chapter 17

Question 1

molecular ions and fragment ions

Answer 1

when an organic compound is placed in the mass spectrometer, it loses an electron and forms a positive ion, the molecular ion. the mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ion which gives the Mr of the compound

Question 2

finding the molecular ion peak

Answer 2

M+ peak
the clear peak at the highest m/z value on the right hand side of the mass spectrum
- you will usually see a very small peak one unit after the M+ peak this is the M+1 peak. It exists because some of the carbon is present as the carbon-13 isotope

Question 3

fragmentation

Answer 3

In the mass spectrometer some molecular ions break down into smaller pieces known as fragments. the other peaks in the mass spectrum are caused by fragment ions.

Question 4

common fragment ions and their m/z values

Answer 4

CH3+ =15
C2H5+ =29
C3H7+ =43
C4H9+ =57

Question 5

effect of IR on bonds

Answer 5

bonds can absorb IR, which makes them vibrate and stretch
amount it stretches depends on:
-mass of atoms in the bond- heavier= more slowly
-strength- stronger bonds vibrate faster

Question 6

infrared spectroscopy

Answer 6

• used as a means of identifying the functional groups present in organic molecules
• the molecule absorbs some of the IR frequencies and the emerging beam is analysed to identify the frequencies that have been absorbed by the sample

Question 7

fingerprint region

Answer 7

• at the end of the IR spectrum

- contains unique peaks which can be used to identify the particular molecule

Question 8

applications of infrared spectroscopy

Answer 8

• pollutants can be identified by their IR spectral fingerprints
• remote sensors analyse the IR spectra of vehicle emissions and measure CO
• breathalysers