further mass spectrometry a guide for a level students knockhardy publishing
TRANSCRIPT
FURTHER MASS FURTHER MASS SPECTROMETRYSPECTROMETRY
A guide for A level studentsA guide for A level students
KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING
INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.argonet.co.uk/users/hoptonj/sci.htm
Navigation is achieved by...
either clicking on the grey arrows at the foot of each page
or using the left and right arrow keys on the keyboard
MASS SPECTROMETRYMASS SPECTROMETRYKNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING
CONTENTS• Fragmentation of molecular ions - theory
• What a mass spectrum tells you
• Molecular ions
• Fragmentation
• Mass spectra of alkanes
• Mass spectra of halogenoalkanes
• Mass spectra of aldehydes and ketones
• Test questions
• Check list
MASS SPECTROMETRYMASS SPECTROMETRY
Before you start it would be helpful to…
• recall the basic principles of a mass spectrometer
• know the different types of functional group in organic chemsitry
MASS SPECTROMETRYMASS SPECTROMETRY
IONISATION
FRAGMENTION
FRAGMENTIONRE-ARRANGEMENT
MOLECULAR MASS DETERMINATIONMOLECULAR MASS DETERMINATIONUSING MASS SPECTROMETRYUSING MASS SPECTROMETRY
Nowadays, mass spectrometry is used to identify unknown or new compounds.
When a molecule is ionised it forms a MOLECULAR ION which can also undergo FRAGMENTATION or RE-ARRANGEMENT to produce particles of smaller mass.
Only particles with a positive charge will be deflected and detected.
The resulting spectrum has many peaks.
The final peak (M+) shows the molecular ion (highest m/z value) and indicates the molecular mass. The rest of the spectrum provides information about the structure.
MOLECULAR ION
Spectra obtained for organic molecules have many peaks. Each peak is due to a particular fragment with a certain m/z value.
highest m/z value usually corresponds to the molecular ion
its position provides information about the molecular mass of a substance
the tallest peaks come from the most stable species
THE MASS SPECTRUMTHE MASS SPECTRUM
Spectra obtained for organic molecules have many peaks. Each peak is due to a particular fragment with a certain m/z value.
highest m/z value usually corresponds to the molecular ion
its position provides information about the molecular mass of a substance
the tallest peaks come from the most stable species
THE MASS SPECTRUMTHE MASS SPECTRUM
Interpretation of thousands of spectra has shown that many classes of organic compound show characteristic fragmentation patterns due to their functional groups.
It is possible to identify the type of compound from its spectrum by looking at the ...
position of peaks
differences between major peaks
In the spectrum of octane, a signal occurs at 114 due to the species C8H18+
THE MASS SPECTRUM - THE MASS SPECTRUM - THE MOLECULAR IONTHE MOLECULAR ION
10 20 30 40 50 60 70 80 90 100 110 120 130 140 0 m/z
20
40
60
8
0
100
Ab
un
dan
ce %
114
.
The species due to the final signal is known as the molecular ion and is usually corresponds to the molecular mass of the compound.
molecular ion
THE MASS SPECTRUM - THE MASS SPECTRUM - THE MOLECULAR IONTHE MOLECULAR ION
The small peak (M+1) at 115 due to the natural abundance (about 1%) of carbon-13. The height of this peak relative to that for the molecular ion depends on the number of carbon atoms in the molecule. The more carbons present, the larger the M+1 peak.
10 20 30 40 50 60 70 80 90 100 110 120 130 140 0 m/z
20
40
60
8
0
100
Ab
un
dan
ce %
114
.
THE MASS SPECTRUM - THE MASS SPECTRUM - FRAGMENTATIONFRAGMENTATION
10 20 30 40 50 60 70 80 90 100 110 120 130 140 0 m/z
20
40
60
8
0
100
Ab
un
dan
ce %
2971
43
57
114
85
.
The rest of the spectrum provides additional information of the molecule’s structure.Peaks appear due to characteristic fragments (e.g. 29 due to C2H5
+) and differences between two peaks also indicates the loss of certain units (18 for H2O, 28 for CO).
The mass spectra of simple hydrocarbons have peaks at m/z values corresponding to the ions produced by breaking C-C bonds. Peaks can occur at ...
m/z 15 29 43 57 71 85 etc. CH3
+ C2H5+ C3H7
+ C4H9+ C5H11
+ C6H13+
• the stability of the carbocation formed affects its abundance
• the more stable the cation the higher the peak
• the more alkyl groups attached to the carbocation the more stable it is
most stable tertiary 3° > secondary 2° > primary 1° least stable
alkyl groups are electron releasing and stabilise the cation
ALKANES
FRAGMENTATION PATTERNSFRAGMENTATION PATTERNS
HALOGENOALKANES
FRAGMENTATION PATTERNSFRAGMENTATION PATTERNS
Multiple peaks occur in the molecular ion region due to different halogen isotopes.There are two peaks for the molecular ion of C2H5Br, one for the molecule containing the isotope 79Br and the other for the one with the 81Br isotope. Because the two isotopes are of similar abundance, the peaks are of similar height.
