Chapter 17

Question 1

What is the molecular ion? How is it written?

Answer 1

• The ion formed when an organic compound loses an electron and forms a positive ion in a mass spectrometer
• Is often written as M^+

Question 2

Show how the molecular ion of propan-1-ol is formed.

Answer 2

• CH3CH2CH2CH2OH -> CH3CH2CH2CH2OH+ + e-

Question 3

Where is the molecular ion peak found?

Answer 3

• It is the peak the furthest right on a mass spectrum

Question 4

What peak can sometimes be found beyond the molecular ion peak, and why?

Answer 4

• The M+1 peak
• It exists due to the fact that 1.1% of carbon atoms exist as carbon-13 (for example, a small proportion of propanol molecules will contain an atom of carbon-13, and so their molecular mass will be 61 instead of 60)

Question 5

What is fragmentation?

Answer 5

• The breaking down of molecular ions into smaller pieces known as fragments in a mass spectrometer

Question 6

What do molecular ions break down into? Give an example.

Answer 6

• In the simplest fragmentation, a molecular ion breaks down into a positively charged fragment ion and a radical
• CH3CH2CH2CH2OH+ -> CH2OH+ + ·CH2CH3

Question 7

What are the other peaks in a mass spectrum caused by?

Answer 7

• Fragment ions

Question 8

What can mass spectrometry be used for, and how?

Answer 8

• To help identify molecules
• From the fragment ions, as these will be different between molecules

Question 9

What are the m/z of the hydroxide, methyl, ethyl, propyl and butyl fragment ions?

Answer 9

• 17
• 15
• 29
• 43
• 57

Question 10

How do you show the difference between the m/z of the molecular ion and fragment ions?

Answer 10

• You show the molecular ion as M+, and the rest as m/z
• The M+1 peak is also shown as M+1

Question 11

What kind of motion do covalent bonds naturally have?

Answer 11

• They vibrate around a central point, either by bending or stretching

Question 12

What does infrared radiation do to covalent bonds?

Answer 12

• It supplies them with more energy, so they bend/ stretch more

Question 13

What restriction is there on the absorption of IR by covalent bonds?

Answer 13

• Bonds can only absorb radiation of the same frequency as their natural frequency

Question 14

How is the absorption of IR by covalent bonds used by chemists?

Answer 14

• They use it to identify functional groups in organic molecules

Question 15

List 3 things to be aware of when interpreting an infrared spectrum.

Answer 15

• The fingerprint region
• Broad peaks
• C-H peak

Question 16

What is the fingerprint region?

Answer 16

• The fingerprint region is below 1500/cm, where there are several peaks
• However, each molecule has a unique fingerprint region

Question 17

How can the fingerprint region be used to identify a molecule?

Answer 17

• It can be ran through a database

Question 18

What is a broad peak?

Answer 18

• A peak that is U-shaped rather than having a sharp point

Question 19

Which peak is broad (but isn’t shown as broad on the data sheet)?

Answer 19

• OH for alcohol

Question 20

How do you identify functional groups using infrared spectroscopy?

Answer 20

• First give the wavenumber range/s
• Use to identify bond/s present
• Using this information you can say the functional group/s (common trick is having an alcohol and a ketone as this can be mistaken for a carboxylic acid)

Question 21

What is important about a C-H peak?

Answer 21

• It is usually not worth identifying as it is in almost all organic molecules
• It is often confused with the OH bond from alcohols

Question 22

What happens when IR radiation enters the atmosphere?

Answer 22

• It originally passes through the atmosphere and gets to the Earth’s surface, where most of it is absorbed
• Some of it is re-emitted as longer wavelength IR radiation

Question 23

What issue does the re-emission of IR radiation cause?

Answer 23

• Greenhouse gases (water, carbon dioxide and methane) have the same natural frequency as this IR radiation, and therefore absorb it
• They then re-emit this radiation, which increases the temperature close to the Earth’s surface (global warming)

Question 24

What are 2 uses of infrared spectroscopy?

Answer 24

• Breathalysers detect ethanol levels
• Remote sensors monitor gases causing air pollution