The origin of Fragmentation PatternsUsing Fragmentation PatternsPeak Heights and also StabilityAcylium ions, +Exercises


After perfect this section, you should be able to

suggest possible molecular formulas for a compound, given the m/z value for the molecule ion, or a mass spectrum from which this value can be obtained. Predict the family member heights the the M+·, (M + 1)+·, etc., peaks in the mass spectrum of a compound, given the natural abundance that the isotopes of carbon and also the other elements present in the compound. Analyze the fragmentation pattern of the mass spectrum the a reasonably simple, well-known compound (e.g., hexane). Use the fragmentation sample in a provided mass spectrum to help in the to know of a fairly simple, unknown compound (e.g., an unknown alkane).

You are watching: Which of the following determines the peak height in a mass spectrum? select the correct answer.

Study Notes

When interpreting fragmentation patterns, you may discover it beneficial to understand that, together you can expect, the weakest carbon-carbon bonds space the ones most likely to break. You might wish to refer to the table of shortcut dissociation energies once attempting troubles involving the translate of mass spectra.

This web page looks at just how fragmentation fads are formed when essential molecules are fed right into a fixed spectrometer, and how girlfriend can gain information native the massive spectrum.

The origin of Fragmentation Patterns

When the vaporized essential sample passes right into the ionization chamber of a fixed spectrometer, the is bombarded through a currently of electrons. This electrons have actually a high enough energy to knock an electron off an organic molecule to kind a hopeful ion. This ion is called the molecular ion - or periodically the parental ion and is often given the prize M+ or

. The period in this second version represents the truth that what in the ion there will be a single unpaired electron. That"s one fifty percent of what was initially a pair of electrons - the other fifty percent is the electron i m sorry was eliminated in the ionization process.

The molecular ions are energetically unstable, and some the them will break up into smaller pieces. The simplest situation is the a molecular ion breaks right into two components - among which is an additional positive ion, and also the various other is one uncharged free radical.


The uncharged cost-free radical will not produce a heat on the massive spectrum. Just charged particles will be accelerated, deflected and detected by the fixed spectrometer. These uncharged particles will simply obtain lost in the an equipment - eventually, they acquire removed through the vacuum pump.

The ion, X+, will travel through the mass spectrometer simply like any type of other optimistic ion - and also will develop a line on the rod diagram. Every sorts the fragmentations of the initial molecular ion are feasible - and also that method that you will acquire a totality host of lines in the massive spectrum. For example, the mass spectrum that pentane looks prefer this:



What causes the line at m/z = 57?

How plenty of carbon atoms are there in this ion? there cannot it is in 5 due to the fact that 5 x 12 = 60. What about 4? 4 x 12 = 48. That leaves 9 to comprise a full of 57. How about C4H9+ then?

C4H9+ would certainly be +, and also this would certainly be produced by the complying with fragmentation:


The methyl radical developed will simply gain lost in the machine.

The line at m/z = 43 deserve to be cleared up similarly. If friend play roughly with the numbers, you will find that this synchronizes to a break developing a 3-carbon ion:


The line at m/z = 29 is typical of one ethyl ion, +:


The various other lines in the mass spectrum are more daunting to explain. For example, lines with m/z worths 1 or 2 much less than among the basic lines are often as result of loss of one or an ext hydrogen atoms during the fragmentation process.

Example 12.2.2: Pentan-3-one

This time the base height (the tallest top - and so the commonest fragment ion) is in ~ m/z = 57. However this is not developed by the same ion as the exact same m/z value height in pentane.


If friend remember, the m/z = 57 height in pentane was created by +. If friend look in ~ the framework of pentan-3-one, it"s impossible to gain that certain fragment native it.

Work along the molecule mentally chopping bits off until you come up v something that adds approximately 57. With a tiny amount that patience, you"ll eventually uncover + - i m sorry is created by this fragmentation:


You would certainly get specifically the same assets whichever next of the CO group you break-up the molecule ion. The m/z = 29 top is created by the ethyl ion - which as soon as again can be developed by dividing the molecule ion either next of the CO group.


Peak Heights and also Stability

The an ext stable an ion is, the much more likely the is to form. The more of a particular sort that ion that"s formed, the higher its peak height will be. We"ll look in ~ two typical examples of this.

Carbocations (carbonium ions)

Summarizing the most crucial conclusion from the web page on carbocations:

Order of stability of carbocations


Applying the logic of this to fragmentation patterns, it way that a separation which to produce a secondary carbocation is going to be an ext successful 보다 one creating a major one. A break-up producing a tertiary carbocation will certainly be much more successful still. Let"s look at the fixed spectrum that 2-methylbutane. 2-methylbutane is one isomer that pentane - isomers room molecules through the very same molecular formula, yet a different spatial arrangement of the atoms.


Look first at the very strong peak in ~ m/z = 43. This is resulted in by a various ion than the matching peak in the pentane mass spectrum. This top in 2-methylbutane is brought about by:


The ion developed is a an additional carbocation - it has two alkyl groups attached to the carbon with the confident charge. Together such, it is reasonably stable. The optimal at m/z = 57 is lot taller 보다 the equivalent line in pentane. Again a secondary carbocation is formed - this time, by:


You would acquire the same ion, that course, if the left-hand CH3 group damaged off rather of the bottom one together we"ve attracted it. In these two spectra, this is more than likely the many dramatic example of the extra security of a an additional carbocation.

See more: Question 35 The Recording Process Occurs, Multiple Choice Flashcards

Acylium ions, +

Ions with the positive charge ~ above the carbon that a carbonyl group, C=O, room also relatively stable. This is fairly plainly seen in the mass spectra that ketones prefer pentan-3-one.