10 20 30 40 50 60 70 80 90 100 110 120 130 140 0 m/z
20
40
60
8
0
100
Ab
un
dan
ce %
molecular ion contains...79Br 81Br
Cleavage of bonds next to the carbonyl group (C=O) is a characteristic fragmentation of aldehydes and ketones. A common fragment is carbon monoxide (CO) but as it is a molecule and thus uncharged it will not produce a peak of its own. However, it will produce an m/z drop of 28 somewhere in the spectrum.
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group
the more stable the acylium ion RCO+, the more abundant it will be andthe more abundant the species the taller its peak in the mass spectrum
FRAGMENTATION PATTERNSFRAGMENTATION PATTERNS
ALDEHYDES AND KETONES
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group.
FRAGMENTATION PATTERNSFRAGMENTATION PATTERNS
Aldehydes and ketones
O
CH3 C C4H9
MOLECULAR IONhas m/z = 100
• +
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group.
FRAGMENTATION PATTERNSFRAGMENTATION PATTERNS
Aldehydes and ketones
O
CH3 C C4H9
O
C4H9 C+ CH3•
O
C4H9 C• CH3+
m/z = 15
m/z = 85
MOLECULAR IONhas m/z = 100
Breaking the bond between the methyl group and the carbonyl group produces two possible ions, depending on how the bond breaks.
Two peaks at m/z values 15 and 85 will appear in the mass spectrum.
• +
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group.
FRAGMENTATION PATTERNSFRAGMENTATION PATTERNS
Aldehydes and ketones
O
CH3 C C4H9
O
CH3 C+ C4H9•
O
CH3 C• C4H9+
m/z = 57
m/z = 43
MOLECULAR IONhas m/z = 100
• +
Breaking the bond between the butyl group and the carbonyl group produces two further ions, depending on how the bond breaks.
Two peaks at m/z values 43 and 57 will appear in the mass spectrum.
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group.
FRAGMENTATION PATTERNSFRAGMENTATION PATTERNS
Aldehydes and ketones
O
CH3 C C4H9
O
CH3 C+ C4H9•
O
CH3 C• C4H9+
m/z = 57
m/z = 43
O
C4H9 C+ CH3•
O
C4H9 C• CH3+
m/z = 15
m/z = 85
MOLECULAR IONhas m/z = 100
• +
A further peak occurs at m/z = 72 (100-28) due to loss of CO
Example;
IDENTIFY THE IDENTIFY THE COMPOUNDSCOMPOUNDS
IDENTIFY THE COMPOUNDIDENTIFY THE COMPOUND
122 124
29
79 81
43
10 20 30 40 50 60 70 80 90 100 110 120 130 140 0 m/z
Ab
un
dan
ce %
20
40
60
8
0
100
IDENTIFY THE COMPOUNDIDENTIFY THE COMPOUND
122 124
29
79 81
43
10 20 30 40 50 60 70 80 90 100 110 120 130 140 0 m/z
Ab
un
dan
ce %
20
40
60
8
0
100
C3H7Br
IDENTIFY THE COMPOUNDIDENTIFY THE COMPOUND
120
28
77
43
51
105
10 20 30 40 50 60 70 80 90 100 110 120 130 140 0 m/z
20
40
60
8
0
100
Ab
un
dan
ce %
IDENTIFY THE COMPOUNDIDENTIFY THE COMPOUND
120
28
77
43
51
105
10 20 30 40 50 60 70 80 90 100 110 120 130 140 0 m/z
20
40
60
8
0
100
Ab
un
dan
ce %
C6H5COCH3
IDENTIFY THE COMPOUNDIDENTIFY THE COMPOUND
0
28
105 106
77
5743
51
10 20 30 40 50 60 70 80 90 100 110 120 130 140 m/z
Ab
un
dan
ce %
20
40
60
8
0
100
IDENTIFY THE COMPOUNDIDENTIFY THE COMPOUND
0
28
105 106
77
5743
51
10 20 30 40 50 60 70 80 90 100 110 120 130 140 m/z
Ab
un
dan
ce %
20
40
60
8
0
100
C6H5CHO
IDENTIFY THE COMPOUNDIDENTIFY THE COMPOUND
0
142
113
71
56 57
43
10 20 30 40 50 60 70 80 90 100 110 120 130 140 m/z
Ab
un
dan
ce %
20
40
60
8
0
100
IDENTIFY THE COMPOUNDIDENTIFY THE COMPOUND
0
142
113
71
56 57
43
10 20 30 40 50 60 70 80 90 100 110 120 130 140 m/z
Ab
un
dan
ce %
20
40
60
8
0
100
C10H22
REVISION CHECKREVISION CHECK
What should you be able to do?
Understand how mass spectrometry can be used to calculate molecular mass
Recall the term molecular ion and understand what information it provides
Interpret simple mass spectra
CAN YOU DO ALL OF THESE? CAN YOU DO ALL OF THESE? YES YES NONO
You need to go over the You need to go over the relevant topic(s) againrelevant topic(s) again
Click on the button toClick on the button toreturn to the menureturn to the menu
WELL DONE!WELL DONE!Try some past paper questionsTry some past paper questions
© 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
FURHER MASS FURHER MASS SPECTROMETRYSPECTROMETRY
The EndThe End