are carbon-13 isotopes and-so a small percentage will have a greater mass number.

Question 6

Fragmentation:

Answer 6

The breaking down of molecular ions into smaller pieces. Normally into a cation and a radical.

Question 7

What is detected by a mass spectrometer?

Answer 7

positive ions

Question 8

How do you identify an organic molecule from a mass spectrum?

Answer 8

Identify the M+ peak to calculate the mass number.

Identify each fragment ion peak and identify the corresponding ions.

Question 9

What is required for a covalent bond to absorb radiation?

Answer 9

The frequency of the radiation must be the same as the bond’s natural frequency.

Question 10

What are the two type of bond vibration?

Answer 10

Stretching and Bending

Question 11

What is bond bending dependant on?

Answer 11

The mass of atoms in the bond (Heavier = Slower)
+
The strength of the bond(Stronger = Faster)

Question 12

How do green house gases absorb radiation from the sun?

Answer 12

The infrared radiation is too short wave to be absorbed, but once some is re-emitted from the earth’s surface it has a longer wavelength and the same frequency as the natural frequency of the atmospheric gases.

Question 13

What is Infrared Spectroscopy used for?

Answer 13

Identifying the functional groups in organic molecules.

Question 14

The process of infrared spectroscopy:

Answer 14

• Sample placed in IR spectrometer.
• A beam of IR radiation in the range 200-4000 is passed through the sample.
• The molecule absorbs some of the IR frequencies and the emerging beam of radiation is analysed to identify which frequencies have been absorbed.
• The IR spectrometer is connected to a computer that plots a graph.

Question 15

What are the dips in an IR graph called?

Answer 15

Peaks

Question 16

C-C wave number:

Answer 16

750-1100

Question 17

C-X (X = Cl, Br, I) wave number:

Answer 17

500-800

Question 18

C-F wave number:

Answer 18

1000-1350

Question 19

C-O wave number:

Answer 19

1000-1300

Question 20

C=C wave number:

Answer 20

1620-1680

Question 21

C=O wave number:

Answer 21

1630-1820(Sharp peak)

Question 22

C=C (Aromatic) wave number:

Answer 22

1450-1650

Question 23

C ≡ N wave number:

Answer 23

2220 - 2260

Question 24

C-H wave number:

Answer 24

2850-3300

Question 25

O-H (Carboxylic acids) wave number:

Answer 25

2500-3000 (Very broad peak)

Question 26

N-H wave number:

Answer 26

3300-3500

Question 27

O-H (alcohols) wave number:

Answer 27

3200-3600 (Broad peak)

Question 28

Application of infrared spectroscopy:

Answer 28

-Identifying pollutants
-Remote sensors can measure the pollutants released by cars
IR based breathalysers - detect presence of ethanol

Question 29

Three techniques to identify a molecule (using spectroscopy):

Answer 29

Elemental analysis
Mass Spectrometry
Infrared spectroscopy

Question 30

What is the base peak in a mass spectrum?

Answer 30

Base peak: The most intense (tallest) peak in a mass spectrum.

Question 31

Why might a molecular ion peak be missing from a spectrum?

Answer 31

The molecular ion may be very unstable and-so will completely fragment.