4.2.4 - Analytical techniques

Question 1

What is a molecular ion (M+) and how is it formed in mass spectrometry?

Answer 1

when placed in a mass spectometer an organic compound loses an electron and** forms a positive ion**

Question 2

Where is the molecular ion peak located?

Answer 2

clear peak at the highest m/z value on the right hand side
- the small peak next to the molecular ion peak is the M+1 peak

Question 3

What is fragmentation?

Answer 3

molecular ions break down into smaller pieces - positive fragment ion and free radical - the small peaks are fragment ions

Question 4

How does absorption of infrared impact covalent bonds?
How does this contribute to global warming

Answer 4

1. causes covalent bonds to vibrate more and absorb more energy
2. water vapour, carbon dioxide and methane absorb IR radiation (same frequency as their bonds)
3. vibrating bonds re-emit energy as radiation which leads to temperature rising in the atmosphere

Question 5

What is the fingerprint region and where is this located?

Answer 5

below 1500 cm-1
- unique peaks which can be used to identify a specific molecule
- can be compared to published spectra

Question 7

How do you identify an alcohol on an infrared spectra ?

Answer 7

O-H bond peak

Question 8

How do you identify an aldehyde or ketone on an infrared spectra?

Answer 8

C=O bond peak

Question 9

How do you identify a carboxylic acid on an infrared spectra?

Answer 9

C=O bond peak and broad O-H bond peak

Question 10

Give two applications of infrared spectroscopy

Answer 10

1. monitoring pollutents
2. modern breathalysers - detect IR absorbance of compounds in breath - bonds present in ethanol are detected - more IR absorbed the higher the ethanol present

Question 11

What is a typical sequence for identification using multiple techniques?

Answer 11

1. elemental analysis - determine empirical formula
2. mass spectroscopy - determine molecular mass and sections of the molecule
3. infrared spectroscopy - identify bonds and functional groups