Chapter 17 - Infrared spectroscopy and mass spectrometry (17.1-17.2)

Question 1

How do you work out the MR from a mass spectrum?

Answer 1

It is the last peak on a mass spectrum formed when a molecule loses an electron.

Question 2

Why will there sometimes be a very small peak after the M+ peak?

Answer 2

This is because 1.1% of carbon is present as a carbon-13 isotope and so the molecular ion peak would be one more.

Question 3

What is fragmentation?

Answer 3

It’s when some molecular ions break down into smaller pieces by the breaking of one bond forming a fragment ion

Question 4

What fragment ion would have the m/z value of 15?

Answer 4

CH3+

Question 5

What fragment ion would have an m/z value of 29?

Answer 5

C2H5+

Question 6

What fragment ion would have an m/z value of 31?

Answer 6

CH3O+

Question 7

What fragment ion would have an m/z value of 43?

Answer 7

C3H7+

Question 8

What fragment ion would have an m/z value of 45?

Answer 8

C2H5O+

Question 9

What fragment ion would have an m/z value of 57?

Answer 9

C4H9+

Question 10

What else can be useful in identifying a compound in mass spectroscopy?

Answer 10

Look at the differences between the m/z value of the molecular ion peak and some fragment ion peaks.

(For example a peak at 15 for CH3+ could be small but there could be a larger peak at 15 less than the molecular ion peak, so you know CH3 has been removed)

Question 11

What happens when a bond absorbs infrared radiation?

Answer 11

It makes them bend or stretch more

Question 12

What does the amount that a bond stretched or bend depend on?

Answer 12

The mass of the atoms in the bond (heavier atoms vibrate more slowly) and the strength of the bond (stronger bonds vibrate faster)

Question 13

What are the three most abundant greenhouse gases?

Answer 13

Water vapour, carbon dioxide and methane.

Question 14

Describe the greenhouse effect

Answer 14

1) IR radiation passes through the atmosphere
2) It reaches the earths surface where most of it is absorbed
3) However some is re-emitted as longer-wavelength IR radiation
4) The greenhouse gases absorb this IR radiation as it has the same frequency as the frequency of their bonds
5) They re-emit this energy which increases the temperature of the earth

Question 15

What is the area on an infrared spectra below 1500?

Answer 15

It is the fingerprint region and contains unique peaks which can be used to identify a particular molecule using computer software

Question 16

How do you recognise the OH- group in an alcohol?

Answer 16

There is a very wide “U” shaped peak between 3200 and 3600 which is only given by an OH- group.

Question 17

How do you recognise an aldehyde or ketone form an infrared spectroscopy?

Answer 17

There is a very sharp peak showing the C=O around 1700cm but there should not be any other significant peaks apart from C-H

Question 18

How do you recognise the infrared spectrum of a carboxylic acid?

Answer 18

Like an alcohol it will have a very broad “U” shaped dip around 3000cm (not just a small peak) but also it will have a very sharp peak at 1700cm like aldehydes and ketones to show the C=O.

Question 19

Give 2 examples of uses of applications of infrared spectroscopy

Answer 19

Analysing vechile emissions - measure CO, CO2 and hydrocarbon levels in towns

Breathalysers - passes a beam of IR through breath and checks absorbance to see if it has characteristics of ethanol are detected.

Question 20

What is a typical sequence of how you would identify a compound?

Answer 20

Elemental analysis - empirical formula
Mass spectrometry - molecular ion and fragment ion peaks
Infrared spectroscopy - identify bonds and functional groups