FURTHER MASS SPECTROMETRY KNOCKHARDY PUBLISHING A guide for A level students KNOCKHARDY PUBLISHING

KNOCKHARDY PUBLISHING MASS SPECTROMETRY 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 SPECTROMETRY 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

Before you start it would be helpful to… MASS 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

MOLECULAR MASS DETERMINATION USING 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. IONISATION MOLECULAR ION FRAGMENTION RE-ARRANGEMENT FRAGMENTION

THE 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

differences between major peaks THE 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 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

THE MASS SPECTRUM - THE MOLECULAR ION In the spectrum of octane, a signal occurs at 114 due to the species C8H18+ The species due to the final signal is known as the molecular ion and is usually corresponds to the molecular mass of the compound. 20 40 60 80 100 Abundance % molecular ion 114 . m/z 10 20 30 40 50 60 70 80 90 100 110 120 130 140

THE MASS SPECTRUM - THE 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. 20 40 60 80 100 Abundance % 114 . m/z 10 20 30 40 50 60 70 80 90 100 110 120 130 140

THE MASS SPECTRUM - FRAGMENTATION 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). 43 20 40 60 80 100 Abundance % 29 57 71 85 114 . m/z 10 20 30 40 50 60 70 80 90 100 110 120 130 140

FRAGMENTATION PATTERNS ALKANES 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

FRAGMENTATION PATTERNS HALOGENOALKANES 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 m/z 20 40 60 80 100 Abundance % molecular ion contains...79Br 81Br

FRAGMENTATION PATTERNS ALDEHYDES AND KETONES 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 and the more abundant the species the taller its peak in the mass spectrum

FRAGMENTATION 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. O CH3 C C4H9 MOLECULAR ION has m/z = 100 • +

FRAGMENTATION 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. O CH3 C C4H9 MOLECULAR ION has m/z = 100 • + O C4H9 C+ CH3• 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. m/z = 85 O C4H9 C• CH3+ m/z = 15

FRAGMENTATION 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. O CH3 C C4H9 MOLECULAR ION has m/z = 100 • + O CH3 C+ 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. C4H9• m/z = 43 O CH3 C• C4H9+ m/z = 57

FRAGMENTATION 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. O CH3 C C4H9 Example; MOLECULAR ION has m/z = 100 • + O C4H9 C+ O CH3 C+ CH3• C4H9• m/z = 85 m/z = 43 O C4H9 C• O CH3 C• CH3+ C4H9+ m/z = 15 m/z = 57 A further peak occurs at m/z = 72 (100-28) due to loss of CO

IDENTIFY THE COMPOUNDS

IDENTIFY THE COMPOUND Abundance % m/z 43 29 122 124 79 81 122 124 29 79 81 43 20 40 60 80 100 Abundance % m/z 10 20 30 40 50 60 70 80 90 100 110 120 130 140

C3H7Br IDENTIFY THE COMPOUND Abundance % m/z 43 29 122 124 79 81 122 124 29 79 81 43 20 40 60 80 100 Abundance % m/z 10 20 30 40 50 60 70 80 90 100 110 120 130 140 C3H7Br

IDENTIFY THE COMPOUND Abundance % m/z 105 77 51 120 43 28 20 40 60 80 100 Abundance % m/z 10 20 30 40 50 60 70 80 90 100 110 120 130 140

C6H5COCH3 IDENTIFY THE COMPOUND Abundance % m/z 105 77 51 120 43 28 20 40 60 80 100 Abundance % m/z 10 20 30 40 50 60 70 80 90 100 110 120 130 140 C6H5COCH3

IDENTIFY THE COMPOUND Abundance % m/z 28 105 106 77 57 43 51 10 20 30 40 50 60 70 80 90 100 110 120 130 140 m/z Abundance % 20 40 60 80 100 28 105 106 77 57 43 51

C6H5CHO IDENTIFY THE COMPOUND Abundance % m/z 28 105 106 77 57 43 51 10 20 30 40 50 60 70 80 90 100 110 120 130 140 m/z Abundance % 20 40 60 80 100 28 105 106 77 57 43 51 C6H5CHO

IDENTIFY THE COMPOUND Abundance % m/z 56 57 113 43 71 142 10 20 30 40 50 60 70 80 90 100 110 120 130 140 m/z Abundance % 20 40 60 80 100 142 113 71 56 57 43

C10H22 IDENTIFY THE COMPOUND Abundance % m/z 56 57 113 43 71 142 10 20 30 40 50 60 70 80 90 100 110 120 130 140 m/z Abundance % 20 40 60 80 100 142 113 71 56 57 43 C10H22

What should you be able to do? REVISION 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? YES NO

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FURHER MASS SPECTROMETRY The End © 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING