Chapter 17 - Spectroscopy (17.1-17.2) Flashcards
How do you work out the MR from a mass spectrum?
It is the last peak on a mass spectrum formed when a molecule loses an electron.
Why will there sometimes be a very small peak after the M+ peak?
This is because 1.1% of carbon is present as a carbon-13 isotope and so the molecular ion peak would be one more.
What is fragmentation?
It’s when some molecular ions break down into smaller pieces by the breaking of one bond forming a fragment ion
What fragment ion would have the m/z value of 15?
CH3+
What fragment ion would have an m/z value of 29?
C2H5+
What fragment ion would have an m/z value of 31?
CH3O+
What fragment ion would have an m/z value of 43?
C3H7+
What fragment ion would have an m/z value of 45?
C2H5O+
What fragment ion would have an m/z value of 57?
C4H9+
What else can be useful in identifying a compound in mass spectroscopy?
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)
How can you use the M and M+1 peaks to find the number of carbon atoms?
Height of M+1 peak/ Height of M peak x100
What happens when a bond absorbs infrared radiation?
It makes them bend or stretch more
What does the amount that a bond stretched or bend depend on?
The mass of the atoms in the bond (heavier atoms vibrate more slowly) and the strength of the bond (stronger bonds vibrate faster)
What are the three most abundant greenhouse gases?
Water vapour, carbon dioxide and methane.
Describe the greenhouse effect
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
What is the area on an infrared spectra below 1500?
It is the fingerprint region and contains unique peaks which can be used to identify a particular molecule using computer software
How do you recognise the OH- group in an alcohol?
There is a very wide “U” shaped peak between 3200 and 3600 which is only given by an OH- group.
How do you recognise an aldehyde or ketone form an infrared spectroscopy?
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
How do you recognise the infrared spectrum of a carboxylic acid?
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.
Give 2 examples of uses of applications of infrared spectroscopy
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.
What is a typical sequence of how you would identify a compound?
Elemental analysis - empirical formula
Mass spectrometry - molecular ion and fragment ion peaks
Infrared spectroscopy - identify bonds and functional groups
